2021/10/29 更新

写真a

アキミツ ジュン
秋光 純
AKIMITSU Jun
所属
異分野基礎科学研究所 教授(特任)
職名
教授(特任)
外部リンク

学位

  • 理学博士 ( 東京大学 )

研究キーワード

  • 強相関電子系

  • 高温超伝導

  • 軌道整列

  • μSR法

  • 中性子回折

研究分野

  • 自然科学一般 / 磁性、超伝導、強相関系

学歴

  • 東京大学    

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    国名: 日本国

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  • 東京大学   Graduate School, Division of Science  

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  • 東京大学   Faculty of Science  

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  • 東京大学   Graduate School, Division of Science  

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  • 東京大学   Faculty of Science  

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    国名: 日本国

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  • 東京大学    

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    国名: 日本国

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経歴

  • 岡山大学   異分野基礎科学研究所   特任教授

    2016年4月 - 現在

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  • 広島大学   キラル物性研究拠点   特任

    2015年4月 - 2016年3月

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  • 岡山大学   エネルギー環境新素材拠点   特任教授

    2015年4月 - 2016年3月

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  • 青山学院大学   理工学部   助教授   青山学院大学名誉教授

    1976年9月 - 2015年3月

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  • 東京大学   The Institute for Solid State Physics

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所属学協会

委員歴

  • 仁科記念財団   評議員  

    2019年10月 - 現在   

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  • 日本原子力   J-PARCセンター利用者協議会委員  

    2018年8月 - 2020年3月   

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  • 国立研究開発法人日本原子力研究開発機構   中性子及び放射光利用研究開発・評価委員会委員  

    2017年2月 - 現在   

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  • 東京大学物性研究所   CCMS(物性研究所計算物質研究センター)  

    2015年10月 - 2019年3月   

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  • 東京大学物性研究所   ポスト「京」重点課題(7)「次世代の産業を支える新機能デバイス・高性能材料の創成」諮問委員  

    2015年10月 - 2019年3月   

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  • 日本原子力研究開発機構・高エネルギー加速器研究機構   物質・生命科学実験施設利用委員会委員  

    2015年5月 - 2019年3月   

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  • 総合科学研究機構   総合科学研究機構中性子科学センター選定委員会委員  

    2015年5月 - 2019年3月   

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  • 日本学術会議   連携会員  

    2011年10月 - 2017年9月   

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  • - 学術会議会員(19期)  

    2003年   

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  • - 科学技術・学術審議会専門委員  

    2002年   

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  • - 高エネルギー加速器研究機構物質構造科学研究所運営協議員  

    2001年   

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  • - 高エネルギー加速器研究機構運営協議員  

    2001年   

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  • - 平成13年度「超伝導材料の新展開に関する調査研究会」委員  

    2001年   

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  • - 物理学研究連絡委員会委員  

    2000年   

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  • - 学術会議物理専門委員会委員  

    1994年   

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  • 固体物理   編集委員  

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  • 凝縮系科学賞   選考委員  

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  • 岩垂奨学会   理事  

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  • 公益財団法人伊藤科学振興会   評議員  

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  • 山田科学振興会   学術参与  

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  • 日本中間子科学会   運営員長  

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論文

  • Controlling avalanche soliton nucleation in a chiral soliton lattice on a monoaxial chiral magnet CrNb<inf>3</inf>S<inf>6</inf>by dynamic strain

    M. Ohkuma, M. Mito, Y. Kousaka, J. Akimitsu, J. Kishine, K. Inoue

    Applied Physics Letters   118 ( 13 )   2021年3月

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    掲載種別:研究論文(学術雑誌)  

    The monoaxial chiral magnet CrNb 3 S 6 exhibits a chiral soliton lattice (CSL) upon application of an external magnetic field (H) perpendicular to the helical axis (c-axis). The increase in the soliton (kink) number of the CSL in the process of decreasing H accompanies soliton penetration after nucleation at the surface. Avalanche soliton nucleation occurs during the initial process of soliton nucleation. We applied dynamic strain with scores of MHz to the crystal with the crystal size along the c-axis of 10 μm from the edge of the c-axis, so that we controlled the avalanche soliton nucleation by modifying the surface barrier. The present results suggest that the acoustic wave is useful for controlling the soliton number.

    DOI: 10.1063/5.0040327

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  • Formations of Narrow Stripes and Vortex–Antivortex Pairs in a Quasi-Two-Dimensional Ferromagnet K2CuF4

    Yoshihiko Togawa, Tetsuya Akashi, Hiroto Kasai, Gary W. Paterson, Stephen McVitie, Yusuke Kousaka, Hiroyuki Shinada, Jun-ichiro Kishine, Jun Akimitsu

    Journal of the Physical Society of Japan   90 ( 1 )   014702 - 014702   2021年1月

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:Physical Society of Japan  

    DOI: 10.7566/jpsj.90.014702

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  • Soliton locking phenomenon over finite magnetic field region in the monoaxial chiral magnet CrNb 3 S 6

    M. Ohkuma, M. Mito, Y. Kousaka, T. Tajiri, J. Akimitsu, J. Kishine, K. Inoue

    Applied Physics Letters   117 ( 23 )   232403 - 232403   2020年12月

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:AIP Publishing  

    DOI: 10.1063/5.0028910

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  • Topological metastability supported by thermal fluctuation upon formation of chiral soliton lattice in $$\hbox {CrNb}_3\hbox {S}_6$$

    T. Honda, Y. Yamasaki, H. Nakao, Y. Murakami, T. Ogura, Y. Kousaka, J. Akimitsu

    Scientific Reports   10 ( 1 )   2020年12月

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:Springer Science and Business Media LLC  

    <title>Abstract</title>
    Topological magnetic structure possesses topological stability characteristics that make it robust against disturbances which are a big advantage for data processing or storage devices of spintronics; nonetheless, such characteristics have been rarely clarified. This paper focused on the formation of chiral soliton lattice (CSL), a one-dimensional topological magnetic structure, and provides a discussion of its topological stability and influence of thermal fluctuation. Herein, CSL responses against change of temperature and applied magnetic field were investigated via small-angle resonant soft X-ray scattering in chromium niobium sulfide (<inline-formula><alternatives><tex-math>$$\hbox {CrNb}_3\hbox {S}_6$$</tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
    <mml:mrow>
    <mml:msub>
    <mml:mtext>CrNb</mml:mtext>
    <mml:mn>3</mml:mn>
    </mml:msub>
    <mml:msub>
    <mml:mtext>S</mml:mtext>
    <mml:mn>6</mml:mn>
    </mml:msub>
    </mml:mrow>
    </mml:math></alternatives></inline-formula>). CSL transformation relative to the applied magnetic field demonstrated a clear agreement with the theoretical prediction of the sine-Gordon model. Further, there were apparent differences in the process of chiral soliton creation and annihilation, discussed from the viewpoint of competing between thermal fluctuation and the topological metastability.

    DOI: 10.1038/s41598-020-74945-6

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    その他リンク: http://www.nature.com/articles/s41598-020-74945-6

  • Superconductivity in Bi2−x Sb x Te3−y Se y (x = 1.0 and y = 2.0) under pressure

    Tong He, Xiaofan Yang, Tomoya Taguchi, Lei Zhi, Takafumi Miyazaki, Kaya Kobayashi, Jun Akimitsu, Hirofumi Ishii, Yen-Fa Liao, Hidenori Goto, Ritsuko Eguchi, Yoshihiro Kubozono

    Journal of Physics: Condensed Matter   32 ( 46 )   465702 - 465702   2020年11月

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:IOP Publishing  

    DOI: 10.1088/1361-648x/abaad2

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    その他リンク: https://iopscience.iop.org/article/10.1088/1361-648X/abaad2/pdf

  • Spontaneous magnetostriction effects in the chiral magnet CrNb3S6

    Takayuki Tajiri, Masaki Mito, Yusuke Kousaka, Jun Akimitsu, Jun-ichiro Kishine, Katsuya Inoue

    PHYSICAL REVIEW B   102 ( 1 )   2020年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    We identified the spontaneous magnetostriction effects in the chiral magnet CrNb3S6 with the Dzyaloshinskii-Moriya interaction. The powdered x-ray diffraction patterns indicate a prominent magnetostriction effect at the earth field. The unit-cell volume remains almost constant, as in the so-called Invar effect, below 170 K in the paramagnetic region, where the atomic positions of Nb and S show characteristic changes. Below the magnetic ordering temperature of 127 K, the lattice constants a and c exhibit an opposite temperature dependence. When a finite magnetic field of 1.2 kOe is applied, a magnetostriction appears that counterbalances the spontaneous magnetostriction. The magnetostriction phenomena in CrNb(3)S(6 )were investigated from the viewpoint of the atomic level of the chiral effect.

    DOI: 10.1103/PhysRevB.102.014446

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  • Superconductivity in 5d transition metal Laves phase SrIr<inf>2</inf>

    R. Horie, K. Horigane, S. Nishiyama, M. Akimitsu, K. Kobayashi, S. Onari, T. Kambe, Y. Kubozono, J. Akimitsu

    Journal of Physics Condensed Matter   32 ( 17 )   2020年4月

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    掲載種別:研究論文(学術雑誌)  

    © 2020 IOP Publishing Ltd. We report here the superconducting properties of a Laves phase superconductor SrIr2, which has a cubic MgCu2 structure. SrIr2 is a type-II superconductor, with a T c of 5.9 K. The estimated superconducting parameters of lower critical field µ 0 H c1 and upper critical field µ 0 H c2, coherence length ξ(0), penetration depth λ(0) and Ginzburg-Landau (GL) parameter κ(0) are approximately µ 0 H c1 = 101 Oe, µ 0 H c2(0) = 5.9 T, ξ(0) = 7.47 nm, λ(0) = 237 nm, and κ(0) = 31.7, respectively. The specific-heat data indicate that SrIr2 is a strong-coupling superconductor because the value of ΔC/γT c is approximately 1.71, which is larger than the value of 1.43 that is expected from the BCS theory. The physical properties obtained in this study are explained well by theoretical calculations including spin-orbit coupling (SOC). This result indicates that the physical properties of SrIr2 are strongly affected by the presence of SOC.

    DOI: 10.1088/1361-648X/ab6a2e

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  • μSR Study of the Magnetic State in Hole and Electron Doped Sr2IrO4

    S. Tsunoda, K. Horigane, H. Okabe, K. Machida, M. Akimitsu, K. Kawashima, R. Horie, K. Kobayashi, A. Koda, R. Kadono, J. Akimitsu

    Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2019)   2020年3月

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    掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:Journal of the Physical Society of Japan  

    DOI: 10.7566/jpscp.30.011145

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  • Superconducting Properties of AgSnSe2 Studied by 77Se-NMR and 119Sn-NMR

    K. Watanabe, Y. Naijo, K. Hada, T. Furukawa, T. Itou, T. Ueno, K. Kobayashi, J. Akimitsu

    Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2019)   2020年3月

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    掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:Journal of the Physical Society of Japan  

    DOI: 10.7566/jpscp.30.011057

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  • Synthesis and physical properties of the new iridium oxyfluoride Sr2Ir(O,F)6−δ using a topochemical reaction method

    K. Kuramochi, T. Shimano, T. Nishio, H. Okabe, A. Koda, K. Horigane, J. Akimitsu, H. Ogino

    Physical Review Materials   4 ( 1 )   2020年1月

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:American Physical Society (APS)  

    DOI: 10.1103/physrevmaterials.4.013403

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    その他リンク: http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevMaterials.4.013403/fulltext

  • Superconducting behavior of a new metal iridate compound, SrIr2, under pressure

    Xiaofan Yang, Huan Li, Tong He, Tomoya Taguchi, Yanan Wang, Hidenori Goto, Ritsuko Eguchi, Rie Horie, Kazumasa Horigane, Kaya Kobayashi, Jun Akimitsu, Hirofumi Ishii, Yen-Fa Liao, Hitoshi Yamaoka, Yoshihiro Kubozono

    JOURNAL OF PHYSICS-CONDENSED MATTER   32 ( 2 )   2020年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:IOP PUBLISHING LTD  

    Herein, we investigated the pressure dependence of electric transport in a new type of superconducting metal iridate compound, SrIr2, that exhibits a superconducting transition temperature, T-c, as high as 6.6 K at ambient pressure, in order to complete the T-c-pressure (p) phase diagram. Very recently, this sample's superconductivity was discovered by our group, but the superconducting behavior has not yet been clarified under pressure. In this study, we fully investigated this sample's superconductivity in a wide pressure range. The T-c value decreased with an increase in pressure, but the onset superconducting transition temperature, T-c(onset), increased above a pressure of 8 GPa, indicating an unconventional superconductivity different from a BCS-type superconductor. The magnetic field dependence of electric resistance (R) against temperature (R - T plot) recorded at 7.94 and 11.3 GPa suggested an unconventional superconductivity, followed by a p-wave polar model, supporting the deviation from a simple s-wave pairing. Moreover, we fully investigated the pressure dependence of crystal structure in SrIr2 and discussed the correlation between superconductivity and crystal structure. This is the first systematic study on superconducting behavior of a new type of metal iridate compound, MIr2 (M: alkali-earth metal atom), under pressure.

    DOI: 10.1088/1361-648X/ab4605

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  • Fermi level tuning of Ag-doped Bi2Se3 topological insulator

    Eri Uesugi, Takaki Uchiyama, Hidenori Goto, Hiromi Ota, Teppei Ueno, Hirokazu Fujiwara, Kensei Terashima, Takayoshi Yokoya, Fumihiko Matsui, Jun Akimitsu, Kaya Kobayashi, Yoshihiro Kubozono

    Scientific Reports   9 ( 1 )   2019年12月

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:Springer Science and Business Media LLC  

    DOI: 10.1038/s41598-019-41906-7

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    その他リンク: http://www.nature.com/articles/s41598-019-41906-7

  • Anomalous spiked structures in ESR signals from the chiral helimagnet CrNb3S6

    Daichi Yoshizawa, Yuya Sawada, Yusuke Kousaka, Jun-ichiro Kishine, Yoshihiko Togawa, Masaki Mito, Katsuya Inoue, Jun Akimitsu, Takehito Nakano, Yasuo Nozue, Masayuki Hagiwara

    PHYSICAL REVIEW B   100 ( 10 )   2019年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    We have performed X-band electron spin resonance (ESR) measurements on a single crystal of the metallic chiral helimagnet CrNb3S6 from 3.5 to 180 K and for the external magnetic fields H-ext, up to 4 kOe, perpendicular to the c axis (the helical axis of CrNb3S6). This field-crystalline configuration is expected to provide the chiral soliton lattice (CSL) state in this system. The main resonance line can be fit with a Dysonian function above T-c = 127 K, but additional features in the spectra were observed below 105 K. Specifically, spiked anomalies superposed on the main signals were observed for magnetic fields between H-c1 and H-c2 that are the appearing and disappearing fields of the spiked anomalies, respectively. The resulting magnetic field vs temperature phase diagram possesses three regions, which are interpreted as different dynamical responses in the CSL phase. In addition, the values of H-c2 are close to those reported by the d(2)M/dH(2) curve [Tsuruta et al., Phys. Rev. B 93, 104402 (2016)]. Furthermore, the field range between H-c1 and H-c2, where the spiked anomalies exist, depends on the field direction and shifts to higher fields when turning to the c axis, thereby providing additional evidence that these spiked anomalies must be related to the chiral soliton dynamics.

    DOI: 10.1103/PhysRevB.100.104413

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  • Surface-size and shape dependencies of change in chiral soliton number in submillimeter-scale crystals of chiral magnet CrNb3S6

    M. Ohkuma, M. Mito, N. Nakamura, K. Tsuruta, J. Ohe, M. Shinozaki, Y. Kato, J. Kishine, Y. Kousaka, J. Akimitsu, K. Inoue

    AIP ADVANCES   9 ( 7 )   2019年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER INST PHYSICS  

    We examine the surface size- and shape-effects of soliton annihilation and soliton nucleation in chiral magnet CrNb3S6. We measure magnetization (M) curves of submillimeter-sized single crystals with an equal length along the c-axis (L-c = 10 mu m) but with different cross sections in the ab-plane (S-ab = 0.120-0.014 mm(2)). We find a ferromagnetic type of magnetizing (FMM) with a convex curve (d(2)M/dH(2) < 0) near zero field (H = 0) and a major jump in M near the forced ferromagnetic state, which are more conspicuous, compared with earlier samples with submillimeter L-c [K. Tsuruta et al. J. Phys. Soc. Jpn. 85, 013707 (2016)]. A new finding is that the major jump in M occurs at lower fields in samples with the smaller S-ab. We further perform numerical simulation of the magnetization process with the Landau-Lifshitz-Gilbert equation of the Langevin-type. Based on the numerical results, we attribute the FMM at small fields to rapid annihilation of soliton assisted by the reduction of Dzyaloshinskii-Moriya interaction near the surfaces. We also discuss possible penetration processes of chiral soliton through the ac-(bc-)plane as well as ab-plane, and its relation to the major jump in M. Our experimental and calculated results will contribute to understanding of the effects of topological metastability in chiral magnets.

    DOI: 10.1063/1.5092366

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  • Observation of orbital angular momentum in the chiral magnet CrNb3S6 by soft x-ray magnetic circular dichroism

    M. Mito, H. Ohsumi, T. Shishidou, F. Kuroda, M. Weinert, K. Tsuruta, Y. Kotani, T. Nakamura, Y. Togawa, J. Kishine, Y. Kousaka, J. Akimitsu, K. Inoue

    PHYSICAL REVIEW B   99 ( 17 )   2019年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    The chiral magnet CrNb3S6 with its solitonic objects has novel magnetic and transport properties, in which the spin-orbit coupling (SOC) plays a central role. Aiming to address the possible existence of orbital moments driven by SOC, we perform soft x-raymagnetic circular dichroism spectroscopy at the Cr L-2,L-3 edges with in-plane magnetization. The dichroic signals provide direct experimental evidence that the Cr orbital magnetic moment is not quenched and is coupled antiparallel to the spin counterpart. Application of the orbital sum rule reveals that the magnitude of the Cr orbital moment is about 1% of the total magnetization. These findings are consistent with the first-principles electronic structure calculations that utilize the Cr 2p core radial function to define the Cr local 3d quantities. The distinct roles of the atomic SOC among the Cr 3d and Nb 4d states are discussed.

    DOI: 10.1103/PhysRevB.99.174439

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  • Superconductivity of carbides

    Kaya Kobayashi, Kazumasa Horigane, Rie Horie, Jun Akimitsu

    Physics and Chemistry of Carbon-Based Materials: Basics and Applications   149 - 209   2019年3月

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    掲載種別:論文集(書籍)内論文  

    © Springer Nature Singapore Pte Ltd. 2019. Carbon-based superconductors can be roughly categorized into two groups. The first includes relatively new superconductors, such as boron-doped diamond, graphite-intercalated compounds (GIC), carbon nanotube (CNT) superconductors and fullerene superconductors. Here, we review relatively "old materials," which are materials that can be described within the "BCS (Bardeen-Cooper-Schrieffer) frame." The contents of this chapter are as follows: 1. MX-type compounds (B1-type superconductors and SiC) 2. MC2 compounds (YC2 and LaC2) 3. Ln2C3 (La2C3 and Y2C3) 4. Boron carbide (RNi2B2C and YB2C2) We introduce the present experimental and theoretical understanding of carbonbased superconductors.

    DOI: 10.1007/978-981-13-3417-7_6

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  • Anomalous Temperature Behavior of the Chiral Spin Helix in CrNb3S6 Thin Lamellae

    Y. Togawa, J. Kishine, P. A. Nosov, T. Koyama, G. W. Paterson, S. McVitie, Y. Kousaka, J. Akimitsu, M. Ogata, A. S. Ovchinnikov

    PHYSICAL REVIEW LETTERS   122 ( 1 )   2019年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    Using Lorentz transmission electron microscopy and small-angle electron scattering techniques, we investigate the temperature-dependent evolution of a magnetic stripe pattern period in thin-film lamellae of the prototype monoaxial chiral helimagnet CrNb3S6. The sinusoidal stripe pattern appears due to formation of a chiral helimagnetic order (CHM) in this material. We found that as the temperature increases, the CHM period is initially independent of temperature and then starts to shrink above the temperature of about 90 K, which is far below the magnetic phase transition temperature for the bulk material T-c (123 K). The stripe order disappears at around 140 K, far above T-c. We argue that this cascade of transitions reflects a threestage hierarchical behavior of melting in two dimensions.

    DOI: 10.1103/PhysRevLett.122.017204

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  • Determination of the local structure of Sr2-xMxIrO4 (M = K, La) as a function of doping and temperature

    Kensei Terashima, Eugenio Paris, Eduardo Salas-Colera, Laura Simonelli, Boby Joseph, Takanori Wakita, Kazumasa Horigane, Masanori Fujii, Kaya Kobayashi, Rie Horie, Jun Akimitsu, Yuji Muraoka, Takayoshi Yokoya, Naurang Lal Saini

    PHYSICAL CHEMISTRY CHEMICAL PHYSICS   20 ( 36 )   23783 - 23788   2018年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ROYAL SOC CHEMISTRY  

    The local structure of correlated spin-orbit insulator Sr2-xMxIrO4 (M = K, La) has been investigated by Ir L-3-edge extended X-ray absorption fine structure measurements. The measurements were performed as a function of temperature for different dopings induced by substitution of Sr with La or K. It is found that Ir-O bonds have strong covalency and they hardly show any change across the Neel temperature. In the studied doping range, neither Ir-O bonds nor their dynamics, measured by their mean square relative displacements, show any appreciable change upon carrier doping, indicating the possibility of nanoscale phase separation in the doped system. On the other hand, there is a large increase of the static disorder in Ir-Sr correlation, larger for K doping than La doping. Similarities and differences with respect to the local lattice displacements in cuprates are briefly discussed.

    DOI: 10.1039/c8cp03756f

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  • Crystal structure and superconducting properties of KSr2Nb3O10 査読

    Kawaguchi T, Horigane K, Itoh Y, Kobayashi K, Horie R, Kambe T, Akimitsu J

    PHYSICA B-CONDENSED MATTER   536   830 - 832   2018年5月

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    掲載種別:研究論文(学術雑誌)  

    DOI: 10.1016/j.physb.2017.09.060

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  • Neutron diffraction study of antiferromagnetic ErNi3Ga9 in magnetic fields 査読

    Hiroki Ninomiya, Takaaki Sato, Yuji Matsumoto, Taketo Moyoshi, Akiko Nakao, Kazuki Ohishi, Yusuke Kousaka, Jun Akimitsu, Katsuya Inoue, Shigeo Ohara

    Physica B: Condensed Matter   536   392 - 396   2018年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Elsevier B.V.  

    We report specific heat, magnetization, magnetoresistance, and neutron diffraction measurements of single crystals of ErNi3Ga9. This compound crystalizes in a chiral structure with space group R32. The erbium ions form a two–dimensional honeycomb structure. ErNi3Ga9 displays antiferromagnetic order below 6.4 K. We determined that the magnetic structure is slightly amplitude–modulated as well as antiferromagnetic with q=(0,0,0.5). The magnetic properties are described by an Ising–like model in which the magnetic moment is always along the c–axis owing to the large uniaxial anisotropy caused by the crystalline electric field effect in the low temperature region. When the magnetic field is applied along the c–axis, a metamagnetic transition is observed around 12 kOe at 2 K. ErNi3Ga9 possesses crystal chirality, but the antisymmetric magnetic interaction, the so-called Dzyaloshinskii–Moriya (DM) interaction, does not contribute to the magnetic structure, because the magnetic moments are parallel to the DM–vector.

    DOI: 10.1016/j.physb.2017.09.057

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  • Effects of dynamic stress in magnetic superlattice of a monoaxial chiral magnet Cr1/3NbS2 査読

    K. Tsuruta, M. Mito, Y. Togawa, Y. Kousaka, J. Akimitsu, K. Inoue

    Journal of Physics: Conference Series   969 ( 1 )   2018年4月

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:Institute of Physics Publishing  

    A monoaxial chiral magnet forms a kind of spin superlattice structure, termed chiral soliton lattice (CSL), by the application of a magnetic field H perpendicular to a helical chiral axis. It has been reported that the CSL accompanies the magnetoresistance effect as well as a discrete change in magnetization and magnetoresistance. In order to verify the effect of the structural modification on the CSL state, we measured the magnetoresistance under the dynamic stress (DS) with a frequency of the order of MHz, which was applied by a piezoelectric ceramic oscillator. The steady application of DS while decreasing H resulted in a suppression of the insertion of chiral soliton. On the other hand, the application of a pulse-like DS while H decreased assisted the insertion of chiral soliton. These results demonstrate that DS modifies the spin structure of the monoaxial chiral magnet, and we can therefore change the activation energy for the insertion of chiral soliton while H is decreased.

    DOI: 10.1088/1742-6596/969/1/012132

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  • Enhanced superconducting transition temperatures in the rocksalt-type superconductors In1-xSnxTe (x≤0.5) ENHANCED SUPERCONDUCTING TRANSITION ... KOBAYASHI, AI, JESCHKE, and AKIMITSU 査読

    Kaya Kobayashi, Yukio Ai, Harald O. Jeschke, Jun Akimitsu

    Physical Review B   97 ( 10 )   2018年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Physical Society  

    We investigate superconductivity in In1-xSnxTe (x≤0.5) synthesized at high pressures of up to 2 GPa and observe an enhancement of the superconducting transition temperature Tc for increasing tin concentration x. These compounds have not been accessible in rocksalt structure via conventional ambient pressure synthesis. While the lattice constant smoothly increases with x, Tc saturates around x=0.4. Electronic structure calculations indicate that the Tc modulation is brought on by the change of the density of states in the vicinity of the Fermi energy [N(EF)]. However, differences between the calculated N(EF) and the observed electronic specific-heat coefficient indicate that the phonon dispersion plays an important role in the system and that the mechanism of superconductivity may not be the same in the entire doping range.

    DOI: 10.1103/PhysRevB.97.104511

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  • Nonlinear magnetic responses at the phase boundaries around helimagnetic and skyrmion lattice phases in MnSi: Evaluation of robustness of noncollinear spin texture 査読

    K. Tsuruta, M. Mito, H. Deguchi, J. Kishine, Y. Kousaka, J. Akimitsu, K. Inoue

    Physical Review B   97 ( 9 )   2018年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Physical Society  

    The phase diagram of a cubic chiral magnet MnSi with multiple Dzyaloshinskii-Moriya (DM) vectors as a function of temperature T and dc magnetic field Hdc was investigated using intensity mapping of the odd-harmonic responses of ac magnetization (M1ω and M3ω), and the responses at phase boundaries were evaluated according to a prescription [J. Phys. Soc. Jpn. 84, 104707 (2015)JUPSAU0031-901510.7566/JPSJ.84.104707]. By evaluating M3ω/M1ω appearing at phase boundaries, the robustness of noncollinear spin texture in both the helimagnetic (HM) and the skyrmion lattice (SkL) phases of MnSi was discussed. The robustness of vortices-type solitonic texture SkL in MnSi is smaller than those of both the single DM HM and chiral soliton lattice phases of a monoaxial chiral magnet Cr1/3NbS2, and furthermore the robustness of the multiple DM HM phase in MnSi is smaller than that of its SkL. Through magnetic diagnostics over the wide T-Hdc range, we found a new paramagnetic (PM) region with ac magnetic hysteresis, where spin fluctuations have been observed via electrical magnetochiral effect. The anomalies observed in the previous ultrasonic attenuation measurement correspond to the peak positions of out-of-phase M1ω. The appearance of a new PM region occurs at a characteristic magnetic field, above which indeed the SkL phase appears. It has us suppose that the new PM region could be a phase with spin fluctuation like the skyrmion gas phase.

    DOI: 10.1103/PhysRevB.97.094411

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  • Pressure-induced superconductivity in AgxBi2-xSe3 査読

    Tong He, Xiaofan Yang, Takahiro Terao, Takaki Uchiyama, Teppei Ueno, Kaya Kobayashi, Jun Akimitsu, Takafumi Miyazaki, Takumi Nishioka, Koji Kimura, Kouichi Hayashi, Naohisa Happo, Hitoshi Yamaoka, Hirofumi Ishii, Yen-Fa Liao, Hiromi Ota, Hidenori Goto, Yoshihiro Kubozono

    Physical Review B   97 ( 10 )   2018年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Physical Society  

    We investigated the pressure dependence of electric transport and crystal structure of Ag-doped Bi2Se3. In the sample prepared by Ag doping of Bi2Se3, the Bi atom was partially replaced by Ag, i.e., Ag0.05Bi1.95Se3. X-ray diffraction patterns of Ag0.05Bi1.95Se3 measured at 0-30 GPa showed three different structural phases, with rhombohedral, monoclinic, and tetragonal structures forming in turn as pressure increased, and structural phase transitions at 8.8 and 24 GPa. Ag0.05Bi1.95Se3 showed no superconductivity down to 2.0 K at 0 GPa, but under pressure, superconductivity suddenly appeared at 11 GPa. The magnetic field (H) dependence of the superconducting transition temperature Tc was measured at 11 and 20.5 GPa, in order to investigate whether the pressure-induced superconducting phase is explained by either p-wave polar model or s-wave model.

    DOI: 10.1103/PhysRevB.97.104503

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  • Magnetic phase diagram of S r2-x L axIr O4 synthesized by mechanical alloying 査読

    K. Horigane, M. Fujii, H. Okabe, K. Kobayashi, R. Horie, H. Ishii, Y. F. Liao, Y. Kubozono, A. Koda, R. Kadono, J. Akimitsu

    Physical Review B   97 ( 6 )   2018年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Physical Society  

    We report the crystal structure and physical properties of Sr2-xLaxIrO4 synthesized by mechanical alloying. The magnetic transition temperature TN and electrical resistivity decreased with increasing La doping, consistent with previous studies involving single-crystalline samples. We also identified the relationship between TN and tetragonal distortion (c/a) in this system. This result suggests that the magnetism of the Sr214 system is strongly correlated with its crystal structure. Zero-field muon spin rotation/relaxation studies revealed that short-range antiferromagnetic ordering is realized in Sr1.9La0.1IrO4
    also, the spin-glass state is stabilized in the low-temperature region. The Ir moment estimated from the longitudinal field μSR results is 0.045μB, ten times smaller than that of Sr2IrO4 (∼0.4μB), indicating that electrons are introduced into the Ir atoms.

    DOI: 10.1103/PhysRevB.97.064425

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  • YBa2Cu3O x superconducting nanorods 査読

    William Rieken, Atit Bhargava, Rie Horie, Jun Akimitsu, Hiroshi Daimon

    Japanese Journal of Applied Physics   57 ( 2 )   2018年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Japan Society of Applied Physics  

    Herein, we report the synthesis of YBa2Cu3O x superconducting nanorods performed by solution chemistry. Initially, a mixture of fine-grained coprecipitated powder was obtained and subsequently converted to YBa2Cu3O x nanorods by heating to 1223 K in oxygen for 12 h. The nanorods are superconducting without the need for any further sintering or oxygenation, thereby providing an avenue for direct application to substrates at room temperature or direct use as formed nanorods. A critical superconducting transition temperature T c of about 92 K was achieved at a critical magnetic field of 10 Oe.

    DOI: 10.7567/JJAP.57.023101

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  • Geometrical protection of topological magnetic solitons in microprocessed chiral magnets 査読

    Masaki Mito, Hiroyuki Ohsumi, Kazuki Tsuruta, Yoshinori Kotani, Tetsuya Nakamura, Yoshihiko Togawa, Misako Shinozaki, Yusuke Kato, Jun-Ichiro Kishine, Jun-Ichiro Ohe, Yusuke Kousaka, Jun Akimitsu, Katsuya Inoue

    Physical Review B   97 ( 2 )   2018年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Physical Society  

    A chiral soliton lattice stabilized in a monoaxial chiral magnet CrNb3S6 is a magnetic superlattice consisting of magnetic kinks with a ferromagnetic background. The magnetic kinks are considered to be topological magnetic solitons (TMSs). Changes in the TMS number yield discretized responses in magnetization and electrical conductivity, and this effect is more prominent in smaller crystals. We demonstrate that, in microprocessed CrNb3S6 crystals, TMSs are geometrically protected through element-selected micromagnetometry using soft x-ray magnetic circular dichroism (MCD). A series of x-ray MCD data is supported by mean-field and micromagnetic analyses. By designing the microcrystal geometry, TMS numbers can be successfully changed and fixed over a wide range of magnetic fields.

    DOI: 10.1103/PhysRevB.97.024408

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  • Monochiral helimagnetism in homochiral crystals of CsCuCl3 査読

    Y. Kousaka, T. Koyama, K. Ohishi, K. Kakurai, V. Hutanu, H. Ohsumi, T. Arima, A. Tokuda, M. Suzuki, N. Kawamura, A. Nakao, T. Hanashima, J. Suzuki, J. Campo, Y. Miyamoto, A. Sera, K. Inoue, J. Akimitsu

    PHYSICAL REVIEW MATERIALS   1 ( 7 )   2017年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    We report a crystal growth method to obtain homochiral single crystals of CsCuCl3 and polarized neutron diffraction studies to examine the chiral helimagnetism of this compound. The homochiral crystals were grown by two-step crystallization. First, millimeter-sized seed crystals were synthesized by spontaneous crystallization with stirring. The handedness of the seed crystals was determined by x-ray diffraction. Then, centimeter-sized homochiral crystals were obtained from the selected homochiral seed crystals. The large homochiral crystals made it possible to perform polarized neutron diffraction. We clarified a strong correlation between the crystal and magnetic chiralities, which governs the nature of antisymmetric Dzyaloshinskii-Moriya interactions.

    DOI: 10.1103/PhysRevMaterials.1.071402

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  • Magnetic solitons and magnetic phase diagram of the hexagonal chiral crystal CrNb3 S6 in oblique magnetic fields 査読

    Jun-Ichiro Yonemura, Yusuke Shimamoto, Takanori Kida, Daichi Yoshizawa, Yusuke Kousaka, Sadafumi Nishihara, Francisco Jose Trindade Goncalves, Jun Akimitsu, Katsuya Inoue, Masayuki Hagiwara, Yoshihiko Togawa

    Physical Review B   96 ( 18 )   2017年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Physical Society  

    We investigate the magnetic torque and magnetoresistance (MR) responses in oblique magnetic fields in micrometer-sized specimens of the hexagonal chiral magnetic crystal CrNb3S6. The results exhibit hysteresis over a wide range of applied field angles, while reversible behavior appears only when the magnetic field is closely aligned to the helical axis of the crystal. Stepwise changes of the magnetic torque and MR detected in the hysteresis region indicate the existence of chiral solitons in the oblique magnetic fields. A magnetic phase diagram is derived from the experimental results, and the stability of the chiral magnetic phases, such as the chiral soliton lattice and chiral conical phase, and the nature of the phase transition between them are discussed.

    DOI: 10.1103/PhysRevB.96.184423

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  • Magneto-chiral dichroism of CsCuCl3 査読

    N. Nakagawa, N. Abe, S. Toyoda, S. Kimura, J. Zaccaro, I. Gautier-Luneau, D. Luneau, Y. Kousaka, A. Sera, M. Sera, K. Inoue, J. Akimitsu, Y. Tokunaga, T. Arima

    Physical Review B   96 ( 12 )   2017年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Physical Society  

    In this Rapid Communication, we investigate the magneto-chiral dichroism (MChD) of right- and left-handed CsCuCl3 crystals in a strong magnetic field up to 14.5 T. We perform near-infrared optical measurements, and the results show a change in the absorption coefficient of an intra-atomic d-d transition in a Cu2+ ion at approximately 1.5 eV. The magnitude of MChD was found to be proportional to the magnetization, which depends on the magnetic field and temperature. We verify this relation by developing a simple CuCl6 cluster model.

    DOI: 10.1103/PhysRevB.96.121102

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  • Pressure-induced quantum phase transitions in the S=1/2 triangular lattice antiferromagnet CsCuCl3 査読

    A. Sera, Y. Kousaka, J. Akimitsu, M. Sera, K. Inoue

    PHYSICAL REVIEW B   96 ( 1 )   2017年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    We investigated the pressure effect on the magnetization of the soft material CsCuCl3. We also measured the lattice distortion under the longitudinal magnetic fields at the ambient pressure. While the ab plane shrinks in all the quantum phases below T-N at the ambient pressure, its magnitude is much larger in the intermediate 2-1-coplanar or the IC3 phase with a large quantum spin fluctuation than in the low field phase. We found the pressure induced quantum phases; the uud phase for H parallel to c and the IC5 phase for H parallel to b*. We also found the large reduction of the magnetization both below and above T-N and the enhancement of T-N by pressure. dT(N)/dP is much larger in the intermediate field phase than in the low field phase. From these results, we could draw the rough magnetic phase diagram under pressure in a high field region. While all the quantum phases below T-N are stabilized by pressure, the degree of the stability by pressure is much larger in the quantum phase with a large quantum spin fluctuation than in the low field phase. In the soft material such as CsCuCl3, we propose that the ab plane shrinks spontaneously so as to enhance T-N and the quantum spin fluctuation in high field quantum phases under pressure.

    DOI: 10.1103/PhysRevB.96.014419

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  • Evolution of the remnant Fermi-surface state in the lightly doped correlated spin-orbit insulator Sr2-xLaxIrO4 査読

    K. Terashima, M. Sunagawa, H. Fujiwara, T. Fukura, M. Fujii, K. Okada, K. Horigane, K. Kobayashi, R. Horie, J. Akimitsu, E. Golias, D. Marchenko, A. Varykhalov, N. L. Saini, T. Wakita, Y. Muraoka, T. Yokoya

    PHYSICAL REVIEW B   96 ( 4 )   2017年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    The electronic structure of the lightly electron-doped correlated spin-orbit insulator Sr2IrO4 has been studied by angle-resolved photoelectron spectroscopy. We have observed the coexistence of a lower Hubbard band and an in-gap band; the momentum dependence of the latter traces that of the band calculations without on-site Coulomb repulsion. The in-gap state remained anisotropically gapped in all observed momentum areas, forming a remnant Fermi-surface state, evolving towards the Fermi energy by carrier doping. These experimental results show a striking similarity with those observed in deeply underdoped cuprates, suggesting the common nature of the nodal liquid states observed in both compounds.

    DOI: 10.1103/PhysRevB.96.041106

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  • Unusual upper critical field behavior in Nb-doped bismuth selenides 査読

    Kaya Kobayashi, T. Ueno, H. Fujiwara, T. Yokoya, J. Akimitsu

    Physical Review B   95 ( 18 )   2017年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Physical Society  

    We investigate superconductivity in the candidate topological superconductor NbxBi2Se3 from x=0 to 1. We observe superconductivity for all x&gt
    0 with a superconducting transition temperature TC∼ 3 K similar to that reported in Cu- and Sr-intercalated Bi2Se3. Structurally, for small x we find introduction of Nb modifies the Bi2Se3 separation while for larger x the system is driven to the BiSe structure. While all compounds are found to be superconducting, two distinct behaviors for the upper critical field Hc2 are observed. From photoemission spectroscopy we find that the enhanced Hc2 corresponds to compounds with a low surface Dirac point energy (-0.67 eV) relative to the Fermi level
    in absolute scale this value is lower than previously reported superconductors in this materials family. We comment on the utility of this system for the study of topological superconductivity in a new structure type combining a doping (Bi2 type) layer and layers with topologically protected gapless state (Bi2Se3 type).

    DOI: 10.1103/PhysRevB.95.180503

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  • Collective resonant dynamics of the chiral spin soliton lattice in a monoaxial chiral magnetic crystal 査読

    F. J.T. Goncalves, T. Sogo, Y. Shimamoto, Y. Kousaka, J. Akimitsu, S. Nishihara, K. Inoue, D. Yoshizawa, M. Hagiwara, M. Mito, R. L. Stamps, I. G. Bostrem, V. E. Sinitsyn, A. S. Ovchinnikov, J. Kishine, Y. Togawa

    Physical Review B   95 ( 10 )   2017年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Physical Society  

    The magnetic resonance properties of microsized monoaxial chiral crystals of CrNb3S6 are investigated. We observed that the resonance of the chiral soliton lattice is sensitive to the polarization of the driving microwave field. When the microwave field is parallel to the helical axis, the resonance is symmetric with regards to the magnetic field direction. In contrast, asymmetric field dependence emerges when the microwave field is perpendicular to the helical axis. The robustness of the chiral magnetic order, due to topological protection, allows tuning the resonance frequency in ways hardly accessible using nanopatterned films.

    DOI: 10.1103/PhysRevB.95.104415

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  • The electronic structure of Ag1-: XSn1+ xSe2 (x = 0.0, 0.1, 0.2, 0.25 and 1.0) 査読

    Takanori Wakita, Eugenio Paris, Kaya Kobayashi, Kensei Terashima, Muammer Yasin Hacisalihoǧlu, Teppei Ueno, Federica Bondino, Elena Magnano, Igor Píš, Luca Olivi, Jun Akimitsu, Yuji Muraoka, Takayoshi Yokoya, Naurang L. Saini

    Physical Chemistry Chemical Physics   19 ( 39 )   26672 - 26678   2017年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Royal Society of Chemistry  

    We have studied the valence electronic structure of Ag1-xSn1+xSe2 (x = 0.0, 0.1, 0.2, 0.25) and SnSe (x = 1.0) by a combined analysis of X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS) measurements. Both XAS and XPS reveal an increase in electron carriers in the system with x (i.e. excess Sn concentration) for 0 ≤ x ≤ 0.25. The core-level spectra (Sn 3d, Ag 3d and Se 3d) show that the charge state of Ag is almost 1+, while that of of Sn splits into Sn2+ and Sn4+ (providing clear evidence of valence skipping for the first time) with a concomitant splitting of Se into Se2- and Se2-δ states. The x dependence of the split components in Sn and Se together with the Se-K edge XAS reveals that the Se valence state may have an essential role in the transport properties of this system.

    DOI: 10.1039/c7cp05369j

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  • S=1/2 triangular-lattice antiferromagnets Ba3CoSb2O9 and CsCuCl3: Role of Spin-orbit coupling, crystalline electric field effect, and Dzyalosinskii-Moriya interaction 査読

    A. Sera, Y. Kousaka, J. Akimitsu, M. Sera, T. Kawamata, Y. Koike, K. Inoue

    Phys. Rev. B   94   214408   2016年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    DOI: 10.1103/PhysRevB.94.214408

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  • Size dependence of discrete change in magnetization in single crystal of chiral magnet Cr1/3NbS2 査読

    K. Tsuruta, M. Mito, Y. Kousaka, J. Akimitsu, J. Kishine, Y. Togawa, K. Inoue

    Journal of Applied Physics   120 ( 14 )   2016年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Institute of Physics Inc.  

    The single crystal of a chiral magnet Cr1/3NbS2 exhibits discrete changes in magnetization (M) in response to changes in magnetic field (H) triggered by the formation of a chiral soliton lattice (CSL). In order to provide evidence of this phenomenon, the study of the size effect is indispensable. We investigated the effects of size on this phenomenon by the use of two single crystals, (A) and (B), whose crystal sizes along the c-axis were 110 μm and 60 μm, respectively. First, in (A), the large jumps of M observed in the process of decreasing H exhibited inconsistent features, whereas the largest and second-largest jumps in (B) exhibited reproducibility for both the value of H and the magnitude of the M jumps. This confirms that these large jumps do not originate from the Barkhausen effect, as this effect would result in M jumps appearing at random values of H. When the system size of a sample becomes smaller, the features of the Barkhausen effect are suppressed. Second, as for the successive jumps observed in the H region where the rapid change in M is entirely seen for both the samples, the number of observed M jumps for (B) is more than that for (A). Indeed, the number of the domain wall due to the 2π-soliton in the CSL increases, as the c-axis length of the single crystal increases. However, a series of M jumps must appear in a limited H region below the critical field of the order of 2 kOe. The greater the number of M jumps, the more difficult the detection of the M jump will be in the H resolution of the present setup. Thus, the effects of size on the M jumps observed in the present setup can be understood within the framework of the CSL formation.

    DOI: 10.1063/1.4964427

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  • Long Periodic Helimagnetic Ordering in CrM 3S6 (M = Nb and Ta) 査読

    Y. Kousaka, T. Ogura, J. Zhang, P. Miao, S. Lee, S. Torii, T. Kamiyama, J. Campo, K. Inoue, J. Akimitsu

    Journal of Physics: Conference Series   746 ( 1 )   2016年10月

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:Institute of Physics Publishing  

    We report long periodic chiral helimagnetic orderings in ferromagnetic inorganic compounds CrM 3S6 (M = Nb and Ta) with a chiral space group of P6322. Magnetization in polycrystalline samples and high resolution powder neutron diffraction were measured. Our powder neutron diffraction measurements in CrM 3S6 successfully separated nuclear and magnetic satellite peaks, having the period of hundreds of angstroms along the c-axis. Therefore, we propose that the magnetic ordering in ferromagnetic CrM3S6 is not ferromagnetic, but long periodic chiral helimagnetic ordering.

    DOI: 10.1088/1742-6596/746/1/012061

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  • Spin excitations in hole-overdoped iron-based superconductors 査読

    K. Horigane, K. Kihou, K. Fujita, R. Kajimoto, K. Ikeuchi, S. Ji, J. Akimitsu, C. H. Lee

    Scientific Reports   6   2016年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Nature Publishing Group  

    Understanding the overall features of magnetic excitation is essential for clarifying the mechanism of Cooper pair formation in iron-based superconductors. In particular, clarifying the relationship between magnetism and superconductivity is a central challenge because magnetism may play a key role in their exotic superconductivity. BaFe2As2 is one of ideal systems for such investigation because its superconductivity can be induced in several ways, allowing a comparative examination. Here we report a study on the spin fluctuations of the hole-overdoped iron-based superconductors Ba1-xKxFe2As2 (x = 0.5 and 1.0
    Tc = 36 K and 3.4 K, respectively) over the entire Brillouin zone using inelastic neutron scattering. We find that their spin spectra consist of spin wave and chimney-like dispersions. The chimney-like dispersion can be attributed to the itinerant character of magnetism. The band width of the spin wave-like dispersion is almost constant from the non-doped to optimum-doped region, which is followed by a large reduction in the overdoped region. This suggests that the superconductivity is suppressed by the reduction of magnetic exchange couplings, indicating a strong relationship between magnetism and superconductivity in iron-based superconductors.

    DOI: 10.1038/srep33303

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  • Chiral Surface Twists and Skyrmion Stability in Nanolayers of Cubic Helimagnets 査読

    A. O. Leonov, Y. Togawa, T. L. Monchesky, A. N. Bogdanov, J. Kishine, Y. Kousaka, M. Miyagawa, T. Koyama, J. Akimitsu, Ts. Koyama, K. Harada, S. Mori, D. McGrouther, R. Lamb, M. Krajnak, S. McVitie, R. L. Stamps, K. Inoue

    Physical Review Letters   117 ( 8 )   2016年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Physical Society  

    Theoretical analysis and Lorentz transmission electron microscopy (LTEM) investigations in an FeGe wedge demonstrate that chiral twists arising near the surfaces of noncentrosymmetric ferromagnets [Meynell et al., Phys. Rev. B 90, 014406 (2014)] provide a stabilization mechanism for magnetic Skyrmion lattices and helicoids in cubic helimagnet nanolayers. The magnetic phase diagram obtained for freestanding cubic helimagnet nanolayers shows that magnetization processes differ fundamentally from those in bulk cubic helimagnets and are characterized by the first-order transitions between modulated phases. LTEM investigations exhibit a series of hysteretic transformation processes among the modulated phases, which results in the formation of the multidomain patterns. Published by the American Physical Society under the terms of the «http://creativecommons.org/licenses/by/3.0/» Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

    DOI: 10.1103/PhysRevLett.117.087202

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  • Suppression of spin-exciton state in hole overdoped iron-based superconductors 査読

    C. H. Lee, K. Kihou, J. T. Park, K. Horigane, K. Fujita, F. Waßer, N. Qureshi, Y. Sidis, J. Akimitsu, M. Braden

    Scientific Reports   6   2016年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Nature Publishing Group  

    The mechanism of Cooper pair formation in iron-based superconductors remains a controversial topic. The main question is whether spin or orbital fluctuations are responsible for the pairing mechanism. To solve this problem, a crucial clue can be obtained by examining the remarkable enhancement of magnetic neutron scattering signals appearing in a superconducting phase. The enhancement is called spin resonance for a spin fluctuation model, in which their energy is restricted below twice the superconducting gap value (2Δ s), whereas larger energies are possible in other models such as an orbital fluctuation model. Here we report the doping dependence of low-energy magnetic excitation spectra in Ba1-xKxFe2As2 for 0.5 &lt
    x &lt
    0.84 studied by inelastic neutron scattering. We find that the behavior of the spin resonance dramatically changes from optimum to overdoped regions. Strong resonance peaks are observed clearly below 2Δ s in the optimum doping region, while they are absent in the overdoped region. Instead, there is a transfer of spectral weight from energies below 2δs to higher energies, peaking at values of 3δs for x = 0.84. These results suggest a reduced impact of magnetism on Cooper pair formation in the overdoped region.

    DOI: 10.1038/srep23424

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  • Single-crystal growth of Ba1-xKxFe2As2 by KAs self-flux method 査読

    Kunihiro Kihou, Taku Saito, Kay Fujita, Shigeyuki Ishida, Masamichi Nakajima, Kazumasa Horigane, Hideto Fukazawa, Yoh Kohori, Shin-Ichi Uchida, Jun Akimitsu, Akira Iyo, Chul-Ho Lee, Hiroshi Eisaki

    Journal of the Physical Society of Japan   85 ( 3 )   2016年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Physical Society of Japan  

    Single crystals of Ba1-xKxFe2As2 with 0.15 = x = 1 have been successfully synthesized by a KAs self-flux method. The potassium (K) concentration x of the grown crystals was systematically changed by changing the mixing ratio of Ba to Fe in the starting materials. The crystals have flat surfaces corresponding to the crystallographic (001) planes, whose planar dimensions increase with decreasing thickness when x increases. The superconducting transition temperature systematically changes with x, ranging from 38K at x = 0.4 to 3.4K at x = 1.0, in good agreement with the results for polycrystalline samples.

    DOI: 10.7566/JPSJ.85.034718

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  • Phase diagram of the chiral magnet Cr1/3NbS2 in a magnetic field 査読

    K. Tsuruta, M. Mito, H. Deguchi, J. Kishine, Y. Kousaka, J. Akimitsu, K. Inoue

    Physical Review B   93 ( 10 )   2016年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Physical Society  

    We construct the phase diagram of the chiral magnet Cr1/3NbS2 in a dc magnetic field (Hdc) using ac magnetic susceptibility measurements. At Hdc=0, the ac response at the transition from the helical magnetic (HM) state to the paramagnetic (PM) state consists of a giant third-order harmonic component (M3ω) and a first-order harmonic component (M1ω). By applying Hdc perpendicular to the c axis, the HM state is transformed to the chiral soliton lattice (CSL) state, which is a superlattice tuned by Hdc. The above giant M3ω is markedly suppressed at small Hdc. The CSL state is found to consist of CSL-1, with dominant helical texture and a poor ferromagnetic array, and CSL-2, with a large ferromagnetic array. The transition between CSL-1 and the PM state causes a linear magnetic response, the dominant component of which is the in-phase M1ω. With increasing temperature, CSL-2 is transformed into the forced ferromagnetic (FFM) state, and ultimately the PM state is reached. The transition between CSL-2 and the FFM state consists of a large M3ω and large out-of-phase M1ω as well as in-phase M1ω. The transition between the FMM and PM states also yields a linear magnetic response, like the CSL-1-PM-state transition. Five typical magnetic dynamics in the transitions among the HM state, CSL-1, CSL-2, FFM state, and PM state were identified according to the equivalent dynamical motion equation of a nonlinear spring model.

    DOI: 10.1103/PhysRevB.93.104402

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  • Discrete Change in Magnetization by Chiral Soliton Lattice Formation in the Chiral Magnet Cr1/3NbS2 査読

    Kazuki Tsuruta, Masaki Mito, Yusuke Kousaka, Jun Akimitsu, Jun-Ichiro Kishine, Yoshihiko Togawa, Hiroyuki Ohsumi, Katsuya Inoue

    Journal of the Physical Society of Japan   85 ( 1 )   2016年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Physical Society of Japan  

    In the chiral magnet Cr1/3NbS2, discrete changes in the magnetization (M) caused by the formation of a chiral soliton lattice (CSL) were observed in magnetization curve measurements using a single crystal of submillimeter thickness. When M is measured with a minimal increment of the magnetic field H, 0.15 Oe, discrete changes in M are observed in the H region that exhibits definite magnetic hysteresis. In particular, enormous discrete changes in M are observed as H decreases from the field above the saturation field, while fine M steps are also found in the intermediate H range independently of the sweeping direction of the field. The former is considered as a type of enormous Barkhausen effect associated with the CSL formation. The latter originates from the change in soliton number during the CSL formation.

    DOI: 10.7566/JPSJ.85.013707

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  • Crystal and magnetic structures of Cr1/3NbSe2 from neutron diffraction 査読

    A. F. Gubkin, E. P. Proskurina, Y. Kousaka, E. M. Sherokalova, N. V. Selezneva, P. Miao, S. Lee, J. Zhang, Y. Ishikawa, S. Torii, T. Kamiyama, J. Campo, J. Akimitsu, N. V. Baranov

    Journal of Applied Physics   119 ( 1 )   2016年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Institute of Physics Inc.  

    Neutron diffraction measurements of the Cr intercalated niobium diselenide Cr1/3NbSe2 together with magnetization measurements have revealed that this compound exhibits ferromagnetic ordering below TC = 96 K unlike a chiral helimagnetic order observed in the sulfide compound Cr1/3NbS2. As derived from neutron diffraction data, the Cr magnetic moments μCr = 2.83 ± 0.03 μB in Cr1/3NbSe2 are aligned within basal plane. The discrepancy in the magnetic states of Cr1/3NbS2 and Cr1/3NbSe2 is ascribed to the difference in the preferential site occupation of Cr ions in crystal lattices. In Cr1/3NbSe2, the Cr ions are predominantly distributed over 2b Wyckoff site, which determines a centrosymmetric character of the crystal structure unlike Cr1/3NbS2, where the Cr ions are mainly located in 2c position and the crystal structure is non-centrosymmetric.

    DOI: 10.1063/1.4939558

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  • 三角格子反強磁性体CsCuCl<sub>3</sub>の圧力下THz-ESR測定

    櫻井 敬博, 大木 瑛登, 平尾 祐樹, 大久保 晋, 太田 仁, 上床 美也, 田中 秀数, 世良 文香, 高阪 勇輔, 秋光 純, 世良 正文, 井上 克也

    日本物理学会講演概要集   71   846 - 846   2016年

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    記述言語:日本語   出版者・発行元:一般社団法人 日本物理学会  

    <p>三角格子反強磁性体CsCuCl_3_は磁場中で量子相転移を生ずる物質として非常に注目を集めた物質である。最近、圧力下での磁化過程において1/3プラトーが見出され再び注目を集めている。我々はその詳細をミクロな観点から探ることを目的に、圧力下でESR測定を行っている。現在、圧力に伴う反強磁性ギャップの増大を観測し、これに寄与するいくつかの相互作用を分離して評価する試みを行っている。</p>

    DOI: 10.11316/jpsgaiyo.71.2.0_846

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  • KSr<sub>2</sub>Nb<sub>3</sub>O<sub>10</sub>の構造と超伝導

    小林 夏野, 川口 貴弘, 堀金 和正, 秋光 純

    日本物理学会講演概要集   71   2235 - 2235   2016年

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    記述言語:日本語   出版者・発行元:一般社団法人 日本物理学会  

    <p>ペロブスカイト構造を持つKCa2Nb3O10の超伝導が構造によって変化することが報告されている。本研究では類似物質の構造を変化させることによって現れた超伝導の発現を報告する。</p>

    DOI: 10.11316/jpsgaiyo.71.2.0_2235

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  • S=1/2三角格子反強磁性体Ba<sub>3</sub>CoSb<sub>2</sub>O<sub>9</sub>とCsCuCl<sub>3</sub>の異なる磁気挙動とその起源

    世良 文香, 高阪 勇輔, 秋光 純, 世良 正文, 井上 克也, 川股 隆行, 小池 洋二

    日本物理学会講演概要集   71   996 - 996   2016年

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    記述言語:日本語   出版者・発行元:一般社団法人 日本物理学会  

    <p>Ba_3_CoSb_2_O_9_は理想的な三角格子反強磁性体として知られており、実験と理論の両面から近年注目を集めている。一方で、CsCuCl_3_は古くから盛んに研究されているが、DM相互作用が存在するためにその物性は非常に複雑である。今回、我々はこれら2つの磁性体の詳細な磁化測定を行い比較することで、構造や相互作用の違いからくる磁気挙動の違いとその起源を明らかにした。学会ではその詳細について報告する。</p>

    DOI: 10.11316/jpsgaiyo.71.2.0_996

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  • キラル磁性体Yb(Ni,Cu)<sub>3</sub>Al<sub>9</sub>の中性子回折測定

    高阪 勇輔, 二宮 博樹, 松本 裕司, 大原 繁男, 大石 一城, 鈴木 淳市, 井上 克也, 秋光 純

    日本物理学会講演概要集   71   1876 - 1876   2016年

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    記述言語:日本語   出版者・発行元:一般社団法人 日本物理学会  

    <p>キラル磁性体YbNi3Al9はYbサイトが磁性を担っており、キラルらせん磁気構造を形成する。本物質のNiサイトをCuで数パーセント置換することでTc及びHcが大幅に増加することが報告されている。我々は本物質群の磁気構造を検証するために小角中性子回折測定をJ-PARC, TAIKANにて実施した。Cu置換によりらせん磁気構造の周期が増加していることを観測した。</p>

    DOI: 10.11316/jpsgaiyo.71.2.0_1876

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  • Sr<sub>2-x</sub>La<sub>x</sub>IrO<sub>4</sub> (x=0.04, 0.08)の電子状態:角度分解光電子分光

    寺嶋 健成, 砂川 正典, 藤原 弘和, 藤井 政徳, 堀金 和正, 小林 夏野, 堀江 理恵, 秋光 純, 村岡 祐治, 横谷 尚睦

    日本物理学会講演概要集   71   1791 - 1791   2016年

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    記述言語:日本語   出版者・発行元:一般社団法人 日本物理学会  

    <p>Sr_2_IrO_4_は反強磁性絶縁体であり、実験的・理論的に銅酸化物高温超伝導体との類似性が指摘されている。今回我々は、Sr_2-x_La_x_IrO_4_単結晶試料の角度分解光電子分光を行い、電子ドープに伴うギャップ内状態発達の過程を調べた。その結果、ハバードバンドとギャップ内状態が共存するドープ量域が存在し、両者のウェイトがドープに伴い推移することを見出した。講演では銅酸化物との比較を行う。</p>

    DOI: 10.11316/jpsgaiyo.71.2.0_1791

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  • カルコゲンを含む超伝導体の探索

    小林 夏野, 上野 哲平, 秋光 純

    日本物理学会講演概要集   71   2040 - 2040   2016年

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    記述言語:日本語   出版者・発行元:一般社団法人 日本物理学会  

    <p>立方晶の超伝導体AgSnSe2は、Snの価数が2または4しか取れないことからバレンススキップ機構によって超伝導が実現していると考えられている。我々はカルコゲンまたはAg/Snを他元素に置換することにより超伝導性がどのように変化するかを調べた。いくつかの元素において超伝導転移温度が上昇することを発見した。</p>

    DOI: 10.11316/jpsgaiyo.71.2.0_2040

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  • キラルソリトン格子磁気共鳴の磁場依存性

    十河 忠幸, Goncalves F., 吉澤 大智, 萩原 政幸, Stamps R., 高阪 勇輔, 秋光 純, 西原 禎文, 井上 克也, Bostrem I. G., Sinitsyn Vl. E., Ovchinnikov A. S., 岸根 順一郎, 戸川 欣彦

    日本物理学会講演概要集   71   2221 - 2221   2016年

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    記述言語:日本語   出版者・発行元:一般社団法人 日本物理学会  

    <p>結晶構造がキラリティを有する磁性体では、らせん軸に垂直に磁場を印加するとキラルソリトン格子(CSL)が基底状態として発現する。CSLは結晶キラリティに保護された頑強なスピン位相秩序であり、スピン位相コヒーレンスを持つ系の集団運動を調べることは大変興味深い。本講演では単軸性キラル磁性体CrNb_3_S_6_における磁気共鳴実験の詳細を示す。磁場依存性などを通じ、CSL集団運動の特性を議論する。</p>

    DOI: 10.11316/jpsgaiyo.71.2.0_2221

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  • Magnetic soliton confinement and discretization effects arising from macroscopic coherence in a chiral spin soliton lattice 査読

    Y. Togawa, T. Koyama, Y. Nishimori, Y. Matsumoto, S. McVitie, D. McGrouther, R. L. Stamps, Y. Kousaka, J. Akimitsu, S. Nishihara, K. Inoue, I. G. Bostrem, Vl E. Sinitsyn, A. S. Ovchinnikov, J. Kishine

    Physical Review B - Condensed Matter and Materials Physics   92 ( 22 )   2015年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Physical Society  

    We present how macroscopically coherent ordering within a chiral state can be manifested in the physical properties within the context of an archetypical system - the chiral spin soliton lattice in a monoaxial chiral magnet CrNb3S6. Using magnetotransport measurements and state-of-the-art Lorentz electron microscopy, we demonstrate spin soliton confinement in 1-μm-wide grains with different crystalline chirality and discretized magnetoresistance in 10-μm-wide crystals. Discretization effects are found to be prominent when the system size is reduced to the order of 10μm along the chiral axis. A consequence that we identify is a robust coherence of the chiral soliton lattice against deformation. The spin configuration at the grain boundaries, which leads to soliton confinement, is experimentally clarified.

    DOI: 10.1103/PhysRevB.92.220412

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  • Study of magnetic domain dynamics using nonlinear magnetic responses: Magnetic diagnostics of the itinerant magnet MnP 査読

    Masaki Mito, Hideaki Matsui, Kazuki Tsuruta, Hiroyuki Deguchi, Jun-Ichiro Kishine, Katsuya Inoue, Yusuke Kousaka, Shin-Ichiro Yano, Yuya Nakao, Jun Akimitsu

    Journal of the Physical Society of Japan   84 ( 10 )   2015年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Physical Society of Japan  

    The nonlinear and linear magnetic responses to an ac magnetic field H are useful for the study of the magnetic dynamics of both magnetic domains and their constituent spins. In particular, the third-harmonic magnetic response M&lt
    inf&gt
    3ω&lt
    /inf&gt
    reflects the dynamics of magnetic domains. Furthermore, by considering the ac magnetic response as a function of H, we can evaluate the degree of magnetic nonlinearity, which is closely related to M&lt
    inf&gt
    3ω&lt
    /inf&gt
    . In this study, a series of approaches was used to examine the itinerant magnet MnP, in which both ferromagnetic and helical phases are present. On the basis of this investigation, we systematize the diagnostic approach to evaluating nonlinearity in magnetic responses.

    DOI: 10.7566/JPSJ.84.104707

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  • Unconventional superconductivity in Y5Rh6Sn18 probed by muon spin relaxation 査読

    Amitava Bhattacharyya, Devashibhai Adroja, Naoki Kase, Adrian Hillier, Jun Akimitsu, Andre Strydom

    SCIENTIFIC REPORTS   5   2015年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:NATURE PUBLISHING GROUP  

    Conventional superconductors are robust diamagnets that expel magnetic fields through the Meissner effect. It would therefore be unexpected if a superconducting ground state would support spontaneous magnetics fields. Such broken time-reversal symmetry states have been suggested for the high-temperature superconductors, but their identification remains experimentally controversial. We present magnetization, heat capacity, zero field and transverse field muon spin relaxation experiments on the recently discovered caged type superconductor Y5Rh6Sn18 (T-C=3.0 K). The electronic heat capacity of Y5Rh6Sn18 shows a T-3 dependence below T-c indicating an anisotropic superconducting gap with a point node. This result is in sharp contrast to that observed in the isostructural Lu5Rh6Sn18 which is a strong coupling s-wave superconductor. The temperature dependence of the deduced superfluid in density Y5Rh6Sn18 is consistent with a BCS s-wave gap function, while the zero-field muon spin relaxation measurements strongly evidences unconventional superconductivity through a spontaneous appearance of an internal magnetic field below the superconducting transition temperature, signifying that the superconducting state is categorized by the broken time-reversal symmetry.

    DOI: 10.1038/srep12926

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  • Investigation of structural changes in chiral magnet Cr&lt;inf&gt;1/3&lt;/inf&gt;NbS&lt;inf&gt;2&lt;/inf&gt; under application of pressure 査読

    M. Mito, T. Tajiri, K. Tsuruta, H. Deguchi, J. Kishine, K. Inoue, Y. Kousaka, Y. Nakao, J. Akimitsu

    Journal of Applied Physics   117 ( 18 )   2015年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Institute of Physics Inc.  

    We perform structural analysis experiments on the chiral magnet Cr&lt
    inf&gt
    1/3&lt
    /inf&gt
    NbS&lt
    inf&gt
    2&lt
    /inf&gt
    , in which Cr&lt
    sup&gt
    3+&lt
    /sup&gt
    ions are inserted between hexagonal NbS&lt
    inf&gt
    2&lt
    /inf&gt
    layers. The noncentrosymmetrical nature of the inserted Cr&lt
    sup&gt
    3+&lt
    /sup&gt
    appears as a distorted CrS&lt
    inf&gt
    6&lt
    /inf&gt
    octahedron. Under the application of hydrostatic pressure, the lattice shrinks significantly along the c-axis rather than the a-axis. However, at a pressure P of approximately 3-4 GPa, a kink in the rate of decrease in the lattice parameters is observed, and the slight movement of a Nb atom along the c-axis brings about a decrease in the distortion of the CrS&lt
    inf&gt
    6&lt
    /inf&gt
    octahedron. This structural change qualitatively suggests a decrease in the strength of the Dzyaloshinskii-Moriya (D-M) interaction. Under hydrostatic pressure, the magnetic ordering temperature T&lt
    inf&gt
    C&lt
    /inf&gt
    decreases, and dT&lt
    inf&gt
    C&lt
    /inf&gt
    /dP exhibits a slight change at around 3 GPa. A series of experiments indicates that the change in the structural symmetry of the CrS&lt
    inf&gt
    6&lt
    /inf&gt
    octahedron influences the exchange network between Cr&lt
    sup&gt
    3+&lt
    /sup&gt
    ions as well as the D-M interaction.

    DOI: 10.1063/1.4919833

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  • Broken time-reversal symmetry probed by muon spin relaxation in the caged type superconductor Lu5Rh6Sn18 査読

    A. Bhattacharyya, D. T. Adroja, J. Quintanilla, A. D. Hillier, N. Kase, A. M. Strydom, J. Akimitsu

    PHYSICAL REVIEW B   ( 91 )   060503(R)   2015年2月

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    記述言語:英語  

    DOI: 10.1103/PhysRevB.91.060503

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  • Superstructure-dependent electronic states in CaAlSi superconductors studied by angle-resolved photoemission spectroscopy 査読

    T. Sato, Y. X. Xiao, S. Souma, K. Nakayama, K. Sugawara, S. Kuroiwa, J. Akimitsu, T. Takahashi

    Phys. Rev. B   91   195114   2015年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    DOI: 10.1103/PhysRevB.91.195114

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  • Metallic Binary Copper Chalcogenides with Orthorhombic Layered Structure 査読

    Kaya Kobayashi, Shinya Kawamoto, Jun Akimitsu

    Materials Research Society Symposium Proceedings   1726   2015年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:Materials Research Society  

    Chalcogenide materials have regained attention after the recent recognition of the compatibility of transition metal dichalcogenides with graphene. Additionally, there has been a recent appreciation for the rich variety of properties they support due to the anomalies in the materials' intrinsic band structure. These materials generally have layered structures and weak interlayer connection through the chalcogen layer and its van der Waals type bonding. We have synthesized orthorhombic copper telluride and measured its electrical transport properties. The results of these measurements reveal that the conduction is metallic in both the in-plane and out-of-plane directions. The range of stability of this structure is examined along with the lattice constants. The independence of the resistivity in samples to changes in excess copper indicates that the transport is essentially within the conducting planes. This result shows that the material hosts two-dimensional character likely due to its covalent interlayer bonding.

    DOI: 10.1557/opl.2015.467

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  • Magnetic Resonance in the Chiral Helimagnet CrNb3S6 査読

    Daichi Yoshizawa, Jun-Ichiro Kishine, Yusuke Kousaka, Yoshihiko Togawa, Masaki Mito, Jun Akimitsu, Katsuya Inoue, Masayuki Hagiwara

    Physics Procedia   75   926 - 931   2015年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:Elsevier B.V.  

    Recently, magnetic substances with chirality, namely the handedness of the magnetic structure, have attracted considerable attention because of the anomalous phenomena which appear in magnetic fields. CrNb3S6 is one of the chiral magnets formed by exchange and Dzyaloshinsky-Moriya (DM) interactions. Electron spin resonance (ESR) measurements of CrNb3S6 in magnetic fields parallel to the c-axis (helical axis) have been performed to evaluate the exchange and the DM constants that determine the helical structure. Fitting the ESR data to a calculated mode based on a spin wave theory yields values for the ferromagnetic inter-plane exchange constant J/kB = 16.2 K, the DM constant D/kB = 1.29 K, and the single-ion anisotropy constant. From the Curie-Weiss temperature θCW ∼ 145 K, large intra-plane ferromagnetic exchange interactions are suggested.

    DOI: 10.1016/j.phpro.2015.12.127

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  • 22aPS-127 カイラル磁性体CrNb_3S_6における磁気トルク測定

    米村 潤一朗, 木田 孝則, 吉澤 大智, 高阪 勇輔, 秋光 純, 西原 禎文, 井上 克也, 岸根 順一郎, 戸川 欣彦, 萩原 政幸

    日本物理学会講演概要集   70   2215 - 2215   2015年

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    記述言語:日本語   出版者・発行元:一般社団法人日本物理学会  

    DOI: 10.11316/jpsgaiyo.70.1.0_2215

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  • 22aPS-129 ベルリナイト型構造を有する新規カイラル磁性体TPO_4(T=遷移金属)の合成

    井上 渓介, 小椋 隆弘, 高阪 勇輔, 秋光 純

    日本物理学会講演概要集   70   2216 - 2216   2015年

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    記述言語:日本語   出版者・発行元:一般社団法人日本物理学会  

    DOI: 10.11316/jpsgaiyo.70.1.0_2216

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  • 21aBE-5 無機キラル磁性体CsCuCl_3のキラルらせん磁気構造の検証

    高阪 勇輔, 大石 一城, 小山 珠美, 宮本 幸乃, 世良 文香, 鈴木 淳市, 井上 克也, 秋光 純

    日本物理学会講演概要集   70   1979 - 1979   2015年

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    記述言語:日本語   出版者・発行元:一般社団法人日本物理学会  

    DOI: 10.11316/jpsgaiyo.70.1.0_1979

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  • 21aBE-3 カイラルソリトン格子のソリトン閉じ込め効果

    戸川 欣彦, McVitie Stephen, McGouther Damien, Stamps Robert, 小山 司, 西森 祐太, 松本 悠, 高阪 勇輔, 秋光 純, 西原 禎文, 井上 克也, Bostrem I. G., Sinitsyn Vl. E., Ovchinnikov A. S., 岸根 順一郎

    日本物理学会講演概要集   70   1977 - 1977   2015年

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    記述言語:日本語   出版者・発行元:一般社団法人日本物理学会  

    DOI: 10.11316/jpsgaiyo.70.1.0_1977

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  • 21aBE-4 カイラル磁性体CsCuCl_3の磁気秩序

    大石 一城, 高阪 勇輔, Proskurina E., 小椋 隆弘, 幸田 章宏, 秋光 純, 井上 克也

    日本物理学会講演概要集   70   1978 - 1978   2015年

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    記述言語:日本語   出版者・発行元:一般社団法人日本物理学会  

    DOI: 10.11316/jpsgaiyo.70.1.0_1978

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  • Tunneling spectra of break junctions involving Nb 3Sn 査読

    Toshikazu Ekino, Akira Sugimoto, Yuta Sakai, Alexander M. Gabovich, Jun Akimitsu

    Low Temperature Physics   40 ( 10 )   2014年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Institute of Physics Inc.  

    The electronic gap structure of Nb3Sn was measured by the break-junction (BJ) tunneling technique. The superconducting gap values are estimated to be in the range 2{increment} = 4-5.5 meV at T = 4.2 K as follows from the observed distinct conductance peaks. In addition to the superconducting gap structure, we observed reproducible hump-like structures at the biases of about ±20 and ±50 mV. Such a coexistence of gap and hump structures resembles the situation found in the high-Tc copper-oxide superconductors. Above the superconducting critical temperature Tc ~ 18 K, the humps appear as the only gap-like structures. Their possible origin is discussed in connection to the structural phase transition occurring in Nb3Sn.

    DOI: 10.1063/1.4897415

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  • Anomalous local spin susceptibilities in noncentrosymmetric La2 C3 superconductor 査読

    Anton Potočnik, Peter Jeglič, Kaya Kobayashi, Kenji Kawashima, Saori Kuchida, Jun Akimitsu, Denis Arčon

    Physical Review B - Condensed Matter and Materials Physics   90 ( 10 )   2014年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:American Physical Society  

    We report on La139 and C13 nuclear magnetic resonance (NMR) study of noncentrosymmetric superconductor La2C3 with superconducting critical temperature Tc=11.8 K. In the normal state La139 NMR spectra show a characteristic quadrupole polycrystalline broadening yielding a quadrupole frequency νQ=3.1 MHz and a very large shift of ∼3000 ppm. A significant part of this shift arises from the Van Vleck-like susceptibility as a result of a sizable spin-orbit coupling. In the superconducting phase La2C3 shows abnormal behavior that dramatically deviates from the conventional s-wave superconductors. Namely, (i) La139 and C13 NMR shifts in the superconducting state do not change from their normal-state values, and (ii) the La139 spin-lattice relaxation rate is strongly enhanced below Tc. Such unconventional response in the local static and dynamic spin susceptibilities is discussed in terms of possible mixture of spin-singlet and spin-triplet Cooper pairs, which is promoted by the asymmetric spin-orbit coupling in the system without the center of inversion.

    DOI: 10.1103/PhysRevB.90.104507

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  • Magnetic Properties of Chalcopyrite Cu1−xFeS2 at Low Temperatures

    K. Kobayashi, Y. Azuma, K. Kawashima, K. Horigane, M. Akaki, M. Tokunaga, J. Akimitsu

    Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013)   2014年6月

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    掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:Journal of the Physical Society of Japan  

    DOI: 10.7566/jpscp.3.017020

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  • Anisotropic Superconductivity of the Caged Compound Y5Rh6Sn18 with Unusual Normal-State Electrical Resistivity 査読

    N. Kase, S. Kittaka, T. Sakakibara, J. Akimitsu

    JPS Conference Proceedings   ( 3 )   015042   2014年6月

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    記述言語:英語  

    DOI: 10.7566/JPSCP.3.015042

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  • Superconductivity in the Ternary Boride Cr2Re3B with the beta-Mn-Type Structure 査読

    Haru Niimura, Kenji Kawashima, Keigo Inoue, Masaaki Yoshikawa, Jun Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   83 ( 4 )   2014年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We report the superconducting properties of a new intermetallic superconductor, Cr2Re3B. Cr2Re3B has a cubic structure called the beta-Mn-type structure with the space group P4(1)32 (No. 213), and shows a superconducting transition at around T-c = 4.8 K. The magnetization curve shows a typical type-II superconducting behavior. The estimated superconducting parameters of lower and upper critical fields, coherence length xi(0), penetration depth lambda(0), and Ginzburg-Landau (GL) parameter kappa(0), determined using GL relations, are about H-c1(0) = 52 Oe, H-c2(0) = 100 kOe, lambda(0) = 356 nm, xi(0) = 5.7 nm, and kappa(0) = 62.5, respectively. The specific heat data indicates that the superconductivity in Cr2Re3B can be described in the BCS theory. Finally, we discuss the superconducting state of Cr2Re3B using the determined parameters.

    DOI: 10.7566/JPSJ.83.044702

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  • 27pPSA-57 μSR法による無機カイラル磁性体CsCuCl_3の磁気秩序状態(27pPSA 領域8ポスターセッション(低温),領域8(強相関系))

    大石 一城, 高阪 勇輔, Proskurina E, 小椋 隆弘, 秋光 純, 井上 克也

    日本物理学会講演概要集   69 ( 1 )   2014年3月

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:一般社団法人日本物理学会  

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  • Orbital occupancies and the putative j eff = 1 2 ground state in Ba 2 IrO 4: A combined oxygen K -edge XAS and RIXS study 査読

    M. Moretti Sala, M. Rossi, S. Boseggia, J. Akimitsu, N. B. Brookes, M. Isobe, M. Minola, H. Okabe, H. M. Rønnow, L. Simonelli, D. F. McMorrow, G. Monaco

    Physical Review B - Condensed Matter and Materials Physics   89 ( 12 )   2014年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    The nature of the electronic ground state of Ba2IrO4 has been addressed using soft x-ray absorption and inelastic scattering techniques in the vicinity of the oxygen K edge. From the polarization and angular dependence of x-ray absorption spectroscopy (XAS) we deduce an approximately equal superposition of xy, yz, and zx Ir4+ 5d orbitals. By combining the measured orbital occupancies, with the value of the spin-orbit coupling provided by resonant inelastic x-ray scattering (RIXS), we estimate the crystal field splitting associated with the tetragonal distortion of the IrO6 octahedra to be small, Δ≈50(50) meV. We thus conclude definitively that Ba2IrO4 is a close realization of a spin-orbit Mott insulator with a jeff=12 ground state, thereby overcoming ambiguities in this assignment associated with the interpretation of x-ray resonant scattering experiments. © 2014 American Physical Society.

    DOI: 10.1103/PhysRevB.89.121101

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  • Bulk nature of layered perovskite iridates beyond the Mott scenario: An approach from a bulk-sensitive photoemission study

    A. Yamasaki, S. Tachibana, H. Fujiwara, A. Higashiya, A. Irizawa, O. Kirilmaz, F. Pfaff, P. Scheiderer, J. Gabel, M. Sing, T. Muro, M. Yabashi, K. Tamasaku, H. Sato, H. Namatame, M. Taniguchi, A. Hloskovskyy, H. Yoshida, H. Okabe, M. Isobe, J. Akimitsu, W. Drube, R. Claessen, T. Ishikawa, S. Imada, A. Sekiyama, S. Suga

    PHYSICAL REVIEW B   89 ( 12 )   2014年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    We present genuine bulk Ir 5d j(eff) states of layered perovskite iridates obtained by hard-x-ray photoemission spectroscopy (HAXPES) with s- and p-polarized lights. HAXPES spectra of Sr2IrO4 and Ba2IrO4 are well reproduced by the quasiparticle density of states calculated by the local density approximation with dynamical mean-field theory (LDA + DMFT). It is demonstrated that the insulating nature of the iridates is triggered by antiferromagnetic correlation (Slater type) combined with electron correlation (Mott type). The extremely-lowenergy bulk-sensitive photoemission spectroscopy reveals "bad metallic" states in the paramagnetic phase of the iridates, suggesting strongly renormalized metallic states above the Neel temperature as predicted by the LDA + DMFT.

    DOI: 10.1103/PhysRevB.89.121111

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  • The electronic structure of the high-symmetry perovskite iridate Ba 査読

    S. Moser, L. Moreschini, A. Ebrahimi, B. Dalla Piazza, M. Isobe, H. Okabe, J. Akimitsu, V. V. Mazurenko, K. S. Kim, A. Bostwick, E. Rotenberg, J. Chang, H. M. Rønnow, M. Grioni

    New Journal of Physics   16   2014年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We report angle-resolved photoemission (ARPES) measurements,density functional and model tight-binding calculations on (Ba-214),an antiferromagnetic insulator.Ba-214 does not exhibit the rotational distortion of the Iroctahedra that is present in its sister compound (Sr-214),and is therefore an attractive reference material to study the electronic structure of layered iridates.We find that the band structures of Ba-214 and Sr-214 are qualitatively similar, hinting at the predominant role of the spin-orbit interaction in these materials.Temperature-dependent ARPES data show that the energy gap persists well above TN, and favor a Mott over a Slater scenario for this compound. © 2014 IOP Publishing and Deutsche Physikalische Gesellschaft.

    DOI: 10.1088/1367-2630/16/1/013008

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  • Chiral soliton lattice in chiral magnetic crystal crnb3s6 査読

    Y. Togawa, T. Koyama, S. Mori, Y. Kousaka, J. Akimitsu, S. Nishihara, K. Inoue, A. S. Ovchinnikov, J. Kishine

    Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy   61 ( 1 )   S34 - S36   2014年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Journal of the Japan Society of Powder and Powder Metallurgy  

    We microscopically investigate chiral magnetic orders in the absence and presence of magnetic field in a chiral magnetic crystal CrNb3S6 by means of low-temperature Lorenz transmission electron microscopy and small-angle electron scattering method. Based on detailed analyses in both real and reciprocal space, we directly observe that chiral soliton lattice (CSL) emerges in small magnetic fields applied perpendicular to the chiral crystallographic axis. CSL develops from chiral helimagnetic structure (CHM) with increasing the spatial period from 48 nm toward sample size in rising magnetic fields. Chiral magnetic orders of CSL and CHM do not exhibit any structural dislocation, indicating their high stability and robustness. This is because chiral magnetic orders are macroscopically induced by monoaxial Dzyaloshinkii-Moriya exchange interaction that is allowed in hexagonal CrNb3S6 crystals belonging to noncentrosymmetric chiral space group. Present observations of periodic, nonlinear, tunable, and robust CSL will be the first step to explore fascinating functions of CSL for magnetic and spintronic device applications using chiral magnets.

    DOI: 10.2497/jjspm.61.S34

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  • Crystal growth of chiral magnetic material in CsCuCl3 査読

    Yusuke Kousaka, Takemi Koyama, Marina Miyagawa, Kohei Tanaka, Jun Akimitsu, Katsuya Inoue

    Journal of Physics: Conference Series   502 ( 1 )   2014年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:Institute of Physics Publishing  

    We report a crystallization technique to make single crystals with homochiral crystallographic domain in CsCuCl3. By adapting crystallization technique with stirring, we succeeded in obtaining the mm-ordered enantiopure single crystals. The samples obtained without the stirring crystallization technique had zigzag shape, suggesting many crystalline nuclei combined during the crystallization process. The samples done with the stirring crystallization technique formed quart-crystal-like shape, suggesting only one crystalline nucleus grew. Absolute structure analysis using X-ray diffraction showed that the samples grown only by our technique had a homochiral crystalline domain.

    DOI: 10.1088/1742-6596/502/1/012019

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  • Spin Waves in Ferromagnetic Phase of MnP 査読

    S. Itoh, S. Yano, T. Yokoo, S. Satoh, D. Kawana, Y. Kousaka, J. Akimitsu, Y. Endoh

    1ST CONFERENCE ON LIGHT AND PARTICLE BEAMS IN MATERIALS SCIENCE 2013 (LPBMS2013)   502   2014年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:IOP PUBLISHING LTD  

    Inelastic neutron scattering experiments on an intermetallic compound, MnP, were performed by using a chopper spectrometer as well as triple axis spectrometers. Spin waves were observed in the ferromagnetic phase in the entire Brillouin zone along the a*- and b*-axes. The observed dispersion relations of spin waves were well described by an isotropic Heisenberg interaction adding a single ion anisotropy with two sub-lattices.

    DOI: 10.1088/1742-6596/502/1/012044

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  • Spin and Hole Dynamics in Carrier-Doped Quantum Haldane Chain 査読

    T. Yokoo, S. Itoh, S. Ibuka, H. Yoshizawa, J. Akimitsu

    27TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT27), PTS 1-5   568   2014年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:IOP PUBLISHING LTD  

    Quantum spins in one-dimensional (1D) chains exhibit characteristic features such as the Haldane gap (S = 1) and the spin-Peierls state (S = 1/2). Multiple degrees of freedom associated with quantum spins have recently become a focus of attention. We present here the spin dynamics in lightly hole-doped Nd2-xCaxBaNiO5 (x = 0.1), a 1D Haldane system, using pulsed neutron inelastic scattering. The entire dispersion of the 1D Haldane chain was clearly observed with a similar to 14 meV spin gap (Haldane gap) at the magnetic zone center. Below the gap, much spectral weight was observed, indicating an in-gap state possibly originating from the doped holes in Haldane chains.

    DOI: 10.1088/1742-6596/568/4/042035

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  • Dynamical Properties of Spins and Holes in Carrier Doped Quantum Haldane Chain 査読

    T. Yokoo, S. Itoh, D. Kawana, H. Yoshizawa, J. Akimitsu

    1ST CONFERENCE ON LIGHT AND PARTICLE BEAMS IN MATERIALS SCIENCE 2013 (LPBMS2013)   502   2014年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:IOP PUBLISHING LTD  

    Quantum spins in one-dimensional (1D) chains exhibit characteristic features such as the Haldane gap (S=1) and the spin-Peierls state (S=1/2). Recently multi degrees of freedom in cooperation with quantum spins are focused. We present here spin dynamics in the lightly hole doped Nd2-xCaxBaNiO5 (x=0.035), 1D Haldane system by means of pulsed neutron inelastic scattering method. One-magnon band with a spin gap (Haldane gap) was visible with the decrease of the zone boundary energy to 60 meV. In contrast, the spin gap slightly increases. Dynamical structures inside spin gap upon carrier doping showing incommensurate structure centered at the magnetic zone center was newly observed which is possibly originating from the doped holes in Haldane chains.

    DOI: 10.1088/1742-6596/502/1/012045

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  • Antiferromagnetic transition of the caged compound TmTi2Al20 査読

    Naoki Kase, Yasuyuki Shimura, Shunichiro Kittaka, Toshiro Sakakibara, Satoru Nakatsuji, Tomohito Nakano, Naoya Takeda, Jun Akimitsu

    Journal of Physics: Conference Series   592 ( 1 )   2014年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:Institute of Physics Publishing  

    We examined the low temperature magnetic properties of a single crystal of TmTi2Al20, a hole analogue of PrTi2Al20, by means of specific heat C(T) and magnetization M(T, H) measurements. At low field, an anomaly that can be ascribed to an antiferromagnetic ordering is observed in M(T) at TN =0.7 K. C(T) shows a very sharp λ-type anomaly at 0.7 K, confirming the phase transition. From the calculated magnetic entropy S(T), we found that the ground state of TmTi2Al20 is likely to be a magnetic Γ5 triplet in the J =6 multiplet.

    DOI: 10.1088/1742-6596/592/1/012052

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  • Interlayer Magnetoresistance due to Chiral Soliton Lattice Formation in Hexagonal Chiral Magnet CrNb3S6 査読

    Togawa, Y, Kousaka, Y, Nishihara, S, Inoue, K, Akimitsu, J, Ovchinnikov, A. S, Kishine, J

    PHYSICAL REVIEW LETTERS   111 ( 19 )   2013年11月

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    We investigate the interlayer magnetoresistance (MR) along the chiral crystallographic axis in the hexagonal chiral magnet CrNb3S6. In a region below the incommensurate-commensurate phase transition between the chiral soliton lattice and the forced ferromagnetic state, a negative MR is obtained in a wide range of temperature, while a small positive MR is found very close to the Curie temperature. Normalized data of the negative MR almost falls into a single curve and is well fitted by a theoretical equation of the soliton density, meaning that the origin of the MR is ascribed to the magnetic scattering of conduction electrons by a nonlinear, periodic, and countable array of magnetic soliton kinks.

    DOI: 10.1103/PhysRevLett.111.197204

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  • New germanide superconductors with the type-I clathrate type structure 査読

    T. Ipponjima, K. Kawashima, S. Miyazaki, D. Hyakumura, M. Yoshikawa, J. Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   494   74 - 76   2013年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We discovered that the new germanide superconductors of A(8)(Al,Ge)(46) (A = Rb, Cs). A(8)(Al,Ge)(46) have the type-I clathrate type structure (cubic, space group: Pm (3) over barn (No. 223)). The superconducting transition temperature is confirmed at around 3.6 K for Rb-8(Al0.4Ge0.6)(46) and 4 K for Cs-8(Al0.4Ge0.6)(46), respectively. The lattice constants of these compounds are estimated to be a = 1.082(6) nm for Rb-8(Al0.4Ge0.6)(46) and a = 1.087(8) nm for Cs-8(Al0.4Ge0.6)(46). The M-H curve at 1.8 K shows that the Rb-8(Al0.4Ge0.6)(46) is categorized to be a conventional type-II superconductor. The superconducting parameters of Rb-8(Al0.4Ge0.6)(46) are determined using the thermodynamic data and Ginzburg-Landau relations. (C) 2013 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physc.2013.03.053

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  • Carrier doping effect for transport properties of a spin-orbit Mott insulator Ba2IrO4 査読

    H. Okabe, M. Isobe, E. Takayama-Muromachi, N. Takeshita, J. Akimitsu

    Physical Review B - Condensed Matter and Materials Physics   88 ( 7 )   2013年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    Heavily potassium-substituted barium iridates Ba2-xK xIrO4 (x≤0.5) and lanthanum-substituted Ba 2-yLayIrO4 (y≤0.05) were prepared for the study of the carrier doping effects on the transport properties of the spin-orbit Mott state. The carrier type is holelike for the nondoped (x = y = 0) and K-doped (x&gt
    0) phases, while it is electronlike for the La-doped (y&gt
    0) phases. It was found that electron doping is more effective in decreasing the electrical resistivity. This suggests an asymmetry of the density of states between the upper and lower energies of the Fermi level. A semimetallic state emerges for the K-doped phases with x≥0.3 at ambient pressure. More conducting metallic states (ρ∼10-2Ωcm, dρ/dT&gt
    0) were achieved under high pressure for both the K- and La-doped phases. Notwithstanding, no superconductivity was observed in the metallic states down to 4.2 K. The experimental results are discussed with respect to the electronic phase diagram calculated by Watanabe. © 2013 American Physical Society.

    DOI: 10.1103/PhysRevB.88.075137

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  • Three-dimensional near-surface imaging of chirality domains with circularly polarized X-rays 査読

    Hiroyuki Ohsumi, Akihisa Tokuda, Soshi Takeshita, Masaki Takata, Motohiro Suzuki, Naomi Kawamura, Yusuke Kousaka, Jun Akimitsu, Taka-Hisa Arima

    Angewandte Chemie - International Edition   52 ( 33 )   8718 - 8721   2013年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    Imaging of chirality domains: Resonant circularly polarized X-ray diffraction distinguishes crystalline enantiomers (see picture). Control of the penetration depth and microfocusing of X-rays leads to a new visualization technique of the three-dimensional chirality-domain morphology, providing support to the increasing demand for the improvement of inorganic chiral crystal engineering. Copyright © 2013 WILEY-VCH Verlag GmbH &amp
    Co. KGaA, Weinheim.

    DOI: 10.1002/anie.201303023

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  • Superconductivity in KSn2 with the MgZn2-type structure 査読

    Shota Miyazaki, Kenji Kawashima, Tsukasa Ipponjima, Michinori Fukuma, Daiki Hyakumura, Jun Akimitsu, Masaaki Yoshikawa

    Journal of the Korean Physical Society   63 ( 3 )   475 - 476   2013年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We report here the sysnthesis and discovery of a new superconductor potassium distannide KSn2. KSn2 has a hexagonal crystal structure with a space group of P63 /mmc, which is called the MgZn2-type (C14) structure belonging to the famous family of Laves phases. The calculated lattice constants a and c are 0.6427(4) and 1.043(6) nm, respectively. From the magnetic susceptibility measurement, KSn2 shows a superconducting transition at around 3.2 K. © 2013 The Korean Physical Society.

    DOI: 10.3938/jkps.63.475

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  • NMR and NQR Studies on Non-centrosymmetric Superconductors Re7B3, LaBiPt, and BiPd 査読

    Kazuaki Matano, Satoki Maeda, Hiroki Sawaoka, Yuji Muro, Toshiro Takabatake, Bhanu Joshi, Srinivasan Ramakrishnan, Kenji Kawashima, Jun Akimitsu, Guo-qing Zheng

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   82 ( 8 )   2013年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We report the nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements for non-centrosymmetric superconductors Re7B3, LaBiPt, and BiPd containing heavy elements. For all three compounds, the spin-lattice relaxation rate 1/T-1 shows a coherence peak just below T-c and decreases exponentially at low temperatures, which indicates that an isotropic superconducting gap is dominant in these compounds. In BiPd, the height of the coherence peak just below T-c is much suppressed, which suggests that there exists a substantial component of gap with nodes in this compound. Our results indicate that heavy element is not the only factor, but the extent of inversion symmetry breaking is also important to induce a large spin-orbit coupling and an unconventional superconducting state.

    DOI: 10.7566/JPSJ.82.084711

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  • Magnetic states in quasi-2-D iridium oxides with large spin-orbit coupling 査読

    Masaaki Isobe, Hirotaka Okabe, Jun Akimitsu

    Journal of the Korean Physical Society   63 ( 3 )   394 - 397   2013年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    The magnetic states in the novel K2NiF4-type quasi-2-D spin-orbit-induced Mott insulator Ba2IrO4 was studied using magnetic susceptibility, muon spin rotation (μSR), and high-pressure resistivity measurements. In Ba2IrO4, a Néel ordered state was found to be stabilized at temperatures below T N (Néel temperature) ∼ 240 K. The critical exponent β ∼ 0.18 suggests a quasi-2-D Heisenberg antiferromagnetic state on the IrO2 planar square lattice. The magnetic moment is significantly reduced to ∼0.34 μB/Ir-atom, probably due to a low-dimensional quantum spin fluctuation with a large magnetic intra-plane correlation {pipe}J{pipe}. A non-Fermi liquid-like behavior with carrier scattering due to the spin fluctuation was observed in the metallic state under high pressures of more than 13.8 GPa. © 2013 The Korean Physical Society.

    DOI: 10.3938/jkps.63.394

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  • Thermo-electronic properties of the copper-oxide Cu6O8YCI1-xBrx 査読

    Kenji Kawashima, Hiroki Takeda, Hazuki Tozawa, Jun Akimitsu

    Journal of the Korean Physical Society   63 ( 3 )   420 - 422   2013年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We report the thermal conductivity and the Seebeck coefficient of polycrystalline samples of Cu6O8YCl1-xBrx. The lattice constant a linearly increases with increasing Br- concentration x, indicating an expansion of the Cu6O8 cage. At 350 K, the thermal conductivity κ is small and undergoes very little change with x, suggesting the existence of an anharmonic vibration. On the other hand, the Seebeck coefficient S is enhanced by substituting Br- for Cl-. These facts indicate that the thermoelectronic properties are sensitive to the Cu6O8 cage size in the Cu6O8YCl system. © 2013 The Korean Physical Society.

    DOI: 10.3938/jkps.63.420

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  • Emergence of highly degenerate excited states in the frustrated magnet MgCr2O4 査読

    K. Tomiyasu, T. Yokobori, Y. Kousaka, R. I. Bewley, T. Guidi, T. Watanabe, J. Akimitsu, K. Yamada

    Physical Review Letters   110 ( 7 )   2013年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    High degeneracy in ground states leads to the generation of exotic zero-energy modes, a representative example of which is the formation of molecular spin-liquid-like fluctuations in a frustrated magnet. Here we present single-crystal inelastic neutron scattering results for the frustrated magnet MgCr2O4, which show that a common set of finite-energy molecular spin excitation modes is sustained in both the liquid-like phase above magnetic ordering temperature TN and an ordered phase with an extremely complex magnetic structure below TN. Based on this finding, we propose the concept of high degeneracy in excited states, which promotes local resonant elementary excitations. This concept is expected to have ramifications on our understanding of excitations in many complex systems, including not only spin but also atomic liquids, complex order systems, and amorphous systems. © 2013 American Physical Society.

    DOI: 10.1103/PhysRevLett.110.077205

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  • Tunneling break-junction measurements of the superconducting gap in Y2C3 査読

    T. Ekino, A. Sugimoto, A. M. Gabovich, H. Kinoshita, J. Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   484   52 - 55   2013年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Tunneling measurements of the electron spectrum in the superconducting state have been carried out in Y2C3 samples with the bulk (resistive and diamagnetic) critical temperature T-c = 15-16 K. A break-junction technique was crucial to find the intrinsic gap features. The representative gap structure at 4 K reveals distinct BCS-like peaks with the gap value of 2 Delta(4 K) = 3-4 meV. Assuming local T-c values of 10-13 K, the characteristic gap to T-c ratio 2 Delta/k(B)T(c) (k(B) is the Boltzmann constant) was found to possess the BCS weak-coupling value 3.5 +/- 0.2, appropriate to isotropic s-wave superconductors. (C) 2012 Elsevier B. V. All rights reserved.

    DOI: 10.1016/j.physc.2012.03.044

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  • Effect of the counter-anion on the spin-transition properties of a family of Fe(ii) tetrazole complexes, [Fe(i4tz)&lt;inf&gt;6&lt;/inf&gt;]X&lt;inf&gt;2&lt;/inf&gt; (X = ClO &lt;inf&gt;4&lt;/inf&gt;&lt;sup&gt;-&lt;/sup&gt;, PF&lt;inf&gt;6&lt;/inf&gt;&lt;sup&gt;-&lt;/sup&gt;, SbF&lt;inf&gt;6&lt;/inf&gt; &lt;sup&gt;-&lt;/sup&gt;, BF&lt;inf&gt;4&lt;/inf&gt;&lt;sup&gt;-&lt;/sup&gt;) 査読

    Myrvete Tafili-Kryeziu, Matthias Weil, Takahiro Muranaka, Azzedine Bousseksou, Miki Hasegawa, Akimitsu Jun, Wolfgang Linert

    Dalton Transactions   42 ( 44 )   15796 - 15804   2013年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Royal Society of Chemistry  

    The monodentate ligand 1-isobutyl-1H-tetrazole (i4tz) was used to synthesize a series of iron(ii) spin-crossover (SCO) complexes with the general formula [Fe(i4tz)&lt
    inf&gt
    6&lt
    /inf&gt
    ]X&lt
    inf&gt
    2&lt
    /inf&gt
    , where X = ClO&lt
    inf&gt
    4&lt
    /inf&gt
    &lt
    sup&gt
    -&lt
    /sup&gt
    (1), PF&lt
    inf&gt
    6&lt
    /inf&gt
    &lt
    sup&gt
    -&lt
    /sup&gt
    (2), SbF&lt
    inf&gt
    6&lt
    /inf&gt
    &lt
    sup&gt
    -&lt
    /sup&gt
    (3), or BF&lt
    inf&gt
    4&lt
    /inf&gt
    &lt
    sup&gt
    -&lt
    /sup&gt
    (4). Upon cooling, magnetic susceptibility measurements of compounds 1, 2 and 3 show a reversible one-step spin crossover (SCO) behaviour between HS (S = 2) and LS (S = 0) states without hysteresis. The ClO&lt
    inf&gt
    4&lt
    /inf&gt
    &lt
    sup&gt
    -&lt
    /sup&gt
    compound shows a gradual spin transition at T &lt
    inf&gt
    1/2&lt
    /inf&gt
    = 208 K at a considerably higher temperature than the other compounds. The larger anions PF&lt
    inf&gt
    6&lt
    /inf&gt
    &lt
    sup&gt
    -&lt
    /sup&gt
    (T&lt
    inf&gt
    1/2&lt
    /inf&gt
    = 114 K) and SbF&lt
    inf&gt
    6&lt
    /inf&gt
    &lt
    sup&gt
    -&lt
    /sup&gt
    (T&lt
    inf&gt
    1/2&lt
    /inf&gt
    = 116 K) show more gradual transitions, and a "frozen-in effect" was observed after fast cooling. The crystal structure of [Fe(i4tz)&lt
    inf&gt
    6&lt
    /inf&gt
    ](SbF&lt
    inf&gt
    6&lt
    /inf&gt
    )&lt
    inf&gt
    2&lt
    /inf&gt
    (3) was determined at 296, 200 and 100 K, revealing a reversible first-order phase transition between 200 and 100 K, accompanied by a triplication of the unit cell volume at 100 K. Temperature-dependent vibration and electronic spectroscopic measurements of compound 1 were compared with the magnetic susceptibility measurements. © The Royal Society of Chemistry 2013.

    DOI: 10.1039/c3dt52339j

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  • Superconductivity in Lu2SnC 査読

    S. Kuchida, T. Muranaka, K. Kawashima, K. Inoue, M. Yoshikawa, J. Akimitsu

    Physica C: Superconductivity and its Applications   494   77 - 79   2013年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We discovered the superconductivity in Lu2SnC with the T c of 5.2 K. Lu2SnC crystallizes in a hexagonal structure (Cr2AlC-type) with the space group of P63/mmc. We measured the physical properties of Lu2SnC in the superconducting state as functions of temperature and magnetic field. The M-H curve shows the typical type-II superconducting behavior. Superconducting parameters Hc1(0), Hc2(0), λ(0), ξ(0) and κGL are determined to be about 110 Oe, 4.5 kOe, 250 nm, 27 nm, and 9.2, respectively. © 2013 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physc.2013.04.050

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  • Magnetic excitations in ferromagnetic phase of MnP 査読

    Shin-Ichiro Yano, Shinichi Itoh, Tetsuya Yokoo, Setsuo Satoh, Daichi Kawana, Yusuke Kousaka, Jun Akimitsu, Yasuo Endoh

    Journal of Magnetism and Magnetic Materials   347   33 - 38   2013年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    Inelastic neutron scattering experiments were performed on an intermetallic compound, MnP. We used a newly developed High Resolution Chopper Spectrometer, HRC, for energy transfers E≤75meV, besides various triple axis spectrometers
    LTAS for energy transfers E≤2meV, TOPAN for E≤7.5meV and TAS-1 for E≤35meV. Spin wave excitations were observed in the ferromagnetic phase of MnP in the entire Brillouin zone along the a*- and b*-axes. The zone boundary energies of spin waves were determined to be around 60 meV along the a*-axis and around 75 meV along the b*-axis, and the dispersion relations showed two branches for both axes. The observed dispersion relations of spin waves were well described by an isotropic Heisenberg interaction adding a single ion anisotropy with two sub-lattices. © 2013 Published by ElsevierB.V.

    DOI: 10.1016/j.jmmm.2013.07.019

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  • 高温超電導材料Ⅵ

    48 ( 3 )   110 - 118   2013年

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    掲載種別:研究論文(学術雑誌)  

    DOI: 10.2221/jcsj.48.110

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  • Superconductivity in Ba(TM,Si)(2) (TM = Pd, Pt, Cu, Ag, and Au) with the AlB2-Type Structure 査読

    Kenji Kawashima, Keigo Inoue, Takuya Ishikawa, Michinori Fukuma, Masaaki Yoshikawa, Jun Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   81 ( 11 )   2012年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We succeeded in synthesizing BaTMxSi2-x (TM = Pd, Pt, Cu, Ag, and Au) superconductors with the AlB2-type structure and performed detailed thermodynamic measurements of the superconducting state in BaTMxSi2-x to determine the superconducting parameters [H-c1(0), H-c2(0), lambda(0), xi(0), and kappa(GL)] using Ginzburg-Landau relations. The observed parameters indicated that BaTMxSi2-x can be categorized into a conventional type-II superconductor. The AlB2-type structure appears at x between 0.2 and 0.8, and the lattice parameters systematically changed with x. The superconductivity was observed at a TM concentration x = 0.2-0.6. T-c was almost constant between x = 0.2 and 0.3 and observed to decrease with increasing TM concentration (0.3 &lt;= x &lt;= 0.6). The specific heat data indicated that the superconductivity in BaTMxSi2-x exists in a weak coupling regime in the BCS theory. Moreover, we discussed the role of the TM element in BaTMxSi2-x using the observed data.

    DOI: 10.1143/JPSJ.81.114717

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  • Unconventional Magnetic Domain Structure in the Ferromagnetic Phase of MnP Single Crystals 査読

    Tsukasa Koyama, Shin-ichiro Yano, Yoshihiko Togawa, Yusuke Kousaka, Shigeo Mori, Katsuya Inoue, Jun-ichiro Kishine, Jun Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   81 ( 4 )   2012年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We have studied ferromagnetic (FM) structures in the FM phase of MnP single crystals by low-temperature Lorentz transmission electron microscopy and small-angle electron diffraction analysis. In Lorentz Fresnel micrographs, striped FM domain structures were observed at an external magnetic field less than 10 Oe in specimens with the ab-plane in their plane. From real-and reciprocal-space analyses, it was clearly identified that striped FM domains oriented to the c-axis appear with Bloch-type domain walls in the b-direction and order regularly along the a-axis with a constant separation less than 100 nm. Moreover, the magnetic chirality reverses in alternate FM domain walls. These specific spin configuration of striped FM domains will affect the magnetic phase transition from the FM phase to the proper screw spiral phase at low temperature or to the FAN phase in magnetic fields in MnP.

    DOI: 10.1143/JPSJ.81.043701

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  • Chiral Magnetic Soliton Lattice on a Chiral Helimagnet 査読

    Togawa, Y, Koyama, T, Takayanagi, K, Mori, S, Kousaka, Y, Akimitsu, J, Nishihara, S, Inoue, K, Ovchinnikov, A. S, Kishine, J

    PHYSICAL REVIEW LETTERS   108 ( 10 )   2012年3月

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    Using Lorenz microscopy and small-angle electron diffraction, we directly present that the chiral magnetic soliton lattice (CSL) continuously evolves from a chiral helimagnetic structure in small magnetic fields in Cr1/3NbS2. An incommensurate CSL undergoes a phase transition to a commensurate ferromagnetic state at the critical field strength. The period of a CSL, which exerts an effective potential for itinerant spins, is tuned by simply changing the field strength. Chiral magnetic orders observed do not exhibit any structural dislocation, indicating their high stability and robustness in Cr1/3NbS2.

    DOI: 10.1103/PhysRevLett.108.107202

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  • Hole depletion of ladders in Sr 14Cu 24O 41 induced by correlation effects 査読

    V. Ilakovac, C. Gougoussis, M. Calandra, N. B. Brookes, V. Bisogni, S. G. Chiuzbaian, J. Akimitsu, O. Milat, S. Tomić, C. F. Hague

    Physical Review B - Condensed Matter and Materials Physics   85 ( 7 )   2012年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    The hole distribution in Sr 14Cu 24O 41 is studied by low-temperature polarization-dependent O K near-edge x-ray absorption fine-structure measurements and state-of-the-art electronic structure calculations that include core-hole and correlation effects in a mean-field approach. Contrary to all previous analysis, based on semiempirical models, we show that correlations and antiferromagnetic ordering favor the strong chain-hole attraction. For the remaining small number of holes accommodated on ladders, leg sites are preferred to rung sites. The small hole affinity of rung sites explains naturally the one-dimensional to two-dimensional crossover in the phase diagram of (La,Y,Sr,Ca) 14Cu 24O 41. © 2012 American Physical Society.

    DOI: 10.1103/PhysRevB.85.075108

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  • Hole depletion of ladders in Sr14Cu24O41 induced by correlation effects 査読

    V. Ilakovac, C. Gougoussis, M. Calandra, N. B. Brookes, V. Bisogni, S. G. Chiuzbaian, J. Akimitsu, O. Milat, S. Tomic, C. F. Hague

    PHYSICAL REVIEW B   85 ( 7 )   2012年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    The hole distribution in Sr14Cu24O41 is studied by low-temperature polarization-dependent O K near-edge x-ray absorption fine-structure measurements and state-of-the-art electronic structure calculations that include core-hole and correlation effects in a mean-field approach. Contrary to all previous analysis, based on semiempirical models, we show that correlations and antiferromagnetic ordering favor the strong chain-hole attraction. For the remaining small number of holes accommodated on ladders, leg sites are preferred to rung sites. The small hole affinity of rung sites explains naturally the one-dimensional to two-dimensional crossover in the phase diagram of (La,Y,Sr,Ca)(14)Cu24O41.

    DOI: 10.1103/PhysRevB.85.075108

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  • Superconducting gap structure of the cage compound Sc5Rh6Sn18 査読

    N. Kase, S. Kittaka, T. Sakakibara, J. Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   ( 81 )   SB016   2012年1月

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    記述言語:英語  

    DOI: 10.1143/JPSJS.81SB.SB016

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  • Magnetic ordering in spin-orbit Mott insulator Ba2IrO4 probed by mu SR 査読

    H. Okabe, M. Isobe, E. Takayama-Muromachi, A. Koda, S. Takeshita, M. Hiraishi, M. Miyazaki, R. Kadono, Y. Miyake, J. Akimitsu

    26TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT26), PTS 1-5   400   2012年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:IOP PUBLISHING LTD  

    Magnetic properties in the novel spin-orbit Mott insulator Ba2IrO4 were studied using muon spin rotation (mu SR) technique. Zero-field mu SR experiments revealed that Ba2IrO4 shows an antiferromagnetic transition at T-N similar to 240 K without any spontaneous magnetization. The most stable mu(+) site was determined by the electrostatic (Madelung) potential calculation. The effective magnetic moment of the iridium ions (vertical bar mu vertical bar) in the antiferromagnetic ordered state was calculated using a dipolar-field model, with an internal field obtained by mu SR experiments. The magnetic moment is significantly reduced (vertical bar mu vertical bar similar to 0.34 mu(B)) due to a low-dimensional quantum spin fluctuation with a large intra-plane correlation. The magnetic ground state of the spin-orbit Mott insulator Ba2IrO4 is quite similar to those in parent materials of high-T-C cuprate superconductors such as La2CuO4.

    DOI: 10.1088/1742-6596/400/3/032071

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  • Magnetic excitations in MnP 査読

    Shinichiro Yano, Jun Akimitsu, Shinich Itoh, Tetsuya Yokoo, Setsuo Satoh, Daichi Kawana, Endoh Yasuo

    Journal of Physics: Conference Series   391 ( 1 )   2012年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:Institute of Physics Publishing  

    We performed inelastic neutron scattering experiments on a three-dimensional magnetic system MnP, which is a ferromagnetic inter-metallic compound below TC 291 K and transitions into a screw state at T * 47 K. Using newly developed high resolution chopper spectrometer (HRC) and triple axis spectrometer LTAS (for E 0-2 meV), 6G (for E 2-8 meV) TAS1 (for E 10-35 meV), we succeeded in observing the excitations from low energy to Brillouin zone center. We assumed isotropic 6 exchange parameters, and calculated dispersion relation using Heisenberg model for 2-sub lattices. We confirm that nearest neighbor (n.n) exchange parameters Jm are J &gt
    0 (ferromagnetic interaction), the next nearest neighbor (n.n.n) exchange parameters are J &lt
    0 (anti-ferromagnetic interaction), We conclude that the competition between n.n and n.n.n would be the cause of helimagnetic structure in MnP. © Published under licence by IOP Publishing Ltd.

    DOI: 10.1088/1742-6596/391/1/012113

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  • Crystal structure refinement of Cu6O8YCl by powder neutron diffraction 査読

    Kenji Kawashima, Kazumasa Horigane, Kenji Ohoyama, Jun Akimitsu

    Journal of Physics: Conference Series   391 ( 1 )   2012年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:Institute of Physics Publishing  

    We report the crystal structure parameters of clathrate-type copper-oxide Cu6O8YCl, which belongs to the Cu6O8MX (M cation, X anion) family, investigated by the powder neutron diffraction technique and the Rietveld method. The crystal structure of Cu6O8YCl was identified to be a cubic structure with the space group Fmm. The Cl atom is located at the 4a site, which is the center position of the Cu6O8 cage, and the Y atom occupies the 4b site in the cuboid space. The Y and Cl atoms have anomalously large atomic displacement parameters (Uiso) at room temperature, which suggests that Y and Cl atoms respectively present in a cuboid and in a Cu6O8 cage exhibit intensive motion such as rattling, as recently observed for clathrate compounds. © Published under licence by IOP Publishing Ltd.

    DOI: 10.1088/1742-6596/391/1/012129

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  • Substitution effect on the superconductivity in W5-xTaxSiB2 with the T-2 phase structure 査読

    M. Fukuma, K. Kawashima, J. Akimitsu

    INTERNATIONAL CONFERENCE ON STRONGLY CORRELATED ELECTRON SYSTEMS (SCES 2011)   391   2012年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:IOP PUBLISHING LTD  

    We report here the magnetic susceptibility and electrical resistivity measurements of W5-xTaxSiB2. W5-xTaxSiB2 crystallizes in a tetragonal (the Mo5SiB2 (T-2 phase) type) structure. We evaluated the effect of the partial substitution of Ta in W5SiB2. The superconductivity was observed below x = 2 in W5-xTaxSiB2. As substituted Ta for W, the superconducting transition temperature (T-c) clearly enhanced from 5.8 K to 6.5 K. The largest volume fraction exhibits at x = 0.2 sample. Thus, this work shows, for the first time, the superconductivity in W5-xTaxSiB2. Moreover, we determined that the dominant carrier sign of W5SiB2 are the hole from the thermoelectric power measurement and discussed about the enhancement of T-c of W5-xTaxSiB2 system.

    DOI: 10.1088/1742-6596/391/1/012090

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  • Magnetic excitations in possible spin-Peierls system TiOBr 査読

    T. Yokoo, S. Itoh, F. Trouw, A. Llobet-Megias, J. Akimitsu

    26TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT26), PTS 1-5   400   2012年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:IOP PUBLISHING LTD  

    Low lying magnetic excitations of newly proposed spin-Peierls system TiOBr was measured by inelastic neutron scattering technique. The correlation between lattice-spin-orbital cooperates to realize the one-dimensional quantum ground state in this system. At antiferromagnetic (AF) zone center localized magnetic signal was observed at the transfer energy of 10 meV. The integrated intensity shows specific Q dependence that can be explained 1D AF dynamical structure.

    DOI: 10.1088/1742-6596/400/3/032123

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  • Superconductivity in BaCuxSi2-x (0.2 &lt;= x &lt;= 0.6) with AlB2-type structure 査読

    K. Inoue, K. Kawashima, T. Ishikawa, M. Fukuma, T. Okumura, T. Muranaka, J. Akimitsu

    ADVANCES IN SUPERCONDUCTIVITY XXIV   27   52 - 55   2012年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:ELSEVIER SCIENCE BV  

    We discovered superconductivity in BaCuxSi2-x (0.2 &lt;= x &lt;= 0.6) with the maximum T-c of 3 K. The synthesized samples have AlB2-type structure. We measured the physical properties of BaCu0.3Si1.7 in the superconducting state as functions of temperature and magnetic field. Magnetization vs. magnetic field (M-H) curves show typical type-II superconducting behavior. Superconducting parameters H-c1(0), H-c2(0), lambda(0), xi(0), and kappa(GL) are determined to be about 20 Oe, 22 kOe, 570 nm, 12 nm, and 48, respectively. Lattice constants a and c and the T-c value depend on Cu concentration, x, T-c decreases with increasing x. (C) 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of ISS Program Committee.

    DOI: 10.1016/j.phpro.2012.03.408

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  • Magnetic structure of Ba2Mg2Fe12O22 under the magnetic field 査読

    S. Yano, Y. Nishikawa, Y. Kousaka, J. Akimitsu, K. Taniguchi, T. Arima, H. Sagayama, H. Kimura, Y. Noda

    26TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT26), PTS 1-5   400   2012年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:IOP PUBLISHING LTD  

    Ba2Mg2Fe12O22 has a strong coupling between electric polarization P and magnetization M. To understand the multiferroic properties of the system, we performed crystal structural and precise magnetic structural analyses of Ba2Mg2Fe12O22 under a magnetic field (T = 4K, H = 0.3T) using 4 circle neutron diffractometer (FONDER) at JRR-3M Tokai. We collected 105 points of magnetic scattering for q= (0,0,3/2) and over 150 points of fundamental scattering which contains ferri magnetic component. Using the part of these data, were investigate magnetic struture of Ba2Mg2Fe12O22, in which 14 (Mg,Fe) sites have been included.

    DOI: 10.1088/1742-6596/400/3/032120

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  • Spin-Orbit Mott State in the Novel Quasi-2D Antiferromagnet Ba2IrO4 査読

    M. Isobe, H. Okabe, E. Takayama-Muromachi, A. Koda, S. Takeshita, M. Hiraishi, M. Miyazaki, R. Kadono, Y. Miyake, J. Akimitsu

    26TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT26), PTS 1-5   400 ( PART 3 )   2012年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:IOP PUBLISHING LTD  

    The spin-orbit Mott state in the novel quasi-2D iridate Ba2IrO4 was studied on crystal structure, electrical resistivity, magnetic susceptibility, and mu SR experiments. Ba2IrO4 crystallizes in a K2NiF4-type structure (I4/mmm, a = 4.030(1) angstrom and c = 13.333(4) angstrom) which includes IrO2 square planer lattices with straight Ir-O-Ir bonds. The minimal Mott-gap size is similar to 70 meV. The magnetic ground state is antiferromagnetic long-range order (T-N similar to 240 K) without spin canting. The magnetic moment (vertical bar mu vertical bar similar to 0.34 mu(B)/Ir) is much reduced by a low-dimensional quantum spin fluctuation with a large intra- plane correlation vertical bar J vertical bar. The critical exponent (beta similar to 0.18) suggests that the magnetic state is classified into 2-D X-Y or (anisotropic) Heisenberg spin systems with weak 3-D interlayer coupling vertical bar J'vertical bar.

    DOI: 10.1088/1742-6596/400/3/032028

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    その他リンク: http://orcid.org/0000-0002-4747-1980

  • Superconducting state in the ternary stannide R3Co4Sn13 (R = Ca, La) 査読

    N. Kase, H. Hayamizu, K. Inoue, J. Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   471 ( 21-22 )   711 - 713   2011年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We observed the superconducting and normal state by using the single crystals of R-3 Co4Sn13 (R = Ca, La). The compounds crystallized in a Yb3Rh4Sn13-type structure with a cubic space group of Pm (3) over barn (No. 223). Superconducting state of R3Co4Sn13 was investigated by magnetic susceptibility, electrical resistivity, and specific heat measurements. From the electrical resistivity measurements under several magnetic fields, the upper critical field H-c2(0) was estimated. From the results, we discovered that the H-c2(T) curves cannot be described as the Wertharmer-Helfand-Hohenberg (WHH) prediction. Specific heat measurements indicated that these compounds were typical BCS superconductors, and categorized as strong-coupling superconductors. (C) 2011 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physc.2011.05.034

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  • Superconductivity in W5SiB2 with the T-2-phase structure studied by the specific heat measurement and band structure calculation 査読

    M. Fukuma, K. Kawashima, M. Maruyama, J. Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   471 ( 21-22 )   714 - 716   2011年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    The superconducting state of W5SiB2 with the T-2-phase structure has been investigated by a specific heat measurement and a density of state calculation. The estimated Delta C-p/gamma T-c and 2 Delta(0)/k(B)T(c) values are 1.49 and 3.32, which are close to 1.43 and 3.53 within the weak coupling regime. The electronic specific heat data clearly indicates the absence of gap nodes in the superconducting order parameter, suggesting an isotropic s-wave superconductor. From the density of state calculations, we found that W d-orbital plays an important role for the superconductivity in W5SiB2. (C) 2011 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physc.2011.05.035

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  • Superconductivity in heavily B-doped diamond layers deposited on highly oriented diamond films 査読

    Takahiro Muranaka, Koji Kobashi, Hirotaka Okabe, Takeshi Tachibana, Yoshihiro Yokota, Kazushi Hayashi, Nobuyuki Kawakami, Jun Akimitsu

    Diamond and Related Materials   20 ( 9 )   1273 - 1281   2011年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We deposited a [100]-oriented B-doped diamond layer by three methods to clarify the effects of film morphology on the transition from metallic to superconducting diamond. Heavily B-doped [100]-oriented diamond layers were deposited on [first method] undoped polycrystalline diamond films with [111] faces, [second method] highly oriented undoped diamond (HOD) thin films with a pyramidal surface, and [third method] thick undoped HOD films with a pyramidal surface. We confirmed that the B-doped layer in the third method was oriented in the [100] direction by scanning electron microscopy (SEM). The highest transition temperatures were Tc(onset) = 5.0 K and T c(zero) = 3.1 K for the B-doped layer deposited on a thick HOD film with a pyramidal surface under a zero magnetic field. By contrast, T c(onset) was 4.1 K for a heavily B-doped diamond layer deposited on a thin HOD film with a pyramidal surface, and was 3.9 K for a heavily B-doped diamond layer deposited on an undoped polycrystalline diamond film. These differences in Tc for our samples are affected by disorder and effective hole-carrier doping in each sample. Using the third method, we successfully deposited a high-quality B-doped [100] layer in three steps: (first step) depositing a [100] HOD film on a Si [100] substrate, (second step) depositing an HOD film with a pyramidal surface, and (third step) depositing a [100]-oriented B-doped layer. The change in the electronic states due to the B-doping of diamond films and the film morphology were investigated by x-ray photoelectron spectroscopy (XPS) measurements and band calculations. © 2011 Elsevier B.V.

    DOI: 10.1016/j.diamond.2011.06.011

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  • Pressure-induced metal-insulator transition in the spin-orbit Mott insulator Ba2IrO4 査読

    H. Okabe, N. Takeshita, M. Isobe, E. Takayama-Muromachi, T. Muranaka, J. Akimitsu

    PHYSICAL REVIEW B   84 ( 11 )   2011年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    Electronic transport properties in the spin-orbit Mott insulator Ba2IrO4 were studied under high pressure (P), up to 15 GPa. Resistivity measurements revealed a metal-insulator (M-I) transition at 13.8 GPa in Ba2IrO4. In the insulating state (P &lt; 13.8 GPa), the resistivity well follows the variable-range-hopping regime, suggesting that the carrier conduction is dominated by Anderson localization. In the metallic state (P &gt;= 13.8 GPa), non-Fermi-liquid behavior was observed at low temperature. Ba2IrO4 exhibits an unconventional critical exponent (delta = 1-2) at the M-I transition. These results suggest that Ba2IrO4 has an unusual electronic state affected by the "marginal quantum critical point (MQCP)."

    DOI: 10.1103/PhysRevB.84.115127

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  • Superconducting properties of Ca3T4Sn13 (T = Co, Rh, and Ir) 査読

    Hiroki Hayamizu, Naoki Kase, Jun Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   80   2011年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We present the results of the superconducting properties of R3T4Sn13 (R = La, Sr, and Ca; T = Co, Rh, and Ir) with quasi skutterudite structure by means of magnetic susceptibility, electrical resistivity, and specific heat measurements. It is shown that the superconductivity is of conventional s-wave type and lies in the extremely strong-coupling regime. Electron-phonon coupling constants lambda(ep) were estimated to be almost 1 for R3T4Sn13, respectively. The result can be categorized as a strong-coupling superconductor; this categorization is in good agreement with the results of specific heat measurements. The lambda(ep) and the electronic density of states at the Fermi level (N(E-F)) increases with the superconducting transition temperature. However, superconducting transition temperature of Co-compound cannot be explained by the difference of N(E-F). From these results, we concluded that T-c of R3T4Sn13 can be described by the strength of the lambda(ep).

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  • Guest Molecular Dynamics in Superconducting Clathrates Ag6O8AgX (X = NO3 and HF2): NMR Investigation 査読

    Fumiya Kanetake, Hirofumi Naruwa, Hidekazu Mukuda, Yoshio Kitaoka, Kenji Kawashima, Masaru Ishii, Jun Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   80 ( 7 )   2011年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We report a microscopic investigation of the superconducting clathrates Ag6O8AgX (X = NO3 and HF2), which are composed of Ag6O8 polyhedral cages and a guest molecule X inside the cages. Site-selective NMR measurements have revealed that the trigonal planar NO3 molecule in Ag6O8AgNO3 is confined in the (001) plane in a low-temperature tetragonal phase, while the uniaxial HF2 molecule in Ag6O8AgHF2 is preferentially directed along the c-axis in a low-temperature phase. These results point to the fact that the distortion of the cages takes place so as to minimize the entropy of the molecular motions of the guest ions inside the cages as the cage volume contracts thermally upon cooling. Note that the difference in T-c between these compounds may be primarily ascribed to a difference in their Debye frequencies, but neither to anharmonic vibrations of the guest molecules called rattling nor to other possible remaining degrees of freedom in the molecular dynamics.

    DOI: 10.1143/JPSJ.80.074706

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  • Highly Anisotropic Gap Function in a Nonmagnetic Superconductor Y5Rh6Sn18 査読

    Naoki Kase, Keigo Inoue, Hiroki Hayamizu, Jun Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   80   2011年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    The low temperature specific heat of R5Rh6Sn18 (R = Y, Lu) was measured under various magnetic fields (H) up to 8 T. In a zero field, electronic specific heat of Y5Rh6Sn18 shows a T-3 dependence, indicating an anisotropic superconducting gap, although that of Lu5Rh6Sn18 shows an exponential dependence. In addition, the coefficient of the T-linear term gamma(H) of Y5Rh6Sn18 in specific heat in the mixed state is found to obey a square root dependence. This is another evidence that Y5Rh6Sn18 has the anisotropic superconducting gap. On the other hand, the gamma(H) of Lu5Rh6Sn18 shows linear dependence, which means the isotropic superconducting gap.

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  • Superconducting state in the ternary stannide superconductors R 3T4Sn13 (R = La, Sr; T = Rh, Ir) with a quasiskutterudite structure 査読

    N. Kase, H. Hayamizu, J. Akimitsu

    Physical Review B - Condensed Matter and Materials Physics   83 ( 18 )   2011年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    Single crystals of R3T4Sn13 (R = La, Sr
    T = Rh, Ir) are synthesized by a Sn-flux method. Superconducting transition temperatures Tc are determined on the basis of magnetic susceptibility and electrical resistivity measurements. For La3T 4Sn13, Tc = 3.1 and 2.5 K when T = Rh and Ir, respectively
    similarly, for Sr3T4Sn13, T c = 4.2 and 5.0 K. Specific heat measurements indicate that superconducting gaps of these compounds are isotropic and that these superconductors can be categorized as strong-coupling superconductors on the basis of the values of ΔCe/γTc and 2Δ(0)/kBTc. The upper critical field shows a clear enhancement with respect to the Werthamer-Helfand-Hohenberg prediction
    this enhancement is consistent with the presence of a strong electron-phonon coupling. We summarized the normal and superconducting parameters of R 3T4Sn13 (R = La, Sr
    T = Rh, Ir) in Table. © 2011 American Physical Society.

    DOI: 10.1103/PhysRevB.83.184509

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  • Large upper critical field in non-centrosymmetric superconductor Y2C3 査読

    H. Q. Yuan, J. Chen, J. Singleton, S. Akutagawa, J. Akimitsu

    JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS   72 ( 5 )   577 - 579   2011年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PERGAMON-ELSEVIER SCIENCE LTD  

    We determine the upper critical field mu H-0(c2)(T-c) of non-centrosymmetric superconductor Y2C3 using two distinct methods: the bulk magnetization M(T) and the tunnel-diode oscillator (TDO) based impedance measurements. It is found that the upper critical field reaches a value of 30 Tat zero temperature which is above the weak-coupling Pauli paramagnetic limit. We argue that the observation of such a large mu H-0(c2)(0) in Y2C3 could be attributed to the admixture of spin-singlet and spin-triplet pairing states as a result of broken inversion symmetry. (C) 2010 Elsevier Ltd. All rights reserved.

    DOI: 10.1016/j.jpcs.2010.10.072

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  • Superconductivity in MgB2 査読

    Takahiro Muranaka, Jun Akimitsu

    Zeitschrift fur Kristallographie   226 ( 4 )   385 - 394   2011年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We review superconductivity in MgB2 in terms of crystal and electronic structure, electron-phonon coupling, two-gap superconductivity and application. Finally, we introduce the development of new superconducting materials in related compounds. © by Oldenbourg Wissenschaftsverlag, München.

    DOI: 10.1524/zkri.2011.1384

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  • Ba2IrO4: A spin-orbit Mott insulating quasi-two-dimensional antiferromagnet 査読

    H. Okabe, M. Isobe, E. Takayama-Muromachi, A. Koda, S. Takeshita, M. Hiraishi, M. Miyazaki, R. Kadono, Y. Miyake, J. Akimitsu

    PHYSICAL REVIEW B   83 ( 15 )   2011年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    A layered iridate Ba2IrO4 was synthesized using a high-pressure synthesis technique. Its electronic state was studied through electrical resistivity, magnetic susceptibility, and mu SR experiments. It was found that Ba2IrO4 crystallizes in a K2NiF4-type structure (space group I4/mmm) with lattice parameters a = 4.030(1) angstrom and c = 13.333(4) angstrom. The structure includes flat IrO2 square planar lattices with straight Ir-O-Ir bonds. Ba2IrO4 is a Mott insulator (activation energy Delta E-a similar to 0.07 eV) driven by a spin-orbit interaction. The magnetic susceptibility and mu SR studies revealed that the magnetic ground state is antiferromagnetic long-range order (T-N similar to 240 K) in which the magnetic moment (similar to 0.34 mu(B)/Ir atom) is significantly reduced by a low-dimensional quantum spin fluctuation with a large intraplane correlation vertical bar J vertical bar. The behavior is similar to that in parent materials of high-T-C cuprate superconductors such as La2CuO4.

    DOI: 10.1103/PhysRevB.83.155118

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    その他リンク: http://orcid.org/0000-0002-4747-1980

  • Evidence of nodal gap structure in the noncentrosymmetric superconductor Y2C3 査読

    J. Chen, M. B. Salamon, S. Akutagawa, J. Akimitsu, J. Singleton, J. L. Zhang, L. Jiao, H. Q. Yuan

    PHYSICAL REVIEW B   83 ( 14 )   2011年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    The magnetic penetration depth lambda(T) and the upper critical field mu H-0(c2)(T-c) of the non-centrosymmetric superconductor Y2C3 have been measured using a tunnel-diode-based resonant oscillation technique. We found that the penetration depth lambda(T) and its corresponding superfluid density rho(s)(T) show linear temperature dependence at very low temperatures (T &lt;&lt; Tc), indicating the existence of line nodes in the superconducting energy gap. Moreover, the upper critical field mu H-0(c2)(T-c) presents a weak upturn at low temperatures with a rather high value of mu H-0(c2)(0) similar or equal to 29 T, which slightly exceeds the weak-coupling Pauli limit. We discuss the possible origins for these nontrivial superconducting properties, and argue that the nodal gap structure in Y2C3 is likely attributed to the absence of inversion symmetry, which allows the admixture of spin-singlet and spin-triplet pairing states.

    DOI: 10.1103/PhysRevB.83.144529

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  • Superconductivity in W5SiB2 with the T-2 Phase Structure 査読

    Michinori Fukuma, Kenji Kawashima, Masaki Maruyama, Jun Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   80 ( 2 )   2011年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We report here the superconducting properties of W5SiB2. W5SiB2 has a T-2 phase structure with the space group I4/mcm. Magnetic susceptibility, electrical resistivity, and specific heat data show that W5SiB2 is a superconductor with a transition temperature (T-c) of 5.8 K. The magnetization versus magnetic field (M-H) curve shows the typical type-II superconducting behavior. The estimated lower critical field H-c1(0) and upper critical field H-c2(0) are about 120 and 5200 Oe, respectively. The penetration depth lambda and coherence length xi are calculated to be approximately 210 and 25 nm, respectively, using Ginzburg-Landau (GL) equations. The estimated Sommerfeld coefficient gamma(N) is 12.8 mJ/(mol K-2). The normalized specific heat jump Delta C/gamma T-N(c) is estimated to be about 1.49, which is near the value predicted by the BCS theory in the weak-coupling limit, i.e., Delta C/gamma T-N(c) = 1.43. This finding indicates that superconductivity in W5SiB2 exists within the week-coupling regime.

    DOI: 10.1143/JPSJ.80.024702

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  • Superconducting state in the ternary germanide Y2AlGe 3 with related ThCr2Si2-type structure 査読

    Naoki Kase, Hironaga Sasaki, Hiroki Hayamizu, Jun Akimitsu

    Journal of the Physical Society of Japan   80 ( 1 )   2011年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    Superconductivity in a ternary germanide, Y2AlGe3, was revealed at the transition temperature of 4.5 K by electrical resistivity and magnetic susceptibility measurements. The magnetization versus magnetic field (M-H) curve indicated that Y2AlGe3 was a type-II superconductor. The lower critical field Hc1 (0) and the upper critical field Hc2(0) were estimated to be 1.66(8) mT and 1.26(15) T. The coherence length ξ(0)GL, penetration depth λ(0) GL, and Ginzburg-Landau (GL) parameter κ(0) obtained from the critical fields were 16.2(5) nm, 631(16) nm, and 38(3), respectively. Specific heat measurements revealed that Y2AlGe3 was a weak-coupling BCS superconductor with an isotropic superconducting gap. The electron-phonon mass enhancement parameter λγ was calculated to be 0.13, which was in good agreement with the weak-coupling limit inferred from the specific heat ΔCe=γTc and 2Δ(0)=kBTc. © 2011 The Physical Society of Japan.

    DOI: 10.1143/JPSJ.80.014716

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  • Stability of hexamer-type spin excitations in the frustrated spinel Mg 1-xCr2O4-x 査読

    Yusuke Kousaka, Keisuke Tomiyasu, Toshio Yokobori, Kazumasa Horigane, Haruhiro Hiraka, Kazuyoshi Yamada, Jun Akimitsu

    Journal of Physics: Conference Series   320   2011年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:Institute of Physics Publishing  

    We performed magnetic susceptibility and inelastic neutron scattering measurements on the frustrated spinel Mg1-xCr2O 4-x (0.02 ≤ x ≤ 0.11). Susceptibility shows that Curie-Weiss temperature is almost independent of x, meaning that exchange interactions sustain. Inelastic neutron scattering reveals that the spin-hexamer excitations also persist with almost the same energy around 4 meV in a lowest temperature phase, whereas the resonance width increases with increasing x. These results suggest that nearest-neighboring Cr⋯Cr direct exchange predominantly forms the hexamers regardless of lattice defect and superexchange via Mg and O sites. © 2011 Published under licence by IOP Publishing Ltd.

    DOI: 10.1088/1742-6596/320/1/012040

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  • Neutron scattering study of spiral-type spin correlations in the frustrated spinel Mn0.07Mg0.93Cr2O4 査読

    T. Yokobori, K. Tomiyasu, Y. Kousaka, H. Matsui, H. Hiraka, K. Iwasa, K. Yamada, J. Akimitsu

    INTERNATIONAL CONFERENCE ON FRUSTRATION IN CONDENSED MATTER (ICFCM)   320   2011年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:IOP PUBLISHING LTD  

    We studied spiral-type spin correlations in Mn0.07Mg0.93Cr2O4 by single-crystalline neutron scattering. The correlation lengths are evaluated to be 12.7 angstrom and 4.4 angstrom along parallel and perpendicular to the propagation vector, respectively. The spiral rotational plane is estimated to be the proper-screw type rather than the cycloid-screw one. This fact suggests that Mn moments stabilize the cycloidal structure, generating the ferroelectricity in the parent material MnCr2O4.

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  • NMR study of pyrochlore lattice antiferromagnet, melanothallite Cu 2OCl2 査読

    Masahide Nishiyama, Akira Oyamada, Tetsuaki Itou, Satoru Maegawa, Hirotaka Okabe, Jun Akimitsu

    Journal of Physics: Conference Series   320   2011年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:Institute of Physics Publishing  

    The melanothallite Cu2OCl2 is a new example of pyrochlore-like antiferromagnet, which is composed of 3d transition metal electrons. We performed Cu- and Cl-NMR experiments on powder samples of Cu 2OCl2 below transition temperature TN = 70 K and we observed six resonant peaks of Cu nuclei, which are composed of three symmetric peaks corresponding to 63Cu and three corresponding to 65Cu. The Cu nuclei feel the strong hyperfine fields because of ordered magnetic moments and the electric field gradients. We determined the spin structure by analyzing the Cu-NMR spectra. The melanothallite has an all-in-all-out spin structure. The spin lattice relaxation rates T 1 -1 of Cu- and Cl-NMR in the ordered phase are proportional to the temperature
    This suggests that although long-range ordering occurs at rather high temperature, the large spin fluctuations caused by the geometrical frustration still remain. © 2011 Published under licence by IOP Publishing Ltd.

    DOI: 10.1088/1742-6596/320/1/012030

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  • Substituted effects of superconducting and magnetic states in reentrant superconductor Tm5Rh6Sn18 査読

    N. Kase, K. Wakui, H. Hayamizu, J. Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   470   S539 - S540   2010年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Substituted effects on superconducting and magnetic states of Tm5Rh6Sn18 were examined by doping Y or Co element. In Y-20% doped material, superconductivity was destroyed above 3.0 kOe with developing magnetic ordering. After destroying superconductivity, electrical resistivity decreased with decreasing temperature again. In addition, magnetic field, which was possible to destroy superconductivity, was higher than that of parent compound. In Co-20% doped material, the magnetic field, destroying the superconductivity, was weaker than that of parent compound. From these results, we concluded that Tm-site was responsible for magnetic properties and Rh-site played a main role for superconductivity. (C) 2010 Elsevier B.V. All rights reserved.

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  • Single composite crystal structure analysis of incommensurate spin-ladder compound Sr2.5Ca11.5Cu24O41 査読

    Y. Gotoh, H. Fujihisa, S. Takeya, H. Yamawaki, J. Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   470   S219 - S220   2010年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Single-crystal X-ray structure analysis of mutually incommensurate (Sr0.36Ca1.64Cu2O3)(0.706)CuO2, "Sr2.5Ca11.5Cu24O41" has been performed on the basis of superspace group approach. The lattice constants of (Sr0.36Ca1.64Cu2O3)(0.706)CuO2 are a = 1.1298(1), b = 1.2582(1), c(1) = 0.2750(1) nm for the CuO2 part and a = 1.1298(1), b = 1.2582(1), c(2) = 0.3897(1) nm for the (Sr0.36Ca1.64Cu2O3) one. From the average structure analysis, it has been expected that the formation of CuO5 with pyramidal coordination by O atoms occurs in the Cu2O3 ladder. The minimum distance between Cu atom in the Cu2O3 ladder and O atom in the CuO2 chain has clearly shown that O in the CuO2 acts as apical oxygen to form the CuO5 coordination. It has been concluded that (Sr0.36Ca1.54Cu2O3)(0.706)CuO2 bears a marked structural resemblance to the cuprate High T-c superconductors. (C) 2009 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physc.2009.11.173

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  • Thermodynamic properties in the clathrate-type silver- and copper-oxide: Ag6O8MX and Cu6O8MX (M = cation, X = anion) 査読

    K. Kawashima, H. Tozawa, M. Maruyama, R. Ichinokawa, T. Sugiyama, J. Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   470   S26 - S28   2010年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We present the thermodynamic properties in the clathrate-type silver- and copper-oxides Ag6O8MX and Cu6O8MX (M = cation, X = anion) compounds, having Ag6O8 (or Cu6O8)-cage in their crystal structure. Ag6O8MX and Cu6O8MX are crystallized in a cubic structure with a space group of Fm (3) over barm. We synthesized high quality samples of Ag6O8MX and Cu6O8MX and examined the electrical resistivity, magnetic susceptibility, and specific heat measurements. The superconductivity in Ag6O8AgX (X = NO3- and HF2- have an isotropic gap symmetry by means of the specific heat data. In Cu6O8MCl, the ground sata of Cu6O8MCl compounds changed metallic to semiconducting by changing the valence state of M site ion. (C) 2009 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physc.2009.10.127

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  • Superconducting properties of R3T4Sn13 (R = La, Sr, Ca, T = Rh, Ir) with a quasi-skutterudite structure 査読

    H. Hayamizu, N. Kase, J. Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   470   S541 - S542   2010年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We measured the superconducting and normal state by using the single crystals of R3T4Sn13 (R = La. Sr, Ca, T = Rh, Ir) The compounds crystallize in a Yb3T4Sn13-type structure with a cubic space group of Pm3-n (No. 223). Superconducting state of R3T4Sn13 was investigated by magnetic susceptibility, electrical resistivity, and specific heat measurements. From the electrical resistivity measurements under several magnetic fields, the upper critical field H-c2(0) was estimated. From the results, we discovered that the H-c2(T) curves cannot be described as the Wertharmer-Helfand-Hohenberg (WHH) prediction. Specific heat measurements indicated that these compounds were typical BCS superconductors, and categorized as strong-coupling superconductors. (C) 2010 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physc.2009.12.062

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  • Tunneling break-junction spectroscopy on the superconductor NdFeAs(O0.9F0.1) 査読

    Toshikazu Ekino, Akira Sugimoto, Hirotaka Okabe, Kazuhiro Shohara, Ryuichi Ukita, Jun Akimitsu, Alexander M. Gabovich

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   470   S358 - S359   2010年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Tunnel break-junction method has been adopted to study polycrystalline samples of iron-oxypnictide superconductor NdFeAs(O0.9F0.1) with T-c = 48K. Measurements were carried out at 4.2 K. Break-junction (BD conductance versus voltage curves showed gap-edge peaks with the peak-to-peak distances Vp-p = 4 Delta/e = 28-40 mV at 4.2 K, where 2 Delta(T) is the superconducting energy gap, e &gt; 0 is the elementary charge. This yields 2 Delta(0) = 14-20 meV, so that the gap ratio 2 Delta(0)/k(B)T(c) is about 4.1 +/- 0.7, k(B) being the Boltzmann constant. This ratio implies strong-coupling superconductivity in the framework of Bardeen-Cooper-Schrieffer theory, being, however, much smaller than that for high-T-c copper oxides. This suggests a significant difference in the pairing mechanism between those classes of materials. (C) 2009 Published by Elsevier B.V.

    DOI: 10.1016/j.physc.2009.10.079

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  • Scanning tunneling spectroscopy and break junction spectroscopy on iron-oxypnictide superconductor NdFeAs(O0.9F0.1) 査読

    A. Sugimoto, T. Ekino, R. Ukita, K. Shohara, H. Okabe, J. Akimitsu, A. M. Gabovich

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   470 ( 20 )   1070 - 1072   2010年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Iron-oxypnictide superconductor NdFeAs(O0.9F0.1) was studied using both low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) and tunnel break junction (BJ) methods. STM topography showed granular and spot structures with a typical size of several nanometers, most probably governed by fluorine atom distribution. The majority of STS conductance, C. versus voltage, V. curves revealed V-shaped structures, whereas some of G(V) dependences possessed coherent gap peaks or kinks at gap energies. At the same time, G(V) dependences obtained by the BJ technique showed clear-cut coherence peaks with peak-to-peak distances V-pp = 4 Delta/e similar to 25 mV at 4.2 K. where A is the superconducting energy gap, e &gt; 0 is the elementary charge. This yields Delta(0) = 6-7 meV, so that the ratio 2 Delta(0)/k(B)T(c) is about 3-4, k(B) being the Boltzmann constant. This value is consistent with the conventional weak-coupling s-wave Bardeen-Cooper-Schrieffer theory. (C) 2010 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physc.2010.05.038

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  • Superconducting state in YSn3 with a AuCu3-type structure 査読

    K. Kawashima, M. Maruyama, M. Fukuma, J. Akimitsu

    Physical Review B - Condensed Matter and Materials Physics   82 ( 9 )   2010年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    In this paper, the superconducting properties of YSn3 are reported. YSn3 has a cubic AuCu3 -type structure with the space group Pm 3̄ m. Magnetic susceptibility, electrical resistivity, and specific-heat data showed that YSn3 is a superconductor with a superconducting transition temperature (Tc) of 7 K. The magnetization versus magnetic field (M-H) curve shows the typical type-II superconducting behavior. The estimated lower critical field Hc1 (0) and the upper critical field Hc2 (0) are about 90 Oe and 3000 Oe, respectively. The penetration depth λ (0) and coherence length ξ (0) are calculated to be approximately 270 nm and 66 nm, respectively, using the Ginzburg-Landau equations. The estimated Sommerfeld coefficient γN is 7.57mJ/mol K2. The normalized specific heat jump ΔC/ γN Tc is estimated to be about 2.19, which is higher than the value predicted by the BCS theory in the weak-coupling limit, i.e., ΔC/ γN Tc =1.43. This fact indicates that superconductivity in YSn3 exists in the strong-coupling regime. The difference between Tc of Ln X3 (X=family of 13 or 14 elements) compounds is discussed to compare their physical properties. © 2010 The American Physical Society.

    DOI: 10.1103/PhysRevB.82.094517

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  • Pressure-induced high-T-c superconducting phase in FeSe: Correlation between anion height and T-c 査読

    H. Okabe, N. Takeshita, K. Horigane, T. Muranaka, J. Akimitsu

    PHYSICAL REVIEW B   81 ( 20 )   2010年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    In this study, we performed high-pressure electrical resistivity measurements of polycrystalline FeSe in the pressure range of 1-16.0 GPa at temperatures of 4-300 K. A precise evaluation of T-c from zero-resistivity temperatures revealed that T-c shows a slightly distorted dome-shaped curve with maximum T-c(offset) (30 K) at 6 GPa. With the application of pressure, the temperature dependence of resistivity above T-c changes dramatically to a linear dependence, that the high-T-c phase appears above 3 GPa. We found a striking correlation between T-c and the Se height: the lower the Se height, the more enhanced is T-c. Moreover, this relation is broadly applicable to other iron pnictides, strongly indicating that high-temperature superconductivity can appear only around the optimum anion height (similar to 1.38 angstrom). On the basis of these results, we suggest that the anion height should be considered as a key determining factor of T-c of iron-based superconductors containing various anions.

    DOI: 10.1103/PhysRevB.81.205119

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  • Nonlinear magnetic susceptibility measurements at GPa-level pressures 査読

    M. Mito, S. Tominaga, Y. Komorida, H. Deguchi, S. Takagi, Y. Nakao, Y. Kousaka, J. Akimitsu

    Journal of Physics: Conference Series   215   2010年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)  

    The second nonlinear susceptibility, χ(2), reflects magnetic domain formation. At zero DC field, the temperature dependence of χ(2) can be obtained through that of the third-harmonic AC susceptibility, χ3ω. Generally, the magnitude of χ3ω is at most a few percent of the linear component, χ1ω, connected with the linear susceptibility, χ(0). At GPa-level pressures generated using a diamond anvil cell, the detection of χ3ω becomes more difficult, because of the small amount of measurement sample. We propose an effective method of measuring χ3ω using a superconducting quantum interference device magnetometer and a statistics average technique. The experimental results for an inorganic chiral magnet, Cr1/3NbS2, indicate the efficiency of this technique and demonstrate that magnetic signals of the order of 10-9 emu can be detected. © 2010 IOP Publishing Ltd.

    DOI: 10.1088/1742-6596/215/1/012182

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  • AC susceptibility study of superconducting aluminum-doped silicon carbide 査読

    M. Kriener, T. Muranaka, J. Akimitsu, Y. Maeno

    Physica C: Superconductivity and its Applications   470 ( SUPPL.1 )   S602-S603   2010年

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    記述言語:英語  

    DOI: 10.1016/j.physc.2009.12.009

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  • Two distinct ferroelectric phases in the multiferroic y -type hexaferrite Ba2 Mg2 Fe12 O22 査読

    Hajime Sagayama, Kouji Taniguchi, Nobuyuki Abe, Taka-Hisa Arima, Yusaku Nishikawa, Shin-Ichiro Yano, Yusuke Kousaka, Jun Akimitsu, Masato Matsuura, Kazuma Hirota

    Physical Review B - Condensed Matter and Materials Physics   80 ( 18 )   2009年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    The magnetic phase diagram of the Y -type hexaferrite Ba2 Mg2 Fe12 O22 has been studied using single-crystal neutron diffraction. The result indicates successive phase transitions where the magnetic modulation wave number changed discontinuously when a magnetic field is applied and the temperature is varied. For the low-temperature spin-driven ferroelectric state, we have found a sixfold structure with q= (001/2) in weak magnetic fields and a twofold structure with q= (003/2) in strong magnetic fields between which a first-order transition intervenes accompanied by a hysteresis. © 2009 The American Physical Society.

    DOI: 10.1103/PhysRevB.80.180419

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  • Antiferromagnetic Kondo-Lattice System Ce2RhSi3 with Moderate Heavy-Fermion Behavior 査読

    N. Kase, T. Muranaka, J. Akimitsu

    J MAGN MAGN MATER   ( 321 )   3380 - 3383   2009年11月

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    記述言語:英語  

    DOI: 10.1016/j.jmmm.2009.06.022

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  • Spherical neutron polarimetry studies on the magnetic structure of single crystal Cr1-xMoxB2 (x=0, 0.15) 査読

    E. Kaya, Y. Kousaka, K. Kakurai, M. Takeda, J. Akimitsu

    PHYSICA B-CONDENSED MATTER   404 ( 17 )   2524 - 2526   2009年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    By applying spherical neutron polarimetry technique to Cr1-xMoxB2 (x = 0, 0.15), we successfully determined its precise magnetic structures. We emphasize here that the ratio of magnetic component was determined by measuring the two satellite reflections and rotating the incident neutron spin directions. (C) 2009 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physb.2009.06.010

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  • Unquenched Geometric Frustration Effect on Spiral Spin Correlation in Magnetically Ordered Phase 査読

    Keisuke Tomiyasu, Kazumasa Horigane, Toshio Yokobori, Yusuke Kousaka, Miwako Takahashi, Haruhiro Hiraka, Jun Akimitsu, Kazuyoshi Yamada

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   78 ( 8 )   2009年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We studied magnetic phase diagram and x-dependence of spiral spin correlation in magnetically ordered phase of (Co1-xZnx)Cr2O4 by magnetization and neutron scattering experiments. The spiral correlation survives in a wide x range from 0 to 0.9. As x increases, the correlation length decreases from xi similar to 23 angstrom at x = 0, and saturates at xi similar to 2.5 angstrom at around x = 0.75, which is close to the nearest neighboring Cr3+-Cr3+ distance. The pitch is almost constant from x = 0 to 0.75, then abruptly shortens above around x = 0.75. The correlation is isotropic at x = 0.25 but anisotropic at x = 0.75. We discuss that the spiral correlation is of short-range owing to unquenched geometric frustration, and begins to collapse with the minimization of correlation length above around x = 0.75, possibly as the precursor of dynamic spin hexamer caused by high geometric frustration.

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  • Coexistence of Superconductivity and Magnetism in the Tm-Based Reentrant Superconductor Tm5Rh6Sn18 査読

    Naoki Kase, Jun Akimitsu, Yasuyuki Ishii, Takao Suzuki, Isao Watanabe, Masanori Miyazaki, Masatoshi Hiraishi, Soshi Takeshita, Ryosuke Kadono

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   78 ( 7 )   2009年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We report here the magnetic properties of the thulium-based reentrant superconductor Tm5Rh6Sn18 (T-c = 2.2 K), which were determined by magnetization and mu SR measurements. The spontaneous oscillation signal in the mu SR spectra gradually develops under zero magnetic field below 13 K. The development of a quasi-static local magnetic field is also observed even below T-c, indicating the coexistence of magnetism and superconductivity at zero magnetic field. From the analysis of longitudinal mu SR measurements, magnetic ordering can be considered to be not truly long range but superparamagnetic.

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  • μ SR study of magnetic ground state in Mo<inf>3</inf>Sb<inf>7</inf>

    H. Okabe, J. Akimitsu, S. Takeshita, M. Miyazaki, M. Hiraishi, R. Kadono, R. Kadono

    Physica B: Condensed Matter   404   743 - 745   2009年4月

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    μ SR study on the anomalous magnetic behavior of an intermetallic superconductor Mo3Sb7is reported. Muon depolarization rate due to dynamically fluctuating electronic spins is reduced with decreasing temperature, which is in line with the development of a spin-liquid state. Meanwhile, the detailed analysis of structural deformation below 50 K suggests that the presumed spin-gap is not fully opened, and that the magnetic state just above Tcmight be understood as an itinerant paramagnetic state with localized spin-singlets. © 2008 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physb.2008.11.184

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  • Magnetic response of noncentrosymmetric superconductor La2C3: Effect of double-gap and spin-orbit interaction 査読

    R. Kadono, M. Hiraishi, M. Miyazaki, K. H. Satoh, S. Takeshita, S. Kuroiwa, S. Saura, J. Akimitsu

    PHYSICA B-CONDENSED MATTER   404 ( 5-7 )   737 - 739   2009年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    The presence of spin-orbit (SO) Interaction in a noncentrosymmetric superconductor, La2C3 (T-c similar or equal to 11 K) is demonstrated by muon spin rotation (mu SR) in its normal state, where mu SR spectra exhibit field-induced weak depolarization due to van Vleck-like local susceptibility. In the mixed state, muon spin relaxation due to inhomogeneity of internal field (sigma(v)) exhibits a field dependence that is characterized by a kink, where sigma(v) (and hence the superfluid density) is more strongly reduced at lower fields. This is perfectly in line with the presence of a secondary energy gap previously inferred from the temperature dependence of sigma(v), and also consistent with the possible influence of asymmetric deformation of the Fermi surface due to the SO interaction. (C) 2008 Elsevier B.V. All rights reserved.

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    その他リンク: http://orcid.org/0000-0002-4747-1980

  • Coexistence of superconductivity and magnetism in the Tm-based superconductor probed by muon spin relaxation 査読

    Naoki Kase, Jun Akimitsu, Yasuyuki Ishii, Takao Suzuki, Isao Watanabe, Masanori Miyazaki, Masatoshi Hiraishi, Soshi Takeshita, Ryosuke Kadono

    PHYSICA B-CONDENSED MATTER   404 ( 5-7 )   740 - 742   2009年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    The magnetic property of a thulium-based superconductor, Tm(5)Rh(6)Sn(18) (superconducting transition temperature T(c) = 2.2 K), is investigated by muon spin relaxation (mu SR). Below around 6 K, the development of a quasi-static local magnetic field is clearly inferred from the observation of spontaneous oscillation signal in the mu SR spectra under zero external field, where the magnetism persists even below T(c) The internal magnetic field at the muon site, H(mu) is estimated to be approximately 175 Oe. The fractional volume of the magnetic component is estimated to be 100%, strongly suggesting that the magnetism coexists with superconductivity. (C) 2008 Elsevier B.V. All rights reserved.

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    その他リンク: http://orcid.org/0000-0002-4747-1980

  • Superconducting State of the Binary Boride Ru7B3 with the Noncentrosymmetric Crystal Structure 査読

    Naoki Kase, Jun Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   78 ( 4 )   2009年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    Single crystals of Ru7B3 were grown by the Czochralski puling method in a tetra-arc furnace. The compound crystallizes in a Th7Fe3-type structure (hexagonal, space group: P6(3)mc), which has no inversion symmetry along the c-axis. Measurements in magentic fields indicate type-II superconductivity with upper critical fields of H-c2(0) similar or equal to 17.2 kOe for H parallel to [001] and 15.8 kOe for H parallel to [100], respectively. The coherence length xi(0)(GL), penetration depth lambda(0)(GL), and Ginzburg-Landau (GL) parameter kappa(0) obtained from these values are 144 angstrom, 3110 angstrom, and 21.6 for H parallel to [100], and 138 angstrom, 3520 angstrom, and 25.5 for H parallel to [001], respectively. Specific heat measurements reveal that Ru7B3 is a weak-coupling super-conductor with an isotropic superconducting gap.

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  • Conventional s-Wave Superconductivity in Noncentrosymmetric Ir2Ga9: 71Ga-NQR Evidence 査読

    A. Harada, N. Tamura, H. Mukuda, Y. Kitaoka, K. Wakui, S. Akutagawa, J. Akimitsu

    J. Phys. Soc. Jpn.   2009年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

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  • Hall effect in Sr14-xCaxCu24O41 査読

    E. Tafra, B. Korin-Hamzic, M. Basletic, A. Hamzic, M. Dressel, J. Akimitsu

    PHYSICA B-CONDENSED MATTER   404 ( 3-4 )   385 - 388   2009年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We present Hall effect measurements of two-leg ladder Sr14-xCaxCu24O41 (0 &lt;= x &lt;= 11.5). In these composite materials charge and spin ordering occur within chain and ladder subsystems. They have been investigated intensively during the last years because of their fascinating physical properties, especially after the discovery of superconductivity under pressure (for x&gt;10). They are the first superconducting copper oxides (superconductivity occurs under pressure for x&gt;10) without the CuO2 planes, characteristic for high-temperature superconductors. The question of charge dynamics and hole distribution between the ladders and chains is still open. Our Hall effect results show typical semiconducting behavior for x&lt;10. The x = 11.5 compound shows metallic behavior and our Hall effect results match qualitatively the behavior found in high-T-c cuprates. We have determined the effective number of carriers and its dependence on composition (x) and compared it to the number of holes in the ladders obtained by other experimental techniques at room temperature. (C) 2008 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physb.2008.11.020

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  • Charge and spin ordering in La2-xSrxCoO4 (0.4&lt;x&lt;0.6) 査読

    Kazumasa Horigane, Toru Uchida, Haruhiro Hiraka, Kazuyoshi Yamada, Jun Akimitsu

    Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment   600 ( 1 )   243 - 245   2009年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    Neutron scattering experiments were performed on the single crystals of layered-perovskite cobaltate La2-xSrxCoO4. For a commensurate value of x=0.5, magnetic reflections due to high-spin (HS) state of Co2+ are observed in (H, 0, L) plane. Further, in (H, H, L) plane, we found super-lattice reflections with wave vector qm=(1/4, 1/4, 1). The latter magnetic peaks cannot be understood by the Co2+ spin configurations, indicating that these peaks are originated from Co3+ spin ordering. From the hole-doping dependence, short ranged charge and spin orderings were observed below x=0.5, while those of peaks were suppressed at x=0.6. Our results suggest that double exchange interaction between the HS state of Co2+ and the intermediate spin (IS) state of Co3+ mediate the IS state in this system. © 2008 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.nima.2008.11.141

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  • Chiral helimagnetism in T1/3NbS2 (T=Cr and Mn) 査読

    Y. Kousaka, Y. Nakao, J. Kishine, M. Akita, K. Inoue, J. Akimitsu

    Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment   600 ( 1 )   250 - 253   2009年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We report commensurate-to-incommensurate (C-IC) magnetic ordering in T1/3NbS2 (T=Cr and Mn), of which space group is P6322. We measured the anisotropic magnetization under magnetic fields applied perpendicular and parallel to the hexagonal c-axis. We observed a magnetization anomaly, only when the magnetic field was applied perpendicular to the hexagonal c-axis. The experimental results were in good agreement with the theoretical prediction based on a lattice chiral XY model, which describes the C-IC transition as chiral soliton formation. Therefore, we propose that the magnetic ordering in an incommensurate phase of T1/3NbS2 (T=Cr and Mn) can be chiral helimagnetic. © 2008 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.nima.2008.11.040

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  • Full Gap Superconductivity in Ba0.6K0.4Fe2As2 Probed by Muon Spin Rotation 査読

    Masatoshi Hiraishi, Ryosuke Kadono, Soshi Takeshita, Masanori Miyazaki, Akihiro Koda, Hirotaka Okabe, Jun Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   78 ( 2 )   2009年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    Superfluid density (ns) in the mixed state of an iron pnictide superconductor Ba1-xKxFe2As2 is determined by muon spin rotation for a sample with optimal doping (x = 0.4). The temperature dependence of it, is perfectly reproduced using the conventional BCS model for s-wave paring, where the order parameter can be either it single-gap structure with Delta = 8.35(6) meV [2 Delta/k(B)T(c) = 5.09(4)] or double-gap structure with Delta(1) - 12 meV (fixed) [2 Delta(1)/k(B)T(c) - 7.3] and Delta(2) = 6.8(3) meV [2 Delta(2)/k(B)T(c) = 4.1(2)]. The latter is consistent with the recent result obtained by the angle-resolved photoemission spectroscopy. The large gap parameters (2 Delta/k(B)T(c)) indicate the extremely Strong coupling of carriers to bosons that mediate the Cooper pairing.

    DOI: 10.1143/JPSJ.78.023710

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    その他リンク: http://orcid.org/0000-0002-4747-1980

  • Magnetic and structural phase transitions in Mo<inf>3</inf>Sb<inf>7</inf>

    H. Okabe, S. Yano, T. Muranaka, J. Akimitsu

    Journal of Physics: Conference Series   150   2009年1月

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    We performed the X-ray diffraction measurement to clarify the crystal structure of the intermetallic superconductor Mo3Sb7at low temperature. Mo3Sb7undergoes a symmetry-lowering structural transition from cubic to tetragonal lattice at 50K, which is strongly associated with an opening of a spin-gap. we identified the detailed structure of tetragonal phase using Rietveld refinement, and revealed that the Mo-Mo pair couples to form a dimer only in z direction, the two thirds of Mo tends to remain relatively isolated. This result indicates that the magnetic ground state of Mo3Sb7could be identified as a typical metallic state with localized spin-singlets. © 2009 IOP Publishing Ltd.

    DOI: 10.1088/1742-6596/150/5/052196

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  • Superconductivity in transition metal-silicide W(5)Si(3) 査読

    K. Kawashima, T. Muranaka, Y. Kousaka, S. Akutagawa, J. Akimitsu

    25TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT25), PART 5   150   2009年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:IOP PUBLISHING LTD  

    We report here the superconducting properties of the tungsten silicide W(5)Si(3). Electrical resistivity and magnetic susceptibility measurements as functions of temperature and magnetic field show a bulk superconductivity with T(c) = 2.7 K. The magnetization versus magnetic field (M - H) curve shows a typical type-II superconducting behavior. Furthermore, we present the H - T phase diagram and estimate the upper critical field H(c2)(0) to be about 5200 Oe. We also discuss the superconducting properties in a Ginzburg-Landau framework and estimate the lower critical magnetic field H(c1)(T), penetration depth lambda, and coherence length xi to be about 33 Oe, 450 nm, and 25 nm, respectively.

    DOI: 10.1088/1742-6596/150/5/052106

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  • Superconductivity of hexagonal heavily-boron doped silicon carbide 査読

    M. Kriener, T. Muranaka, Z-A Ren, J. Kato, J. Akimitsu, Y. Maeno

    25TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT25), PART 5   150   2009年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:IOP PUBLISHING LTD  

    In 2004 the discovery of superconductivity in heavily boron-doped diamond (C: B) led to an increasing interest in the superconducting phases of wide-gap semiconductors. Subsequently superconductivity was found in heavily boron-doped cubic silicon (Si:B) and recently in the stochiometric "mixture" of heavily boron-doped silicon carbide (SiC:B). The latter system surprisingly exhibits type-I superconductivity in contrast to the typeII superconductors C:B and Si:B. Here we will focus on the specific heat of two different superconducting samples of boron-doped SiC. One of them contains cubic and hexagonal SiC whereas the other consists mainly of hexagonal SiC without any detectable cubic phase fraction. The electronic specific heat in the superconducting state of both samples SiC: B can be described by either assuming a BCS-type exponentional temperature dependence or a power-law behavior.

    DOI: 10.1088/1742-6596/150/5/052130

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  • Momentum-resolved charge excitations in high-Tc cuprates studied by resonant inelastic X-ray scattering 査読

    K. Ishii, M. Hoesch, T. Inami, K. Kuzushita, K. Ohwada, M. Tsubota, Y. Murakami, J. Mizuki, Y. Endoh, K. Tsutsui, T. Tohyama, S. Maekawa, K. Yamada, T. Masui, S. Tajima, H. Kawashima, J. Akimitsu

    Journal of Physics and Chemistry of Solids   69 ( 12 )   3118 - 3124   2008年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We report a Cu K-edge resonant inelastic X-ray scattering (RIXS) study of high-Tc cuprates. Momentum-resolved charge excitations in the CuO2 plane are examined from parent Mott insulators to carrier-doped superconductors. The Mott gap excitation in undoped insulators is found to commonly show a larger dispersion along the [π, π] direction than the [π, 0] direction. On the other hand, the resonance condition displays material dependence. The Mott gap persists in carrier-doped states. Upon hole doping, the dispersion of the Mott gap excitation becomes weaker associated with the reduction of antiferromagnetic correlation and an intraband excitation appears as a continuum intensity below the gap at the same time. In the case of electron doping, the Mott gap excitation is prominent at the zone center and a dispersive intraband excitation is observed at finite momentum transfer. © 2008 Elsevier Ltd. All rights reserved.

    DOI: 10.1016/j.jpcs.2008.06.041

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  • Muon spin relaxation study of LaTiO3 and YTiO3 査読

    P. J. Baker, T. Lancaster, S. J. Blundell, W. Hayes, F. L. Pratt, M. Itoh, S. Kuroiwa, J. Akimitsu

    Journal of Physics Condensed Matter   20 ( 46 )   2008年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We report muon spin relaxation (μSR) measurements on two Ti3+ containing perovskites, LaTiO3 and YTiO3, which display long-range magnetic order at low temperature. For both materials, oscillations in the time dependence of the muon polarization are observed which are consistent with three-dimensional magnetic order. From our data we identify two magnetically inequivalent muon stopping sites. The μSR results are compared with the magnetic structures of these compounds previously derived from neutron diffraction and μSR studies on structurally similar compounds. © 2008 IOP Publishing Ltd.

    DOI: 10.1088/0953-8984/20/46/465203

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  • Anomalous phonon properties in the silicide superconductors CaAlSi and SrAlSi 査読

    S. Kuroiwa, T. Hasegawa, T. Kondo, N. Ogita, M. Udagawa, J. Akimitsu

    PHYSICAL REVIEW B   78 ( 18 )   2008年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    Lattice-dynamical properties of CaAlSi and SrAlSi with a similar layer structure to MgB(2) have been first investigated by both Raman-scattering and ab initio calculations. All Raman-active phonons with E&apos; symmetry have been clearly observed for both compounds. Their line shapes are asymmetric but their linewidths are similar to 10 cm(-1), which is very narrower than that of MgB(2). In addition to the Raman-active modes, several extra peaks have been observed below 160 cm(-1). These low-energy extra modes can be assigned to the out-of-plane vibrations of Al perpendicular to Al-Si basal plane. Since these peak intensities are strongly affected by the incident energy (resonance Raman process), the electronic state is important for them. Moreover, in both crystals of CaAlSi and SrAlSi, we point out the energy difference for the different propagation directions along the c axis and the c plane, in spite of the very close wave vector to the Brillouin-zone center. This energy difference cannot be explained by a usual Raman-scattering scenario at this stage.

    DOI: 10.1103/PhysRevB.78.184303

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  • Crossover in charge transport from one-dimensional copper-oxygen chains to two-dimensional ladders in (La,Y)(y)(Sr,Ca)(14-y)Cu(24)O(41) 査読

    T. Ivek, T. Vuletic, B. Korin-Hamzic, O. Milat, S. Tomic, B. Gorshunov, M. Dressel, J. Akimitsu, Y. Sugiyama, C. Hess, B. Buechner

    PHYSICAL REVIEW B   78 ( 20 )   2008年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    The charge transport in the copper-oxygen chain/ladder layers of (La,Y)(y)(Sr,Ca)(14-y)Cu(24)O(41) is investigated along two crystallographic directions in the temperature range from 50 to 700 K and for doping levels from y approximate to 6 (number of holes n(h)&lt; 1) to y=0 (number of holes n(h)=6). A crossover from a one-dimensional hopping transport along the chains for y &gt;= 3 to a quasi-two-dimensional charge conduction in the ladder planes for y less than or similar to 2 is observed. This is attributed to a partial hole transfer from chains to ladders when the hole doping exceeds n(h)approximate to 4 and approaches fully doped value n(h)=6. For y less than or similar to 2 a weak dielectric relaxation at radio frequencies and a microwave mode are detected, which might be recognized as signatures of a charge-density wave phase developed at short length scales in the ladder planes.

    DOI: 10.1103/PhysRevB.78.205105

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  • Significant softening of c66 and hidden order parameter in the magnetic ordered state of La2-xCaxCoO4 (0.4≤x≤0.7) 査読

    K. Horigane, T. Kobayashi, M. Suzuki, K. Abe, K. Asai, J. Akimitsu

    Physical Review B - Condensed Matter and Materials Physics   78 ( 14 )   2008年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We have measured the longitudinal and transverse sound velocities of La2-xCaxCoO4 (0.4≤x≤0.7) propagating along [100], [110], and [001] from 200 K down to 3 K with a frequency of 10 MHz. It was found that c66 shows a remarkable softening below Co 2+ spin-ordering temperature TN in the whole doping range, while c11-c12/2 exhibits a clear hardening below Co 3+ spin-ordering temperature TN2 only when x=0.5. To explain the observed anomaly, the Landau free energy for the La 2-xCaxCoO4 system has been constructed, including the coupling energy between elastic strains and a two-dimensional order parameter (Q1, Q2), having a spatial modulation along [110] or [1̄ 10]. We argued that this represents orbital order, and it is this order that leads to the softening of c66. © 2008 The American Physical Society.

    DOI: 10.1103/PhysRevB.78.144108

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  • Influence of doping on the Hall coefficient in Sr(14-x)Ca(x)Cu(24)O(41) 査読

    E. Tafra, B. Korin-Hamzic, M. Basletic, A. Hamzic, M. Dressel, J. Akimitsu

    PHYSICAL REVIEW B   78 ( 15 )   2008年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    We present Hall-effect measurements of two-leg ladder compounds Sr(14-x)Ca(x)Cu(24)O(41) (0 &lt;= x &lt;= 11.5) with the aim to determine the number of carriers participating in dc transport. Distribution of holes between the ladder and chain subsystems is one of the crucial questions important for understanding the physics of these compounds. Our Hall-effect and resistivity measurements show typical semiconducting behavior for x &lt; 11.5. However, for x=11.5, the results are completely different, and the Hall coefficient and resistivity behavior are qualitatively similar to that of high-temperature copper-oxide superconductors. We have determined the effective number of carriers at room temperature and compared it to the number of holes in the ladders obtained by other experimental techniques. We propose that going from x=0 to x=11.5 less than 1 hole per f.u. is added to the ladders and is responsible for a pronounced change in resistivity with Ca doping.

    DOI: 10.1103/PhysRevB.78.155122

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  • Synthesis and magnetic properties of conjugated radical polymer 査読

    Daisuke Kato, Katsuya Inoue, Jun Akimitsu, Jiro Abe

    Chemistry Letters   37 ( 7 )   694 - 695   2008年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    An unprecedented conjugated radical polymer, poly-BDPI-2Y, consisting of imidazolyl radical was prepared and its magnetic properties were investigated. The SQUID measurement proves the presence of the ferromagnetic fraction at 300 K. Copyright © 2008 The Chemical Society of Japan.

    DOI: 10.1246/cl.2008.694

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  • Specific heat and electronic states of superconducting boron-doped silicon carbide 査読

    M. Kriener, Y. Maeno, T. Oguchi, Z. -A. Ren, J. Kato, T. Muranaka, J. Akimitsu

    PHYSICAL REVIEW B   78 ( 2 )   2008年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    The discoveries of superconductivity in the heavily-boron doped semiconductors diamond (C:B) in 2004 [Ekimov , Nature (London) 428, 542 (2004)] and silicon (Si:B) in 2006 [Bustarret , Nature (London) 444, 465 (2006)] have renewed the interest in the physics of the superconducting state of doped semiconductors. Recently, we discovered superconductivity in the closely related "mixed" system heavily boron-doped silcon carbide (SiC:B) [Ren , J. Phys. Soc. Jpn. 76, 103710 (2007)]. Interestingly, the latter compound is a type-I superconductor whereas the two aforementioned materials are type II. In this paper, we present an extensive analysis of our recent specific-heat study, as well as the band structure and expected Fermi surfaces. We observe an apparent quadratic temperature dependence of the electronic specific heat in the superconducting state. Possible reasons are a nodal gap structure or a residual density of states due to nonsuperconducting parts of the sample. The basic superconducting parameters are estimated in a Ginzburg-Landau framework. We compare and discuss our results with those reported for C:B and Si:B. Finally, we comment on possible origins of the difference in the superconductivity of SiC:B compared to the two "parent" materials C:B and Si:B.

    DOI: 10.1103/PhysRevB.78.024517

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  • Quasiparticle electronic structure of a new surperconductor, Y2C3, in the mixed state investigated by specific heat and flux-flow resistivity 査読

    Akutagawa Satoshi, Ohashi Takeyoshi, Kitano Haruhisa, Maeda Atsutaka, Goryo Jun, Matsukawa Hiroshi, Akimitsu Jun

    Journal of the Physical Society of Japan   77 ( 6 )   064701/1-6   2008年6月

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  • Commensurate to incommensurate transition in the chiral helimagnet CuB 2O4 査読

    Toshiyuki Fujita, Yoshikuni Fujimoto, Seitaro Mitsudo, Toshitaka Idehara, Teruo Saito, Yusuke Kousaka, Shin-Ichiro Yano, Jun Akimitsu, Jun-Ichiro Kishine, Katsuya Inoue, Mitsuhiro Motokawa

    Journal of the Physical Society of Japan   77 ( 5 )   2008年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We have studied a commensurate to incommensurate transition in the novel helimagnet CuB2O4 by means of high-frequency ESR. In the commensurate weak ferromagnetic (CWF) state, two ESR modes have been found at low temperatures, which are explained in terms of an electron paramagnetic resonance of the Cu2+ spins at 8d-sites and an antiferromagnetic resonance of the magnetically ordered Cu2+ spins at 4b-sites with the Dzyaloshinskii-Moriya (DM) interaction. The DM interaction among the Cu 2+ spins at 4b-sites that carries the CWF state is evaluated quantitatively. The incommensurate helimagnetic state is discussed in terms of the magnetic frustration. ©2008 The Physical Society of Japan.

    DOI: 10.1143/JPSJ.77.053702

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  • Superconductivity in the ternary germanide Y3Pt4Ge6 査読

    Naoki Kase, Takahiro Muranaka, Jun Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   77 ( 5 )   2008年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    A new superconducting ternary germanide, Y3Pt4Ge6, was discovered at the transition temperature of 2.6K by the electrical resistivity and magnetic susceptibility measurements. The magnetization versus magnetic field (M-H) curve indicated that Y3Pt4Ge6 is a type-II superconductor. The lower critical field H,1 and the upper critical field H,2 were estimated to be 53 Oe and 2.73 kOe. The coherence length xi(0)(GL), penetration depth lambda(0)(GL), and Ginzburg-Landau (GL) parameter kappa(0) obtained from these values were 347 angstrom., 3230 angstrom, and 9.3, respectively. Specific heat measurements revealed that Y3Pt4Ge6 is a weak-coupling BCS superconductor with an isotropic superconducting gap. The electron-phonon mass enhancement parameter lambda(gamma) was calculated to be 0.19, which was consistent with the weak-coupling limit inferred from the specific jump Delta C-p/gamma T-n(c).

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  • Crystal structure and charge-ordering in La1.5Ca0.5CoO4 studied by neutron and resonant X-ray scattering 査読

    Kazumasa Horigane, Hironori Nakao, Yusuke Kousaka, Tetsuya Murata, Yukio Noda, Youichi Murakami, Jun Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   77 ( 4 )   2008年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    Using neutron and resonant X-ray scattering, we studied crystal structure and charge-ordering pattern in La1.5Ca0.5CoO4. Taking A2mm with a-b type twin structure into account, Co2+/Co3+ checkerboard charge ordering pattern was suggested in La1.5Ca0.5CoO4 by neutron scattering structure analysis. In order to clarify the charge ordering pattern and the valences of the two Co sites, we performed a resonant X-ray scattering (RXS) measurement near the Co K-edge. We found a resonant signal at (3, 0, 0) reflection which corresponds to the charge ordering with a Co2+/Co3+ checkerboard pattern, and the Co valences of charge ordering were fully charge disproportionated. From the X-ray fluorescence spectra, we found the pre-edge peak structures at E = 7.7 10 and 7.713 keV, corresponding to 1S --&gt; t(2g) and 1s ---&gt; e(g) transitions, respectively. The polarization dependence of the latter one suggests that the anisotropy e(g) orbital is realized in La1.5Ca0.5CoO4.

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  • Soft-phonon-driven superconductivity in CaAlSi as seen by inelastic x-ray scattering 査読

    S. Kuroiwa, A. Q. R. Baron, T. Muranaka, R. Heid, K. -P. Bohnen, J. Akimitsu

    PHYSICAL REVIEW B   77 ( 14 )   2008年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    Inelastic x-ray scattering and ab initio calculation are applied to investigate the lattice dynamics and electron-phonon coupling of the ternary silicide superconductor CaAlSi (P (6) over bar m2). A soft c-axis polarized mode is clearly observed along the Gamma-A-L symmetry directions. The soft mode is strongly anharmonically broadened at room temperature, but at 10 K, its linewidth narrows and comes in good agreement with calculations of linear electron-phonon coupling. This establishes a coherent description of the detailed phonon properties into this system and links them clearly and consistently with the superconductivity.

    DOI: 10.1103/PhysRevB.77.140503

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  • Pressure effects on the superconducting transition in nH-CaAlSi 査読

    L. Boeri, J. S. Kim, M. Giantomassi, F. S. Razavi, S. Kuroiwa, J. Akimitsu, R. K. Kremer

    PHYSICAL REVIEW B   77 ( 14 )   2008年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    We present a combined experimental and theoretical study of the effects of pressure on T(c) of the hexagonal layered superconductors nH-CaAlSi (n=1, 5, and 6), where nH denotes the different stacking variants that were recently discovered. Experimentally, the pressure dependence of T(c) has been investigated by measuring the magnetic susceptibility of single crystals up to 10 kbars. In contrast to previous results on polycrystalline samples, single crystals with different stacking sequences display different pressure dependences of T(c). 1H-CaAlSi shows a decrease in T(c) with pressure, whereas 5H- and 6H-CaAlSi exhibit an increase in T(c) with pressure. Ab initio calculations for 1H-, 5H-, and 6H-CaAlSi reveal that an ultrasoft phonon branch associated with out-of-plane vibrations of the Al-Si layers softens with pressure, leading to a structural instability at high pressures. For 1H-CaAlSi, the softening is not sufficient to cause an increase in T(c), which is consistent with the present experiments but adverse to previous reports. For 5H and 6H, the softening provides the mechanism to understand the observed increase in T(c) with pressure. Calculations for hypothetical 2H and 3H stacking variants reveal qualitative and quantitative differences.

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  • Thermodynamic properties of the clathrate-type silver-oxide Ag6O8AgHF2 査読

    K. Kawashima, M. Kriener, M. Ishii, Y. Maeno, J. Akimitsu

    Physica C: Superconductivity and its Applications   468 ( 6 )   464 - 470   2008年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We report normal-state and superconducting properties of the clathrate-type silver-oxide Ag6O8AgHF2. We present electrical resistivity, DC- and AC-susceptibility and specific-heat measurements of single crystalline Ag6O8AgHF2. In the normal state, Ag6O8AgHF2 exhibits metallic conductivity and a phase transition near 110 K, possibly a structural phase transition as observed in the related compound Ag6O8AgNO3. The onset of superconductivity of our samples is observed around 1.2-1.5 K, and the H-T phase diagram is determined for the first time. The upper critical field Hc2(0) is estimated to be about 2000-2200 Oe and the coherence length ξGL(0) to be 40 nm. © 2008 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physc.2008.01.008

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  • Structural phase evolution and superconductivity in the non-stoichiometric intermetallic compound niobium diboride 査読

    Zhi-An Ren, Sogo Kuroiwa, Yoko Tomita, Jun Akimitsu

    Physica C: Superconductivity and its Applications   468 ( 5 )   411 - 416   2008年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    The structural changes and superconductivity in the layered non-stoichiometric niobium diboride compound were systematically studied for both of niobium-deficient and boron-deficient phases by X-ray diffraction analysis and magnetic susceptibility measurements. The niobium and boron vacancies were found to have different effects on the crystal lattice parameters and superconducting properties
    superconductivity only exists in niobium-deficient phases with a clear enhancement of inner-plane B-B bonding and the increase of inter-plane distance, while boron vacancies have the opposite effect of enhancing the Nb-B bonding and breaking the superconductivity. A main superconducting phase with Nb0.83B2 composition was obtained with Tc ∼ 9.3-9.9 K, and a = 3.102(1) Å and c = 3.322(2) Å under the optimum sintering conditions. The low temperature specific heat was measured for three high-pressure synthesized superconducting samples to reveal the origin the increase of Tc with the niobium deficiencies. © 2008 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physc.2008.01.001

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  • Multigap superconductivity in sesquicarbides La2C3 and Y2C3 査読

    S. Kuroiwa, Y. Saura, J. Akimitsu, M. Hiraishi, M. Miyazaki, K. H. Satoh, S. Takeshita, R. Kadono

    PHYSICAL REVIEW LETTERS   100 ( 9 )   2008年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    A complex structure of the superconducting order parameter in Ln(2)C(3) (Ln=La,Y) is demonstrated by muon spin relaxation measurements in their mixed state. The muon depolarization rate [sigma(nu)(T)] exhibits a characteristic temperature dependence that can be perfectly described by a phenomenological double-gap model for nodeless superconductivity. While the magnitude of two gaps is similar between La2C3 and Y2C3, a significant difference in the interband coupling between those two cases is clearly observed in the behavior of sigma(nu)(T).

    DOI: 10.1103/PhysRevLett.100.097002

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    その他リンク: http://orcid.org/0000-0002-4747-1980

  • Superconducting state of silver-oxide clathrate Ag6O8AgNO3 査読

    Kenji Kawashima, Masaru Ishii, Hirotaka Okabe, Jun Akimitsu, Markus Kriener, Hiroshi Takatsu, Shingo Yonezawa, Yoshiteru Maeno

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   77 ( 2 )   2008年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We report the normal-state and superconducting properties of the silver-oxide clathrate Ag6O8AgNO3. Electrical resistivity, specific heat, and DC and AC susceptibility measurements as functions of magnetic field and temperature show phase transitions near 90 and 185 K. We find that Ag6O8AgNO3 is a bulk superconductor with the superconducting transition temperature T-c = 1.04 K. We observe a clear kink structure in the field dependence of the real part of AC susceptibility, suggesting a change in vortex state. Furthermore, we present the H-T phase diagram of this compound and estimate the upper critical field H-c2(0) to be about 770Oe. We also discuss the superconducting properties in a Ginzburg-Landau framework.

    DOI: 10.1143/JPSJ.77.024707

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  • Electrical resistivity of Y2C3 investigated by microwave surface impedance measurement 査読

    Satoshi Akutagawa, Takeyoshi Ohashi, Haruhisa Kitano, Atsutaka Maeda, Jun Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   77 ( 1 )   2008年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    Y2C3 is a superconductor whose T-c changes from 6 to 18 K depending on the synthesis conditions. Since its electronic properties were unknown, we performed the electrical-property study by measuring the microwave complex surface impedance, Z(s), of Y2C3 for different T-c's (13.9 and 15.5 K), and the resistivity and mean free path were determined in the normal state. We observed the typical behavior of a Drude metal in the Hagen-Rubens limit (rho(p) &gt;&gt; rho(2)) in the normal state. We succeeded in observing the occurrence of superconductivity in the electrical property. Furthermore, we found that Y2C3 is a clean superconductor.

    DOI: 10.1143/JPSJ.77.014702

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  • Chiral magnetic ordering and commensurate-to-incommensurate transition in CuB2O4 査読

    Yusuke Kousaka, Shin-Ichiro Yano, Jun-Ichiro Kishine, Yusuke Yoshida, Katsuya Inoue, Koichi Kikuchi, Jun Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   76 ( 12 )   2007年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We measured the anisotropic magnetization processes of copper metaborate CuB2O4 in detail. We propose an interpretation of the magnetization anomaly observed under the magnetic field applied perpendicular to the tetragonal axis, based on the field- and temperature- induced commensurate-to-incommensurate (C-IC) transition (Freedericksz transition) accompanied by the chiral soliton lattice formation.

    DOI: 10.1143/JPSJ.76.123709

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  • Superconductivity in ternary silicide NaAlSi with layered diamond-like structure 査読

    S. Kuroiwa, H. Kawashima, H. Kinoshita, H. Okabe, J. Akimitsu

    Physica C: Superconductivity and its Applications   466 ( 1-2 )   11 - 15   2007年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We have succeeded in synthesizing a new member of ternary silicide NaAlSi with a superconducting transition temperature Tc-onset = 7 K by high pressure technique. The crystal structure of NaAlSi is the layered diamond-like structure with a space group of P4/nmm (no. 129, Z = 3) and without inversion symmetry. The superconducting properties in NaAlSi have been investigated by means of DC and AC magnetic susceptibility measurements. These magnetic responses indicate that NaAlSi belongs to a conventional type-II superconductor with a Ginzbrug-Landau parameter κ = 13.8. From the band structure calculations, we found that the hole band of Si p- and electron Al s-orbitals play an important role for the superconductivity in this system. © 2007 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physc.2007.04.232

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  • Vortex lattice symmetry in hexagonal superconductor CaAlSi probed by small angle neutron scattering 査読

    S. Kuroiwa, Y. Sugiyama, T. Muranaka, H. Kawano-Furukawa, J. Akimitsu

    JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS   68 ( 11 )   2129 - 2132   2007年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PERGAMON-ELSEVIER SCIENCE LTD  

    The small angle neutron scattering diffraction patterns from the flux line lattice state in the layered hexagonal superconductor CaAlSi are observed. Under an applied magnetic field (H) parallel to the crystalline c-axis, a hexagonal vortex structure is observed over the entire temperature/field regions. On the other hand, the vortex configuration under H parallel to a shows an ellipsoidal arrangement of the first-order Bragg peaks due to the anisotropic penetration depth. It was inferred from these results that the vortex state characterized by penetration depth and coherence length in CaAlSi may be described by that of anisotropic uniaxial superconductor using London theory. (c) 2007 Elsevier Ltd. All rights reserved.

    DOI: 10.1016/j.jpcs.2007.08.049

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  • Spin and charge orders and their hole-doping dependence in single layered cobaltate La2-xCaxCoO4 (0.3 &lt;= x &lt;= 0.8) 査読

    Kazumasa Horigane, Haruhiro Hiraka, Toru Uchida, Kazuyoshi Yamada, Jun Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   76 ( 11 )   2007年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    Neutron scattering experiments were performed on single crystals of layered cobalt-oxides La2-xCax-CoO4 (LCCO) to characterize the charge and spin orders in a wide hole-doping range of 0.3 &lt;= x &lt;= 0.8. For a commensurate value of x = 0.5 in (H, 0, L) plane, two types of superlattice reflections concomitantly appear at low temperature; one corresponds to a checkerboard charge ordered pattern of Co2+/ Co3+ ions and the other is magnetic in origin. Further, the latter magnefic-superlattice peaks show two types of symmetry in the reflections, suggesting anti ferromagnetic-stacking (AF-S) and ferromagnetic-stacking (F-S) patterns of spins along the c direction. From the hole-doping dependence, the in-plane correlation lengths of both charge and spin orders are found to give a maximum at x = 0.5. These features are the same with those of x = 0.5 in Lal(1-x)Sri(1+x)MnO(4) (LSMO), a typical checkerboard and spin ordered compound. However, in (H, H, L) plane, we found a magnetic scattering peak at Q = (0.25, 0.25, 0.5) position below T-N. This magnetic peak can not be understood by considering the Co2+ spin configuration, suggesting that this peak is originated from Co3+ spin order. By analyzing these superlattice reflections, we found that they are originated from high-spin state of Co3+ spin order.

    DOI: 10.1143/JPSJ.76.114715

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  • Electronic structures of CaAlSi with different stacking AlSi layers by first-principles calculations 査読

    Sogo Kuroiwa, Akiyoshi Nakashima, Shin Miyahara, Nobuo Furukawa, Jun Akimitsu

    Journal of the Physical Society of Japan   76 ( 11 )   2007年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    The full-potential linear augmented plane-wave calculations have been applied to investigate the systematic change of electronic structures in CaAlSi due to different stacking sequences of AlSi layers. The present ab-initio calculations have revealed that the multistacking, buckling and 60° rotation of AlSi layer affect the electronic band structure in this system. In particular, such a structural perturbation gives rise to the disconnected and cylindrical Fermi surface along the M-L lines of the hexagonal Brillouin zone. This means that multistacked CaAlSi with the buckling AlSi layers increases degree of two-dimensional electronic characters, and it gives us qualitative understanding for the quite different upper critical field anisotropy between specimens with and without superstructure as reported previously. ©2007 The Physical Society of Japan.

    DOI: 10.1143/JPSJ.76.113705

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  • Magneto-optic far-infrared study of Sr14Cu24O41: Triplet excitations in chains 査読

    D. Huevonen, U. Nagel, T. Room, P. Haas, M. Dressel, J. Hwang, T. Timusk, Y. -J. Wang, J. Akimitsu

    PHYSICAL REVIEW B   76 ( 13 )   2007年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    Using far-infrared spectroscopy, we have studied the magnetic field and temperature dependence of the spin gap modes in the chains of Sr14Cu24O41. Two triplet modes T-1 and T-2 were found in the center of the Brillouin zone at Delta(1)=9.65 meV and Delta(2)=10.86 meV in zero magnetic field. The T-1 mode was excited when the electric field vector E of the light was polarized along the b axis (perpendicular to the planes of chains and ladders) and T-2 was excited for E parallel to a (perpendicular to the chains and along the rungs). Up to the maximum magnetic field of 18 T, applied along the chains, the electron g factors of these two modes were similar, g(1c)=2.049 and g(2c)=2.044. Full linewidth at half maximum for both modes was 1 cm(-1) (0.12 meV) at 4 K and increased with T. The temperature dependence of mode energies and line intensities was in agreement with the inelastic neutron scattering results from two groups [M. Matsuda , Phys. Rev. B 59, 1060 (1999); L. P. Regnault , ibid 59, 1055 (1999)]. The T-1 mode has not been observed by inelastic neutron scattering in the points of the k space equivalent to the center of the Brillouin zone. Our study indicates that the zone structure model of magnetic excitations of Sr14Cu24O41 must be modified to include a triplet mode at 9.65 meV in the center of the magnetic Brillouin zone.

    DOI: 10.1103/PhysRevB.76.134418

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  • Terahetz BWO spectroscopy of conductors and superconductors 査読

    B. P. Gorshunov, A. A. Volkov, A. S. Prokhorov, I. E. Spektor, J. Akimitsu, M. Dressel, G. J. Nieuwenhuys, S. Tomic, S. Uchida

    QUANTUM ELECTRONICS   37 ( 10 )   916 - 923   2007年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:TURPION LTD  

    Methods for direct (without the use of the Kramers-Kronig relations) measuring the dielectric response (dynamic conductivity and permittivity) spectra of dielectrics, conductors, and superconductors in the terahertz frequency range (0.03 - 1.5 THz) are described. The methods are realised by using BWO (backward-wave oscillator) spectrometers developed at the A.M. Prokhorov General Physics Institute, RAS. Dielectric measurements can be performed at temperatures from 2 to 1000 K in magnetic fields up to 8 T. The dielectric response spectra are investigated for a number of materials with properties determined by electron correlation effects: superconductors, one-dimensional conductors, heavy fermion systems, spin-ladder cuprates, and spin glasses.

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  • Superconductivity in boron-doped SiC 査読

    Zhi-An Ren, Junya Kato, Takahiro Muranaka, Jun Akimitsu, Markus Kriener, Yoshiteru Maeno

    Journal of the Physical Society of Japan   76 ( 10 )   2007年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We report superconductivity in heavily boron-doped bulk silicon carbide related to the diamond structure. The compound exhibits zero resistivity and diamagnetic susceptibility below a critical temperature T c of ∼1.4 K, and an effective boron doping concentration higher than 10 21 cm -3. We present the H-T phase diagram of this new superconducting compound determined from AC susceptibility. In finite DC magnetic fields a clear hysteresis was observed between cooling and subsequent warming runs. This indicates, in contrast with the type-II superconductivity in boron-doped diamond and silicon, that a type-I superconductivity with a critical field H c(0) of about 100 Oe is realized in boron-doped SiC. Moreover, the specific-heat shows a clear jump at T c, demonstrating bulk nature of the superconductivity. ©2007 The Physical Society of Japan.

    DOI: 10.1143/JPSJ.76.103710

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  • Microwave electrical resistivity of moderately high T-c superconductor, Y2C3 査読

    Satoshi Akutagawa, Takeyoshi Ohashi, Haruhisa Kitano, Atsutaka Maeda, Jun Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   460   649 - 650   2007年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We performed the first electrical property study by measuring microwave complex surface impedance, Z(s) of Y2C3 with T-c = 15.5 K. First of all, the resistivity was determined successfully in the normal state. We observed a typical behavior of the Drude metal in the Hargen-Rubens limit (p(1) &gt;&gt; p(2)) in the normal state, and we succeeded in observing the occurrence of superconductivity in the electrical property for the first time. We found that Y2C3 was a typical type-II clean superconductor. (C) 2007 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physc.2007.04.215

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  • Superconductivity in clathrate-type silver oxides: Ag6O8MX (M = cation, X = anion) 査読

    K. Kawashima, M. Ishii, S. Date, T. Tsutsumi, H. Okabe, J. Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   460   542 - 543   2007年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We have performed physical property measurements of clathrate-type silver oxide: Ag6O8AgNO3, having a face-centered cubic structure with a space group of Fm (3) over barm. We examined the temperature dependence of heat capacity, electrical resistivity and magnetic susceptibility measurements. We confirmed the superconducting state and estimated that the upper critical magnetic field: H-c2 was to be about 710 Oe by the electrical resistivity measurements under various magnetic fields. (c) 2007 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physc.2007.03.417

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  • Superconducting energy gap from break-junction tunneling spectroscopy in the ternary silicide CaAlSi 査読

    S. Kuroiwa, T. Takasaki, T. Ekino, J. Akimitsu

    PHYSICAL REVIEW B   76 ( 10 )   2007年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    Tunneling spectroscopy using a break-junction technique has been conducted to determine the superconducting gap structure of the ternary silicide-layered superconductor CaAlSi with and without superstructure. We find that the tunneling conductance in CaAlSi shows a single gap feature, which can be described by the BCS density of states. The gap size Delta obtained from the tunneling conductance in CaAlSi without superstructure is estimated to be approximately 1.0 meV, corresponding to a weak-coupling gap ratio. On the other hand, for a specimen with superstructure, the largest gap, Delta=1.5 meV, significantly deviates from the BCS value, indicating strong-coupling superconductivity. These results demonstrate systematic variations in the pairing interaction with superstructure formations, which are consistent with heat capacity measurements.

    DOI: 10.1103/PhysRevB.76.104508

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  • Quasi-two-dimensional superconducting characteristics in single-crystalline SrAlSi without superstructure 査読

    S. Kuroiwa, T. Kakiuchi, H. Sagayama, H. Sawa, J. Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   460   154 - 155   2007年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We report on the detailed crystal structure and superconducting properties in the single-crystalline SrAlSi by means of synchrotron Xray diffraction technique and systematic magnetization measurements. Although SrAlSi exhibits a clear no sign of superlattice reflection, the anisotropic parameter for magnetic response in SrAlSi is almost equivalent to that of superstructured CaAlSi. It is inferred from these results that the electronic ground state in SrAlSi shows quasi-two-dimensional characteristics, and its dimensionality is irrelevant to the superstructured system. (c) 2007 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physc.2007.04.005

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  • Thermodynamic properties of ferromagnetic Ce-compound, CeAgAl3 査読

    Takahiro Muranaka, Jun Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   460   688 - 690   2007年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We successfully synthesized a new ferromagnetic Ce-compound, CeAgAl3 (T-C = 3.0 K). From susceptibility measurement using the aliened crystal, effective Bohr magneton, mu(eff), and Weiss temperature, theta, were estimated to be about 2.23 mu(B) and 10.1 K along ab-plane, respectively, indicating the tri-valent state for Cc and weak ferromagnetic interaction between Ce ions. In M-H curve, the magnetization along the ab-plane and the c-axis up to 5 T at 1.8 K exhibited ferromagnetic behaviour saturating over 0.2 T with small magnetic hysteresis and reached the values of about 1.07 mu(B) and 0.75 mu(B), respectively. These values are somewhat smaller than the theoretical value for the saturated moment of free Ce3+ ion (mu(s) = gJ = 2.14 mu(B)). (C) 2007 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physc.2007.03.127

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  • Specific heat and tunneling spectroscopy study of NbB2 with maximum Tc ∼ 10K 査読

    Sogo Kuroiwa, Yoko Tomita, Akira Sugimoto, Toshikazu Ekino, Jun Akimitsu

    Journal of the Physical Society of Japan   76 ( 9 )   2007年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We report on magnetic, transport, specific heat and tunneling measurements of NbB2 with the maximum superconducting critical temperature Tc ∼ 10 K. The essential superconducting properties have been investigated through mapping out the critical field phase diagram, from which it is turned out to be type II superconductor with a small Ginzbrug-Landau parameter κ ∼ 2:2. Both the tunneling and specific heat measurements indicate that our NbB2 specimen has a single gap and s-wave pairing symmetry with a weak-coupling limit. The electron-phonon coupling constant is estimated to be λep = 0:71 ± 0:02, suggesting that the maximum T c of NbB2 can be explained by the McMillan formula. ©2007 The Physical Society of Japan.

    DOI: 10.1143/JPSJ.76.094705

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  • Scanning tunnelling microscopy and spectroscopy of MgB2 査読

    Toshikazu Ekino, Tomoaki Takasaki, Raquel Ribeiro, Takahiro Muranaka, Jun Akimitsu

    Journal of Physics: Conference Series   61 ( 1 )   278 - 282   2007年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Institute of Physics Publishing  

    Experiments of the scanning tunnelling microscopy and spectroscopy (STM/STS) have been carried out on the layered superconductor MgB2 with Tc 39 K. The measurements were done at 5 K using a Pt-Ir tip and the single-crystal grain. The STM images show characteristic hexagonal patterns with the distances of nearest neighbour atoms to be 0.3 - 0.35 nm and 0.15 - 0.2 nm, respectively, which are consistent with the Mg and B lattice sizes. The STS measurements clarify the superconducting gap 2Δ 20 - 24 meV, which leads to the strong-coupling ratio 2Δ/kBTc 5 - 6. The tunnelling spectra can be described by the correlated 2-gap model. These features are consistent with our previous break-junction measurements. The gap structure is homogeneous at least within the range of 200 nm × 200 nm. © 2007 IOP Publishing Ltd.

    DOI: 10.1088/1742-6596/61/1/056

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  • Antiferromagnetic ordering in Cu2OCl2 studied by the muon spin rotation/relaxation technique 査読

    K. Kawashima, H. Okabe, K. Suzuki, S. Kuroiwa, J. Akimitsu, Kh Sato, A. Koda, R. Kadono

    JOURNAL OF PHYSICS-CONDENSED MATTER   19 ( 14 )   2007年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:IOP PUBLISHING LTD  

    We report on muon spin rotation/ relaxation (mu SR) studies of the cuprate mineral melanothallite, Cu2OCl2, over a temperature range between 4.1 and 300 K. This compound has a pyrochlore- like corner- shared OCu4 tetrahedral network with an S = 1/ 2 system. The magnetic susceptibility shows a broad maximum around 140 K and drops at 70 K. In the mu SR measurement, we observed clear muon spin precession spectra below 70 K that is evidence for a long- range magnetic ordered state. However, the time spectra also exhibit exponential damping even in the ordered state, suggesting a possible influence of geometrical frustration in this compound. This renders the need for careful examination of the magnetic structure of the ordered state.

    DOI: 10.1088/0953-8984/19/14/145275

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  • ESR measurements of the chiral spin system CuB2O4 査読

    T. Fujita, Y. Fujimoto, S. Mitsudo, M. Toda, T. Idehara, M. Motokawa, Y. Kousaka, S. Yano, J. Akimitsu

    Journal of Magnetism and Magnetic Materials   310 ( 2 )   1389 - 1391   2007年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We report ESR measurements of the chiral spin system CuB2O4 at 35 GHz in H ∥ c. The g-value is determined as 2.16 for the c-axis. Anomalies in the resonance field and the line width have been observed around 21 and 9 K, which correspond to the successive phase transitions. In the helimagnetic state, a small hysteretic dip appears in ESR spectrum in addition to a helimagnetic resonance absorption. The dip shifts rapidly to the higher field with decreasing temperature below the transition temperature at 9 K. © 2006 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.jmmm.2006.10.408

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  • NMR studies of single crystal chromium diboride 査読

    C. Michioka, Y. Itoh, K. Yoshimura, Y. Watabe, Y. Kousaka, H. Ichikawa, J. Akimitsu

    Journal of Magnetism and Magnetic Materials   310 ( 2 )   e620 - e622   2007年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We report 11B NMR studies of a single crystal CrB2. From the temperature dependence of the Fourier-transformed NMR spectra in the paramagnetic state, the hyperfine coupling constants are estimated to be ACr - B = - 0.64, - 0.74 and - 0.71 kOe / μB for H | | c, H | | a and H | | [2 1 0], respectively. In the magnetically ordered state, the spectra in H | | a and H | | [2 1 0] consist of superposition of a broad hump and five peaks, which correspond to the incommensurate and commensurate spin structures. © 2006 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.jmmm.2006.10.641

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  • Possible chiral magnetism in CuB2 O4 査読

    Y. Kousaka, J. Kishine, S. Yano, J. Akimitsu

    Journal of Magnetism and Magnetic Materials   310 ( 2 )   e463 - e464   2007年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We reported here commensurate-to-incommensurate (C-IC) magnetic ordering in copper metaborate CuB2 O4 on the basis of concomitant chiral soliton lattice formation. We measured the magnetization processes in detail under the applied magnetic fields parallel and perpendicular to the helical axis. We found the anomalous temperature and magnetic field dependence of the magnetization strongly anisotropic with respect to the applied magnetic fields. The experimental results were quite consistent with the theoretical prediction based on the lattice chiral XY model, which describes the C-IC transition as the chiral soliton picture. © 2006 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.jmmm.2006.10.413

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  • Magnetic excitation in La1.5Ca0.5CoO4 査読

    K. Horigane, K. Yamada, H. Hiraka, J. Akimitsu

    Journal of Magnetism and Magnetic Materials   310 ( 2 )   774 - 776   2007年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We examined checkerboard charge and spin order in La1.5Ca0.5CoO4 by elastic and inelastic neutron scattering. We discovered the sharp magnetic peaks with l=half-integer and broad magnetic peaks with l=integer in (0.5, 0, l). The origin of magnetic scattering can be explained by the minority and majority stacking arrangement of Co2+/Co3+ spins along the c-axis since the same magnetic reflections arise from the two stacking vectors in La0.5Sr1.5MnO4. Spin-wave dispersion at majority stacking is seen up to 16 meV, in contrast to 50 meV in La1.5Sr0.5NiO4. This indicates a much weaker exchange interaction in the cobaltates than in nickelates. The dispersion at majority stacking has almost a same one with that of minority stacking. © 2006 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.jmmm.2006.10.530

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  • Superconductivity of Y2C3 investigated by specific heat measurement 査読

    Satoshi Akutagawa, Jun Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   76 ( 2 )   2007年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We report newly synthesized high-purity samples Of Y2C3 with various T-c's (11.6, 13.9, and 15.2 K) and compared their superconducting properties by magnetization and specific heat measurements. From specific heat measurements, the fitting below T-c to the electron specific heat of each sample has an exp(-1/T) dependence, as predicted by BCS theory, rather than a T-n dependence, as for superconductors with an anisotropic symmetry of superconductivity. These results indicate that the symmetry of the superconductivity Of Y2C3 is that of an isotropic s-wave. From the fitting below T-c, we estimated 2 Delta/k(B)T(c), to be 3.6, 3.9, and 4.1 for the 11, 13, and 15 K samples, respectively. We found that T, Of Y2C3 was correlated to the Sommerfeld coefficient, and that the high-T-c, material was in a stronger coupling regime than the low-T-c, material.

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  • スピン梯子系Sr14-xCaxCu24O41の集団的電荷励起--新たな電荷秩序状態か? 査読

    北野晴久, 前田京剛, Boris Gorshunov, Silvia Tomic, 秋光 純

    固体物理   42 ( 4 )   225 - 239   2007年

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    記述言語:日本語  

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  • mu SR measurements on the vortex lattice of CaAlSi: Anisotropic response in magnetic penetration depth 査読

    Kuroiwa S, Satoh KH, Koda A, Kadono R, Ohishi K, Higemoto W, Akimitsu J

    Journal of Physics and Chemistry of Solids   68 ( 11 )   2124 - 2128   2007年

  • Microscopic properties of vortex states in Y B6 probed by muon spin rotation 査読

    Kadono R, Kuroiwa S, Akimitsu J, Koda A, Ohishi K, Higemoto W, Otani S

    Physical Review B - Condensed Matter and Materials Physics   76 ( 9 )   2007年

  • High field ESR measurements on the chiral spin system CuB2O 4 査読

    T. Fujita, Y. Fujimoto, S. Mitsudo, T. Idehara, K. Inoue, J. Kishine, Y. Kousaka, S. Yano, J. Akimitsu, M. Motokawa

    Journal of Physics: Conference Series   51 ( 1 )   111 - 114   2006年12月

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:Institute of Physics Publishing  

    Magnetically ordered states in the chiral spin system CuB2O 4have been studied by ESR measurements. Two resonance absorptions are observed in the helimagnetic state, which imply that the two inequivalent copper sites are in the helimagnetically ordered state and the paramagnetic state with a development of short range order. The frequency-independent dip with hysteresis on the spectrum is also observed in the helimagnetic state. The behavior of the dip can be interpreted by the change of the dielectric properties rather than the magnetic resonance. © 2006 IOP Publishing Ltd.

    DOI: 10.1088/1742-6596/51/1/024

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  • New pyrochlore-like compound Cu2OCl2 with S=1/2

    H. Okabe, K. Suzuki, K. Kawashima, T. Muranaka, J. Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   75 ( 12 )   2006年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We report here on the magnetic properties of Cu2OCl2, the structure of which can be regarded either as a pyrochlore-like corner-shared tetrahedral network with S = 1/2 or cross-linking chains of edge-sharing CuO2Cl2 squares. The magnetic susceptibility exhibits a broad maximum at approximately 140K, indicating a low-dimensional feature, and drops at 70 K accompanied with a small specific-heat jump; however, it does not follow the typical S = 1/2 linear chain model. The magnetic susceptibility can be explained by a mean field corrected chain model with a closed value of the intra- and interchain exchange constants; thus, magnetic frustration would be dominant over a low-dimensional feature in Cu2OCl2. From the band calculation results, we found that the Cu 3d on-site Coulomb interaction is essential to describe the electronic structure of Cu2OCl2. We conclude that Cu2OCl2 should be regarded as a 3D material that has the potential to combine the pyrochlore-like geometrical frustration and the strong correlation among the Cu 3d electrons.

    DOI: 10.1143/JPSJ.75.123705

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  • Temperature dependence of atomic modulation in the incommensurate composite crystal structure of (Sr2Cu2O3)(0.70)CuO2 'Sr14Cu24O41' 査読

    Y. Gotoh, I. Yamaguchi, H. Eisaki, T. Nagata, J. Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   445   107 - 110   2006年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Temperature dependence of atomic modulation in the incommensurate composite crystal structure of (Sr2Cu2O3)(0.70)CuO2, 'Sr14Cu24O41' with two-legged Cu2O3 ladder and edge-shared CuO2 chain has been investigated by single-crystal X-ray diffraction method using centrosymmetric (3 + 1)-dimensional superspace group. The displacive modulations of all constituent atoms at 150 K have been compared with those obtained at room temperature. The hole distribution between the Cu2O3 ladder plane and the one-dimensional CuO2 chain has been considered on the basis of the changes of lattice constants of incommensurate basic structure, the role of the atomic modulation and the interatomic distances between Cu atom in the Cu2O3 and O atom in the CuO2. It is indicated that the small amount of holes doped in the Cu2O3 ladder have been back-transferred to the CuO2 chain and that almost all of the holes in (Sr(2)Cu(2)O3)(0.70)CuO2 are localized in the CuO2 at low temperature. (c) 2006 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physc.2006.03.089

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  • Lattice dynamics calculation of infrared active modes of cuprate superconductors 査読

    T. Hasegawa, N. Ogita, T. Kondo, H. Kawashima, J. Akimitsu, M. Udagawa

    PHYSICA B-CONDENSED MATTER   383 ( 1 )   86 - 88   2006年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We have calculated the highest-energy infrared active E. mode for three cuprate compounds with and without apical ions, in order to investigate the experimental energy-difference between T- and T'-structures. The calculated energies show the similar Cu-O bond length dependence to the experimental results. The calculated spin densities of the T-structure have more electrons in the d(x2-y2) orbital than that of the T'-structure. This means that the CuO2 Plane in the T-structure is compressed, in comparison with the V-structure even with the same Cu-O bond length. This compression increases the energy of the E, mode in the T-structure. This is the clear evidence that the block layer is important for the electronic state on the CuO2 plane. (c) 2006 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physb.2006.03.065

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  • Impurity effect in a novel spin-Peierls compound TiOBr 査読

    T. Sasaki, M. Yamazawa, K. Takahashi, T. Nagai, T. Asaka, Y. Matsui, J. Akimitsu

    PHYSICA B-CONDENSED MATTER   383 ( 1 )   1 - 4   2006年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We have performed magnetic susceptibility and electron diffraction measurements on impurity effect in a novel spin-Peierls system TiOBr, i.e. Ti1-xScxOBr.The impurity-induced antiferromagnetic long-range order found in Cu1-xZnxGeO3 could not be observed in this system. A number of unpaired spins induced by the impurities can be explained by a simple model that one Sc impurity creates one S = (1)/(2) spin up to the threshold concentration, x(c)similar to 0.025. The induced impurity suppresses the spin-Peierls transition temperature, T-c1 and the spin-Peierls phase disappears at x(1)similar to 0.028, although the incommensurate state still persists above x(1). Both values agree well with each other. (c) 2006 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physb.2006.03.033

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  • Low-temperature magnetoresistance of layered cobalt oxides Nax CoO2 査読

    Hirotaka Okabe, Jun Akimitsu, Takashi Kubodera, Masanori Matoba, Toru Kyomen, Mitsuru Itoh

    Physica B: Condensed Matter   378-380   863 - 864   2006年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We measured transport properties in layered cobalt oxides Nax CoO2, that are parent materials of the superconductor Nax CoO2 · y H2 O, in order to reveal so-far unsettled origin of these electronic and magnetic features. Various sodium nonstoichiometric samples Nax CoO2 ( x = 0.47, 0.62, 1.0) have been carefully synthesized to control the carrier density and the electronic state of cobalt ions. We found a large negative magnetoresistance (MR) effect in the vicinity of x ∼ 0.5 and 1.0. The x = 0.47 and 1.0 samples show negative MR, - 45 % and - 10 %, respectively, at 90 kOe and 4 K. These negative MR suggest the pseudogap formation due to the charge ordering in CoO2 layers, such as Ca3 Co4 O9 and Bi2 - x Pbx Sr2 Co2 Oy, all of which exhibit similar behaviors. The suppression of the pseudogap by a magnetic field causes the large negative magnetoresistance. © 2006 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physb.2006.01.319

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  • Superconductivity in M7Re13X (M = W, Mo, X = B, C) compounds 査読

    K Kawashima, A Kawano, T Muranaka, J Akimitsu

    PHYSICA B-CONDENSED MATTER   378-80   1118 - 1119   2006年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We discovered a series of new superconductors, M7Re13X (M = W, Mo, X = B, C), and discussed their superconducting properties. M7Re13X (M = W, Mo, X = B, C) has a beta-Mn-type (Cubic, P(4)I32) structure. The T-c values are 7.1 K for W7Re13B, 7.3 K for W7Re13C, 8.3K for Mo7Re13B and 8.1 K for Mo7Re13C. The magnetization vs. magnetic field curve showed type-II superconducting behavior. These superconductors can be described as strong coupling regime and Re orbital is mainly responsible for the superconductivity of these compounds. (c) 2006 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physb.2006.01.489

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  • Neutron scattering study of phonons in LaCoO3 査読

    Y Kobayashi, TS Naing, M Suzuki, M Akimitsu, K Asai, K Yamada, J Akimitsu, P Manuel, JM Tranquada, G Shirane

    PHYSICA B-CONDENSED MATTER   378-80   532 - 533   2006年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We have investigated phonons with the inelastic neutron scattering in order to study the 100 K spin-state transition (ST) in LaCoO3 The acoustic phonon dispersions show some characteristic features of the folded Brillouin zone (BZ) of the rhombohedrally distorted perovskite structure containing two chemical formula of LaCoO3 in the unit cell. The optical phonons of the E, rotational modes of 0 atoms and the E. vibration mode of La atoms show a remarkable softening above the ST. The softening is most pronounced at R-point of the pseudo-cubic reciprocal lattice but covers almost whole region of the Brillouin zone. In contrast, the acoustic phonons show no such softening associated with the ST. (c) 2006 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physb.2006.01.242

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  • Single crystal growth and magnetic properties of La2-xMxCoO4 (M = Ca, Ba) 査読

    K Horigane, T Uchida, J Akimitsu

    PHYSICA B-CONDENSED MATTER   378-80   334 - 336   2006年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Magnetic properties were investigated for single crystals of La2-xCaxCoO4 (0.5 &lt;= x &lt;= 0.8) and for polycrystalline samples of La2-xBaxCoO4 (0.5 &lt;= x &lt;= 0.9). In the Ca system, the effective moment mu(eff) decreased with increasing x from 4.0 mu(B) (x = 0.5) to 3.0 mu(B) (x = 0.7-0.8), accompanied by a steep reduction of Weiss temperature Theta. These drastic changes of the magnetic properties suggest a spin-state transition of the Co3+ ions from the high-spin state to the intermediate-spin state. On the other hand, the effective moment of La2-xBaxCoO4 was almost 4.0 mu B and there is no change of Weiss temperature Theta. These facts suggest that the intermediate-spin state is related to the Co-O bond length, i.e. crystal fields. (c) 2006 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physb.2006.01.423

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  • Magnetic and superconducting phase diagram in oxybromite cuprate Ca2-xNaxCuO2Br2 査読

    S Kuroiwa, Y Zenitani, M Yamazawa, Y Tomita, J Akimitsu, K Ohishi, A Koda, Saha, SR, R Kadono, Watanabe, I, S Ohira

    PHYSICA B-CONDENSED MATTER   374   75 - 78   2006年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    A comprehensive magnetic and superconducting phase diagram determined by muon spin rotation/relaxation (mu SR) is presented for Ca2-xNaxCuO2Br2 which has apical bromine atoms. Evidence for anti ferromagnetic (AF) order in lightly doped samples and that for quasi-static spin glass (SG)-like state in moderately doped ones are obtained by ZF-mu SR at low temperatures. While the phase diagram is qualitatively similar to that in typical 2-1-4 cuprates including Ca2-xNaxCuO2Cl2 and La2-xSrxCuO4, it exhibits a slight shift (expansion) over the x axis. (c) 2005 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physb.2005.11.019

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    その他リンク: http://orcid.org/0000-0003-1494-6502

  • Charge density waves in Sr14-xCaxCu24O41: Electron correlations versus structural effects 査読

    KY Choi, M Grove, P Lemmens, M Fischer, G Guntherodt, U Ammerahl, B Buchner, G Dhalenne, A Revcolevschi, J Akimitsu

    PHYSICAL REVIEW B   73 ( 10 )   2006年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMERICAN PHYSICAL SOC  

    We report inelastic light scattering experiments in the spin-chain and ladder system Sr14-xCaxCu24O41. A depletion of an electronic background is observed for 0 &lt;= x &lt;= 5 at low temperatures and with light polarization both parallel to the leg and rung directions. This points to the formation of two-dimensional charge density wave in the ladders. Furthermore, the Ca-substitution dependence of the 305 and 249 cm(-1) modes, sensitive to the lattice modulations of the respective chain and ladder system, suggests that the charge dynamics in the ladders is strongly influenced by the structural incommensurability.

    DOI: 10.1103/PhysRevB.73.104428

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  • Superconductivity in intermetallic compound Mo7Re13X(X = B,C) 査読

    K Kawashima, T Muranaka, J Akimitsu

    SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS   7 ( 1 )   9 - 11   2006年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:NATL INST MATERIALS SCIENCE  

    We report here the physical properties of a new intermetallic superconductor Mo7Re13X(X=B,C) (T-c=8.3 K (X=B) and 8.1 K (X=C)). The crystal structure of Mo7Re13X is the beta-Mn-type (cubic, space group: P4(1)32) and the Re atoms form a tilted octahedron. The magnetization (M-H) curves show a typical type-II superconductivity; the lower critical field H-c1(0), upper critical field H-c2(0) and Ginzburg-Landau (GL) parameter value k are, respectively, 3.0 Oe, 15.4 T and 101 for X=B and 3.1 Oe, 14.8 T and 98 for X=C. The superconducting gap versus k(B)T(C): 2 Delta/k(B)T(c) values are 4.4 for X=B and 4.2 for X=C, which implies that superconductivity in Mo7Re13X(X=B,C) can be described by an s-wave strong coupling regime. (c) 2005 Elsevier Ltd. All rights reserved.

    DOI: 10.1016/j.stam.2005.11.001

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  • Superconductivity in Y2C3 with medium Tc 査読

    S Akutagawa, J Akimitsu

    SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS   7 ( 1 )   2 - 5   2006年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:NATL INST MATERIALS SCIENCE  

    Recently, we have reported a relatively high-T-c superconductivity in Y2C3 at 18 K whose T-c could be changed by synthesis conditions from 10 to 18 K [G. Amano, S. Akutagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Superconductivity at 18 K in yttrium sesquicarbide system, Y2C3 J. PhYs. Soc. Jpn. 73 (2004) 530.]. We synthesized a high-purity sample of the medium-T-c phase in Y2C3 and examined its physical properties in detail. From a specific heat measurement, Delta C(T-c)/gamma T-c. value is calculated to be 6.50 and the superconducting gap is estimated to be 2 Delta/k(B) T-c=5.40, indicating that the superconductivity in Y2C3 can be described by an s-wave strong coupling regime. (c) 2005 Elsevier Ltd. All rights reserved.

    DOI: 10.1016/j.stam.2005.11.011

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  • Preface to the special issue on the 21st century COE program of Aoyama Gakuin University 査読

    Jun Akimitsu

    Science and Technology of Advanced Materials   7 ( 1 )   1   2006年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    DOI: 10.1016/j.stam.2005.12.001

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  • Metal-semiconductor transition in novel layered oxychalcogenides 査読

    Kazumasa Horigane, Jun Akimitsu

    Science and Technology of Advanced Materials   7 ( 1 )   6 - 8   2006年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    Magnetic and electrical properties were characterized for newly synthesized La1-xSrxOCuSe (0&lt
    x&lt
    0.25) and La 1-xBaxOCuTe (0&lt
    x&lt
    0.3). Magnetic measurement showed paramagnetic behavior, but it did not obey the Curie-Weiss law, and an ordered phase was not observed down to 1.8 K in the Se and Te systems. The electrical properties of La1-xSrxOCuSe indicated a metal-semiconductor transition at approximately x=0.05 and a metallic state of a higher concentration range x. The electrical conductivity of La 0.8Sr0.2OCuSe was 209 S cm-1 at room temperature. In La1-xBaxOCuTe, electrical resistivity systematically decreased with increasing x and semiconductive behavior was observed for all x values. Metal-semiconductor transition is probably caused by the degree of orientation and difference of acceptor levels in this system. © 2005 Elsevier Ltd. All right Reserved.

    DOI: 10.1016/j.stam.2005.11.013

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  • Infrared absorption study of Ca2-xNaxCuO2X2 (X=Cl, Br) 査読

    Takumi Hasegawa, Norio Ogita, Toshihisa Kondo, Yuji Zenitani, Hirokazu Kawashima, Teruhiko Suzuki, Jun Akimitsu, Masayuki Udagawa

    LOW TEMPERATURE PHYSICS, PTS A AND B   850   533 - +   2006年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:AMER INST PHYSICS  

    IR-active phonon spectra of Ca2-xNaxCuO2X2 (X=Cl, Br) have been measured by a CsI powder method in the energy region between 250 and 4000 cm(-1) at room temperature. Two absorption peaks with the E-u symmetry have been clearly observed for the undoped crystals of Ca2CuO2Cl2 and Ca2CuO2Br2. However, the observed two peaks disappear for the Na-doped superconducting samples. From the comparison of the highest-energy E. phonon, which is the Cu-O stretching vibration, the interaction of the Cu-O bond along the CuO2 plane for the T-structure is stronger by 20% than that of the T-structure in the 2-1-4 family. To understand the effect of the apical ions, first-principles calculations of the E. phonon energy for T- and T'-structure La2CuO4 is performed and the preliminary results agree with the experimental tendency.

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  • NMR study of the orbital ordering in Lu2V2O 7 査読

    Takahisa Shiraoka, Takashi Kiyama, Masayuki Itoh, Luna Kano, Hirohiko Ichikawa, Jun Akimitsu

    AIP Conference Proceedings   850   1231 - 1232   2006年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)  

    We have made 51V NMR measurements on a single crystal to study the orbital ordering in L2V2O7 with the pryochlore structure. The anisotropy of the NMR frequency observed in an external field of 5 T for the ferromagnetic state of Lu2V 2O7 is explained well by an orbital ordering model with dxy+dyz+dzx-type orbitals. © 2006 American Institute of Physics.

    DOI: 10.1063/1.2355149

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  • Possible weak magnetism in MB<inf>6</inf> (M:Ca, Ba) probed by muon spin relaxation and muon level-crossing resonance 査読

    Kuroiwa S, Takagiwa H, Yamazawa M, Akimitsu J, Koda A, Kadono R, Ohishi K, Higemoto W, Watanabe I

    Science and Technology of Advanced Materials   7 ( 1 SPEC. ISS. )   12 - 16   2006年

  • Correlation between crystal structures of CaAlSi with and without superlattice and superconducting properties 査読

    S. Kuroiwa, H. Sagayama, T. Kakiuchi, H. Sawa, Y. Noda, J. Akimitsu

    Physical Review B - Condensed Matter and Materials Physics   74 ( 1 )   2006年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We have succeeded in growing a phase in CaAlSi with Al and Si atoms being distributed regularly in the AlSi layer corresponding to the B2 plane in the AlB2 -type structure without a superlattice along the c axis. The field-induced magnetic response in a phase exhibits an almost isotropic characteristic for the respective crystal axis. Moreover, the 2Δ(0) kB Tc and Δ Cel γN Tc estimated by heat capacity measurements are ∼3.5 and ∼1.4, respectively, indicating a BCS superconductor with a weak coupling. Although CaAlSi has a two-dimensional layered structure, these results strongly suggest that the superconductivity of the phase in CaAlSi is derived from a three-dimensional electronic ground state such as a conventional s -wave superconductor, which is markedly different from those of superstructured CaAlSi. © 2006 The American Physical Society.

    DOI: 10.1103/PhysRevB.74.014517

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  • Direct observation of the orbital state in Lu2 V2 O7: A V51 NMR study 査読

    Takashi Kiyama, Takahisa Shiraoka, Masayuki Itoh, Luna Kano, Hirohiko Ichikawa, Jun Akimitsu

    Physical Review B - Condensed Matter and Materials Physics   73 ( 18 )   2006年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    The orbital ordering of a vanadium pyrochlore compound Lu2 V2 O7 was investigated by V51 NMR measurements. With rotation of a single crystal of Lu2 V2 O7 in an external magnetic field, NMR spectra show a characteristic angle dependence reflecting V 3d orbital states and clearly identify the orbital ordering of Lu2 V2 O7. The large anisotropy of the hyperfine field represents the fact that the quadrupole moment of 3d electrons is almost fully polarized and that the orbital fluctuation is almost quenched in the ground state. The temperature dependence of the NMR spectra indicates that the magnitude of the quadrupole moment is almost invariant below TC and that the orbital ordering would probably persist above TC. © 2006 The American Physical Society.

    DOI: 10.1103/PhysRevB.73.184422

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  • Magnetic field-induced quasiparticle excitation in Nb3Sn: Evidence for anisotropic s-wave pairing 査読

    R. Kadono, K. H. Satoh, A. Koda, T. Nagata, H.K.-Furukawa, J. Suzuki, K. Ohishi, W. Higemoto, S. Kuroiwa, H. Takagiwa, J. Akimitsu

    Phys. Rev. B   74   024513/1 - 5   2006年

  • Crystal symmetry and superlattice reflections in spin-Peierls system TiOBr 査読

    T. Sasaki, T. Nagai, K. Kato, M. Mizumaki, T. Asaka, M. Takata, Y. Matsui, H. Sawa, J. Akimitsu

    Science and Technology of Advanced Materials   7 ( 1 SPEC. ISS. )   17 - 21   2006年

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    掲載種別:研究論文(学術雑誌)  

    DOI: 10.1016/j.stam.2005.11.002

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  • Magnetic response in the superconducting state of 1H-Ca(Al <inf>0.5</inf>Si <inf>0.5</inf>) <inf>2</inf> studied by μSR 査読

    Kuroiwa S, Takagiwa H, Yamazawa M, Tomita Y, Akimitsu J, Ohishi K, Koda A, Saha S.R, Kadono R, Higemoto W

    Physica B: Condensed Matter   374-375   251 - 254   2006年

  • Phase diagrams of (La,Y,Sr,Ca)(14)Cu24O41: Switching between the ladders and the chains 査読

    T Vuletic, T Ivek, B Korin-Hamzic, S Tomic, B Gorshunov, M Dressel, C Hess, B Buchner, J Akimitsu

    JOURNAL DE PHYSIQUE IV   131   299 - 304   2005年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:EDP SCIENCES S A  

    The most comprehensive charge response study of the intrinsically hole doped, spin-chain and spin-ladder composites is overviewed. Results of dc and electric-field-dependent resistivity, low frequency dielectric, and optical spectroscopy in all crystallographic directions are used to build phase diagrams of the underdoped materials (hole count= 6 - y, per formula unit, f.u.) and of the fully doped, 6 holes per f.u. The underdoped materials are insulators with hopping transport along the chains, which behave as a one-dimensional disordered system. For the fully doped materials the charge transport switches to the ladders due to transfer of holes from the chains. Two-dimensional (2D) charge-density wave (CDW) ground state is formed, i.e. besides CDW phason response along the ladders for 0 &lt;= x &lt; 9, we also found it along the ladder rungs for x &lt; 6. However, CDW sliding conductivity, as observed in the standard CDW materials, is not observed in either of directions. Normal to the ladder planes no CDW response was found, and transport is presumably hopping-like. For the first time in any system, CDW response away from the principal direction was identified. For x &gt; 9 both CDW in ladders and CO in chains are supressed.

    DOI: 10.1051/jp4:2005131076

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  • Inelastic neutron scattering study of phonons and magnetic excitations in LaCoO3 査読

    Y Kobayashi, TS Naing, M Suzuki, M Akimitsu, K Asai, K Yamada, J Akimitsu, P Manuel, JM Tranquada, G Shirane

    PHYSICAL REVIEW B   72 ( 17 )   2005年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMERICAN PHYSICAL SOC  

    We have investigated the phonon and the magnetic excitations in LaCoO3 by inelastic neutron scattering measurements. The acoustic phonon dispersions show some characteristic features of the folded Brillouin zone (BZ) for the rhombohedrally distorted perovskite structure containing two chemical formula units of LaCoO3 in the unit cell. We observed two transverse optical (TO) phonon branches along (delta delta delta), consistent with previously reported Raman active E-g modes which show remarkable softening associated with the spin-state transition [Ishikawa , Phys. Rev. Lett. 93, 136401 (2004)]. We found that the softening takes place in the TO mode over the whole BZ. In contrast, the acoustic phonons show no anomalous softening associated with the spin-state transition. The low-energy paramagnetic scattering at 8 K is weak, increasing towards a maximum at E greater than or similar to 15 meV, consistent with excitation of the nonmagnetic low-spin to magnetic intermediate-spin state of Co3+ ions.

    DOI: 10.1103/PhysRevB.72.174405

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  • Point-contact spectroscopy measurements of binary superconductor MgB2 査読

    T Takasaki, T Ekino, RA Ribeiro, T Muranaka, H Fujii, J Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   426   300 - 303   2005年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We studied the superconductor MgB2 by point-contact spectroscopy. The first derivative conductance of the current (I)-voltage (V) characteristics dI/dV(V) displays the Andreev reflection features at the superconductor-normal metal interface. The largest gap values are estimated to be Delta(4.2 K) = 9-12 meV. We also observed clear evidences for the multiple-gaps feature with the small gap Delta(4.2 K) = 1-3 meV. These gap values agree with the previous break-junction data. The observed peak positions in the second derivative conductance (d(2)I/dV(2)(V)) are consistent with the phonon density of states of inelastic neutron scattering measurements. (c) 2005 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physc.2005.01.025

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  • First-principles study of C6B2 査読

    K Kobayashi, Y Zenitani, J Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   426   374 - 380   2005年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We calculate the electronic and lattice properties of C6B2 (Na3As type crystal structure) using the first-principles molecular dynamics (FPMD) method. C6B2 is a hypothetical hexagonal compound, which consists of two B-C layers and two C-C layers. All calculated electronic band structures are metallic. C6B2 has unoccupied flat bands close to the Fermi level at the Gamma-A line. These flat bands correspond to unoccupied sigma-bands of MgB2 at the Gamma-A line. Displacements of B atoms along a B-C bond split unoccupied flat bands at the F-A line into two subbands. A total energy change with displacements of B atoms indicates anharmonicity of a B-C stretching mode. We investigate C6B2 under various compression conditions. The change in the electronic band structure under a, b-axis compression (P,,) is small. In contrast, it is quite remarkable under e-axis compression.(P-z). We find the lattice anomaly of C6B2 under a, b-axis compression. This anomaly shows a kind of negative Poisson ratios. (c) 2005 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physc.2005.01.031

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  • Break-junction tunneling spectra of MgB2: Influence of boron quality 査読

    RA Ribeiro, T Ekino, T Takasaki, T Takabatake, J Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   426   450 - 453   2005年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Tunneling spectra of a superconductor-insulator-superconductor (SIS) junction formed by the break-junction technique have been carried out for two different boron quality MgB2 samples. One is a polycrystalline MgB2 pellet (RRR approximate to 4) and the other is a high purity MgB2 wire (RRR approximate to 25) of approximately 150 mu m in diameter. Both samples exhibit a multiple-gap feature, which can be expressed by the correlated two-gap model. From the conductance fitting at 4.2 K, the pellet gap parameters are Delta(S) = 2.2 +/- 0.3 meV, Delta(M) = 6.0 +/- 1.5 meV and Delta(L) = 10 meV, whereas the only clear and very reproducible gap in the wires is the small gap at Delta(S) = 2.5 +/- 0.1 meV. The wires show almost constant values for the fitting parameters of the two-gap model, in contrast with the pellets that show a large variation. (c) 2005 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physc.2005.02.056

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  • Spatially heterogeneous character of superconductivity in MgB2 as revealed by local probe and bulk measurements 査読

    T Ekino, AM Gabovich, MS Li, T Takasaki, AI Voitenko, J Akimitsu, H Fujii, T Muranaka, M Pekala, H Szymczak

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   426   230 - 233   2005年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Controversial tunnel and point-contact experimental data on superconducting gap values in MgB2 are analyzed and interpreted in terms of the spatial inhomogeneity and the proximity effect appropriate to this layered compound. Existing specific heat measurements are explained in the same way. Break-junction tunnel studies of polycrystalline MgB2 samples are carried out and a clear-cut multiplicity of gaps is observed, although the shapes of the I-V characteristics vary substantially. (c) 2005 Published by Elsevier B.V.

    DOI: 10.1016/j.physc.2005.03.029

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  • Infrared absorption study of Ca2-xNaxCuO2X2 (X = Cl, Br) 査読

    Y Zenitani, H Kawashima, T Suzuki, J Akimitsu, N Ogita, T Kondo, M Udagawa

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   74 ( 6 )   1877 - 1878   2005年6月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    DOI: 10.1143/JPSJ.74.1877

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  • Raman scattering study of rare-earth hexaboride 査読

    N Ogita, S Nagai, M Udagawa, F Iga, M Sera, T Oguchi, J Akimitsu, S Kunii

    PHYSICA B-CONDENSED MATTER   359   941 - 943   2005年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    The excitation-energy, polarization, pressure, and temperature dependences of Raman scattering spectra have been measured for the RB6 crystals (R = Ca2+, La3+, Ce3+, Pr3+, Sm2.6+, Gd3+, Dy3+, and Yb2+). In the Raman spectra, the extra peaks have been clearly observed at around 200cm(-1), except for the Raman-active phonons and CEF excitations. The extra peaks show anomalous behavior in the temperature, polarization, and lattice parameter dependences. These anomalous behaviors originate from the vibration of the rare-earth ions in octahedral B-6 cage, and the extra peaks can be assigned as the second-order Raman excitations of T-1u phonon at Brillouin zone boundary. In the results of pressure dependence, the valence of all Sm ions in SmB6 becomes trivalent above 20 GPa at room temperature. (c) 2005 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physb.2005.01.270

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  • Orbital fluctuations in ground state of YTiO3 : Ti-47,Ti-49 NMR study 査読

    T Kiyama, H Saitoh, M Itoh, K Kodama, H Ichikawa, J Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   74 ( 4 )   1123 - 1126   2005年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    Ti-47.49 NMR spectra of a single crystal of YTiO3 have been obtained at 1.5 K in the ferromagnetically ordered state under all external field of 3 T. With rotation of the sample in the magnetic field, the spectra show a characteristic angle dependence which reflects Ti 3d orbital ordering. The observed spectra indicate that the Ti 3d quadrupole moment is markedly reduced from that predicted by the orbital ordering model based on the Hartree-Fock and GGA band calculations. The reduction of the quadrupole moment is considered to originate from quantum fluctuations in the t(2g) orbital states, taking into account that the effective spin-orbit interaction and itinerancy are not Strong in this compound.

    DOI: 10.1143/JPSJ.74.1123

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  • Manifestations of inhomogeneity in MgB2: from specific heat to tunnel measurements 査読

    T Ekino, AM Gabovich, MS Li, T Takasaki, AI Voitenko, J Akimitsu, H Fujii, T Muranaka, M Pekala, H Szymczak

    PHYSICA B-CONDENSED MATTER   359   460 - 462   2005年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Tunnel conductances of superconducting polycrystalline MgB2 samples were measured by the break-junction technique. Three main gap-like peculiarities are observed, while other ones scatter from sample to sample. These results correlate with heat capacity measurements, an analysis which shows a wide range of energy gaps involved. (c) 2005 Published by Elsevier B.V.

    DOI: 10.1016/j.physb.2005.01.097

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  • Orbital ordering in ferromagnetic Lu2V2O7 査読

    H Ichikawa, L Kano, M Saitoh, S Miyahara, N Furukawa, J Akimitsu, T Yokoo, T Matsumura, M Takeda, K Hirota

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   74 ( 3 )   1020 - 1025   2005年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We have observed the orbital ordering in the ferromagnetic Mott-insulator Lu2V2O7 by the polarized neutron diffraction technique. The orbital ordering pattern determined from the observed magnetic form factors can be explained in terms of a linear combination of wave functions vertical bar yz &gt;, vertical bar zx &gt; and vertical bar xy &gt;;
    vertical bar 0 &gt; = root 1/3 vertical bar xy &gt; + root 1/3 vertical bar yz &gt; +root 1/3 vertical bar zx &gt;
    proportional to vertical bar(x + y + z)(2) - r(2)&gt;,
    where each orbital is extended toward the center-of-mass of the V tetrahedron. We discuss the stability of the ferromagnetic Lu2V2O7, using a Hubbard Hamiltonian with these three orbitals.

    DOI: 10.1143/JPSJ.74.1020

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  • X-ray diffraction study of MgB2 at low temperatures 査読

    Y Xue, S Asada, A Hosomichi, S Naher, J Xue, H Kaneko, H Suzuki, T Muranaka, J Akimitsu

    JOURNAL OF LOW TEMPERATURE PHYSICS   138 ( 5-6 )   1105 - 1115   2005年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:SPRINGER/PLENUM PUBLISHERS  

    We have performed powder X-ray diffraction of MgB2 superconductor between 10 and 300 K. Temperature dependence of integrated intensities of both (002) and (110) X-ray reflections shows a peak at around T-C, superconducting transition temperature. The integrated intensity of the X-ray reflection is related to the phonon frequency through a Debye-Waller factor. Lattice parameters a and c show negative thermal expansions at low temperatures. The negative thermal expansion might be due to an electronic origin and not directly related to the superconducting transition.

    DOI: 10.1007/s10909-004-2903-2

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  • Superconducting phase diagram in Ca2-xNaxCuO2Cl2-yBry 査読

    Y Zenitani, T Suzuki, H Kawashima, J Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   419 ( 1-2 )   32 - 40   2005年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Ca2CuO2Cl2-yBry (0 less than or equal to y less than or equal to 2) has the K2NiF4-tYPe structure with halogens (Cl and Br) at their apical sites and superconductivity can be achieved by substituting Ca2+ sites with Na+. We have succeeded in making a comprehensive superconducting phase diagram of Ca2-xNaxCuO2CI2-yBry by focusing on special attention to the relationship between the superconductivity and the Cu and Cl/Br distance in Ca2-xNaxCuO2Cl2-yBry. We were successful in continuously changing the length of c-axis, which is closely related to the distance between the apical site and the CuO2 plane, or the distance between Cu and Cl/Br. Without changing the bonding distance between Cu and O in the CuO2 plane. This is the first example of this kind of control in cuprate superconductors. (C) 2004 Published by Elsevier B.V.

    DOI: 10.1016/j.physc.2004.12.002

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  • Superconductivity in intermetallic W7Re13X (X = B and C) compounds 査読

    K Kawashima, A Kawano, T Muranaka, J Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   74 ( 2 )   700 - 704   2005年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We discovered superconductivity in W7Re13X (X = B and C) (T-c = 7.1 K and 7.3 K, respectively), and their superconducting properties are discussed. The crystal structure of W7Re13X is beta-Mn-type (cubic, space group: P4(1)32), and Re atoms form a tilted octahedron. The magnetization (M-H) curves show typical type-II superconducting behavior, and the lower critical field H-c1(0), the upper critical field H-c2(0) and the Ginzburg-Landau (GL) parameter kappa are 7.7 mT, 11.4 T and 54 for X = B and 4.0 mT, 12.6 T and 80 for X = C, respectively. From the specific heat measurement, the superconducting gap magnitudes are estimated to be 2Delta/k(B)T(c) = 4.28 and 4.02, respectively.

    DOI: 10.1143/JPSJ.74.700

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  • Anisotropic charge modulation in ladder planes of Sr14-xCaxCu24O41 査読

    T Vuletic, T Ivek, B Korin-Hamzic, S Tomic, B Gorshunov, P Haas, M Dressel, J Akimitsu, T Sasaki, T Nagata

    PHYSICAL REVIEW B   71 ( 1 )   2005年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMERICAN PHYSICAL SOC  

    The charge response of the ladders in Sr14-xCaxCu24O41 is characterized by dc resistivity, low frequency dielectric, and optical spectroscopy in all three crystallographic directions. The collective charge-density wave screened mode is observed in the direction of the rungs for x=0, 3, and 6, in addition to the mode along the legs. For x=8 and 9, the charge-density-wave response along the rungs fully vanishes, while the one along the legs persists. The transport perpendicular to the planes is always dominated by hopping.

    DOI: 10.1103/PhysRevB.71.012508

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  • Hole-induced novel spin state within Haldane gap in Nd2-xCaxBaNiO5 査読

    T Sasaki, T Yokoo, S Katano, J Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   74 ( 1 )   267 - 270   2005年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    Static and dynamical spin properties in a hole-doped one-dimensional Haldane chain system have been investigated by magnetization and neutron scattering measurements on antiferromagnetic Nd2-xCaxBaNiO5. Spin-glass-like weak ferromagnetic behavior and an incommensurate dynamical structure were observed, which are similar to those reported in the Haldane system Y2-xCaxBaNiO5. A novel excitation, moreover, has been revealed at 10 meV because of complete the shift in the Haldane gap due to three-dimensional antiferromagnetic ordering, which possibly provides a total picture of a hole-induced quantum Haldane chain.

    DOI: 10.1143/JPSJ.74.267

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  • Superconductivity in MgB2 査読

    T. Muranaka, Y. Zenitani, J. Shimoyama, J. Akimitsu

    Frontiers in Superconducting Materials   937 - 981   2005年

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    記述言語:英語   掲載種別:論文集(書籍)内論文   出版者・発行元:Springer Berlin Heidelberg  

    DOI: 10.1007/3-540-27294-1_26

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  • Superconductivity in MgB2 and its related materials 査読

    J Akimitsu, S Akutagawa, K Kawashima, T Muranaka

    PROGRESS OF THEORETICAL PHYSICS SUPPLEMENT   ( 159 )   326 - 337   2005年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PROGRESS THEORETICAL PHYSICS PUBLICATION OFFICE  

    After the discovery of MgB2, much attention has been paid to the boride and carbide systems. In this paper, we mainly review the following themes. First, we review the present situation of the superconductivity in MgB2, which can be interpreted in terms of BCS-type phonon mediated pairing. In particular, we focus on the relationship between the superconductivity and the crystal structure in MgB2 and two gap feature. Second, we review our present status for the new superconductors including the boron and carbon systems. Recently, we discovered new superconductivity in related compounds W7Re13X (X = B and C) (T, = 7.1 K and 7.3 K, respectively) and Y2C3 (T = 18 K), and we elucidated their superconducting properties. The crystal structure Of W7Re13X (X = B and C) is beta-Mn type (cubic, space group: P4(1)32), and Re atoms form a distorted octahedron. The superconductivity in W7Re13X system is recognized as strong coupling s-wave symmetry from the specific heat measurement. We discovered the superconductivity at T-c = 18 K in Y2C3, although this material with a maximum T-c of 11.5 K has already investigated by Krupka et al. The crystal structure Of Y2C3 is body centered cubic (Pu2C3-type) structure, and Y atoms are one-dimensionally aligned along [111] axis. The lattice parameter, alpha, obtained in our work is between 8.18-8.23 angstrom, and is smaller than those previously reported (8.21-8.25 angstrom).

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  • Superconductivity and magnetism in ladder and chain compounds physics of (Sr,Ca)14Cu041O41 査読

    M. Uehara, N. Motoyama, M. Matsuda, H. Eisaki, J. Akimitsu

    Frontiers in Magnetic Materials   573 - 609   2005年

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    記述言語:英語   掲載種別:論文集(書籍)内論文   出版者・発行元:Springer Berlin Heidelberg  

    The study of ladder materials has in recent years become one of the central issues in the field of condensed matter physics. Ladder materials, which contain a structural unit composed of coupled arrays of one-dimensional (1D) chains, are considered to be an interesting intermediate step between one- and two-dimensional (2D) systems [1-5]. In fact, it is now recognized that the crossover from 1D to 2D is far from straightforward and that ladder materials by themselves exhibit a variety of new physics. Historically, studies on ladder material were motivated by the discovery of hightransition- temperature (high-Tc) superconductors occurring in 2D spin-1/2 (S=1/2) Heisenberg antiferromagnets [6]. Two theoretical predictions have particularly triggered extensive research [1,2]. First, ladders made from an even number of spin-1/2 Heisenberg chains (even-leg ladders) are expected to have a unique spin-liquid ground state with short range spin correlation, where there exists a finite energy gap ("spin gap") to the lowest excited state. The spin gap, more frequently called the pseudogap, is a key feature of high-Tc superconductors, particularly in the low carrier concentration region. Ladder materials are in this context considered to be good references to gain insights into the pseudogap physics of high-Tc superconductors. Second, and a more intriguing hypothesis, is the possible occurrence of superconductivity in even-leg ladders when they are doped with holes, similar to the high-Tc materials in which holes are doped into 2D copper-oxygen (CuO2) planes. The symmetry of pairing is predicted to be d-wave like [7], adding to the similarities between doped ladders and doped 2D planes. Stimulated by these theoretical suggestions, intensive experimental studies have been carried out, with a variety of cuprate ladder materials having been newly discovered. For instance, (i) the existence of a spin gap was confirmed for SrCu 2O3, a prototype of a twoladder compound [8,9], (ii) hole-doping into two-legged ladders was first achieved on LaCuO2.5 by replacing La with Sr [10], and (iii) superconductivity at 12.5 K was observed in Sr14-xCaxCu24O41 under a pressure of 3 GPa [11]. Here, we review experimental results on cuprate spin ladder materials, emphasizing those for Sr14-xCaxCu 24O41. In Sec. 1.2, brief theoretical background is provided, while Sec. 1.3 introduces typical cuprate spin ladder materials. In Chap. 2, we review the magnetic properties of hole-doped edge-sharing S=1/2 chains, a constituent of (Sr,Ca)14Cu24O41, after which the normal state properties of (Sr,Ca)14Cu24O41 are considered in Chap. 3, with emphasis on the effect of Ca substitution on charge dynamics of hole-doped two-leg ladders. Superconducting properties are then discussed in Chap. 4, being followed by Chap. 5 which discusses the consistency between theory and experiments of ladder and summarizes novel physical insights obtained by a series of experiments on (Sr,Ca)14Cu24O 41. Readers should note that our review is in addition to other excellent reviews on spin ladder system [3-5]. © 2005 Springer-Verlag Berlin Heidelberg.

    DOI: 10.1007/3-540-27284-4_19

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  • Correlation between superconducting carrier density and transition temperature in NbB2+x 査読

    Takagiwa H, Kuroiwa S, Yamazawa M, Akimitsu J, Ohishi K, Koda A, Higemoto W, Kadono R

    Journal of the Physical Society of Japan   74 ( 5 )   1386 - 1389   2005年

  • X-ray angle-resolved photoemission spectroscopy of CaB<SUB>6</SUB> 査読

    S. Souma, T. Takahashi, H. Komatsu, T. Sato, H. Matsui, N. Kimura, H. Aoki, S. Kunii, J. Akimitsu

    Phys. Rev. B   70   073104-1 - 073104-4   2004年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    DOI: 10.1103/PhysRevB.70.073104

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  • Energy gap structure in the binary superconductor MB2 (M = Nb and Mg) 査読

    T Takasaki, T Ekino, H Takagiwa, T Muranaka, H Fujii, J Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   412   266 - 269   2004年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Tunneling spectroscopy measurements have been carried out using a break-junction technique for the AlB2-type superconductors NbB2 (T-c approximate to 7-9 K) and MgB2 (T-c = 39 K). The representative gap value for NbB2 is 2Delta(4.2 K) = 2.4 +/- 0.2 meV, from which the gap ratio of 2Delta(0)/k(B)T(c) approximate to 4.4 is obtained. Such a strong-coupling ratio is comparable to the ratio 2Delta(0)/k(B)T(c)greater than or equal to5 for MgB2, thereby indicating the similar superconducting mechanisms between them. (C) 2004 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.physc.2003.12.038

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  • Raman scattering study of CaB6 and YbB6 査読

    N. Ogita, S. Nagai, N. Okamoto, F. Iga, S. Kunii, T. Akamatsu, J. Akimitsu, M. Udagawa

    Journal of Solid State Chemistry   177   461-465   2004年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    Phonon spectra of CaB6 and RB6 (R=Yb, Ce, and Pr) have been investigated by Raman scattering. We found clear spectral difference between divalent cation hexaboride and trivalent one. E, mode shows the doublet spectra for only the divalent crystals of CaB6 and YbB6. The doublet spectra are understood by the two-dimensional charge distribution on B-6 without lattice distortion. In addition, the scattered intensities of the phonons change at around the ferromagnetic Curie temperature for YbB6 and at T similar or equal to 600 K for CaB6. These are the characteristic temperatures due to the change of the electronic system.

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  • Tunneling measurements of superconducting NbB2 査読

    T Ekino, T Takasaki, H Takagiwa, J Akimitsu, H Fujii

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   408   828 - 829   2004年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We have measured superconducting tunneling spectra of NbB2. This compound is isostructural to the high-T-c superconductor MgB2. The most representative spectra at 4.2 K exhibit intensive gap-edge peaks at +/-2.2-2.6 mV (= +/-2Delta/e) for the superconductor-insulator-superconductor (SIS) break junction. Upon warming, the gap feature disappears at T-c = 6.5-7 K. This leads to the gap ratio 2Delta(0)/k(B)T(c) = 4.3-4.5 with the extrapolated Delta(0) using the BCS prediction. The present result demonstrates that NbB2 is a strong-coupling superconductor with the gap ratio comparable to that for MgB2. (C) 2004 Published by Elsevier B.V.

    DOI: 10.1016/j.physc.2004.03.149

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  • Multiple-gap features from break-junction tunneling in the superconducting MgB2 査読

    T Takasaki, T Ekino, T Muranaka, T Ichikawa, H Fujii, J Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   73 ( 7 )   1902 - 1913   2004年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    Tunneling spectroscopy measurements on polycrystalline MgB2 have been carried out with a break-junction technique. The observed spectra exhibit the multiple-gap feature, which can be expressed by the correlated two-gap model. From the conductance fittings at 4.2 K, the gap parameters can be classified into Delta(S) = 2.2 +/- 0.3 meV, Delta(M) = 6.0 +/- 1.5 meV, and Delta(L) congruent to 10 meV. All these gaps exist up to the same critical temperature T-c. The Delta(M) undergoes various ways of temperature dependence, which can be attributed to the proximity-induced gap. We have also observed the conductances showing the features of normal conducting and/or low-T-c phases. Our tunneling data can be explained by the proximity effect combined with the bulk multiple-gap features.

    DOI: 10.1143/JPSJ.73.1902

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  • Observation of field-induced long-range order in disordered Sr14Cu24O41 査読

    S. Ohsugi, S. Matsumoto, Y. Kitaoka, M. Matsuda, M. Uehara, T. Nagata, J. Akimitsu

    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS   272   E683 - E684   2004年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Field-induced magnetic order in a structural disordered single-crystal Sr14Cu24O41 (Sr-14-B) has been observed in the Cu-nuclear magnetic resonance (NMR) measurements for the ladder sites below 20 K under the magnetic field H similar to 11 T. For the Zhang Rice (ZR) sites in the chains, the spectrum splits below 20 K due to an appearance of the internal field, whereas that of less disordered Sr-14-A does not. This gives an evidence of three-dimensional (3D) long-range (LR) ordering in Sr-14-B below T-N similar to 20 K under H similar to 11 T. (C) 2004 Elsevier B. V. All rights reserved.

    DOI: 10.1016/j.jmmm.2003.11.337

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  • Orbital ordering in YTiO3: TiNMR in a single crystal 査読

    Itoh, I, T Kiyama, K Kodama, J Akimitsu

    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS   272   90 - 91   2004年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Ti-47.49 NMR measurements on a single crystal of a ferromagnetic Mott-Hubbard insulator, YTiO3, have been made to study orbital ordering. In particular we have measured frequency-swept NMR spectra in the ferromagnetic state under an external field of 3 T. We analyze the NMR spectra and show that an orbital ordering model in Hartree-Fock and band calculations might be not suitable. (C) 2003 Elsevier B.V. All rights reserved.

    DOI: 10.1016/j.jmmm.2003.11.392

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  • Evidence of electron-phonon interaction in Al-substituted Mg1-xAlxB2 査読

    T Yokoo, T Muranaka, M Arai, K Papagelis, K Prassides, J Taylor, SM Bennington, J Akimitsu

    JOURNAL OF SUPERCONDUCTIVITY   17 ( 2 )   199 - 203   2004年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:KLUWER ACADEMIC/PLENUM PUBL  

    Phonon density of states in Al substituted Mg1-xAlxB2 has been studied by means of inelastic neutron scattering experiments with pulsed neutron time-of-flight technique. Phonon dynamics have a significant role for realizing superconductivity in the recent discovered superconductor MgB2. We present a systematic investigation on generalized phonon density of states (PDOS) up to E = 120 meV on Al content of x = 0.1, 0.25, and 0.5. We have clearly observed a large effect on the PDOS by Al substitution; a large energy shift occurs around at 70 meV and 89 meV. We have also observed a visible increase of PDOS at a particular energy range below T-c. Those phonon states are related with E-2g mode to be expected to have a strong coupling with the electron states at the Gamma-A region.

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  • Unconventional behavior of field-induced quasiparticle excitation in Ca(Al <inf>0.5</inf>Si <inf>0.5</inf>) <inf>2</inf> 査読

    Kuroiwa S, Takagiwa H, Yamazawa M, Akimitsu J, Ohishi K, Koda A, Higemoto W, Kadono R

    Journal of the Physical Society of Japan   73 ( 10 )   2631 - 2634   2004年

  • Nuclear spin-lattice relaxation of single crystal Sr14Cu24O41 査読

    S Ohsugi, S Matsumoto, Y Kitaoka, M Matsuda, M Uehara, T Nagata, J Akimitsu

    HYPERFINE INTERACTIONS   159 ( 1-4 )   9 - 14   2004年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:SPRINGER  

    Nuclear spin-lattice relaxation rate T-1(-1) has been measured for the ladder sites of two single crystals Sr14Cu24O41 (Sr14-A,B) by Cu-63 NMR/NQR. The hole localization around 100 K appears as a peak in the T variation of T-1(-1) (NQR). Oil the other hand, it is suppressed in the T-1(_-1)(NMR) data Under the magnetic field H similar to 11 T,and a new peak appears around 20 K. T-1(-1)(NMR) around the peak is more enlarged for Sr14-B than for Sr14-A. Hence, holes on the ladders of Sr14-B tend to be more localized. This is considered to be an origin for the occurence of the magnetic order in Sr14-B Under H similar to 11 T.

    DOI: 10.1007/s10751-005-9074-7

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  • Raman scattering investigation of RB6 (R = Ca, La, Ce, Pr, Sm, Gd, Dy, and Yb) 査読

    N. Ogita, S. Nagai, N. Okamoto, M. Udagawa, F. Iga, M. Sera, J. Akimitsu, S. Kunii

    Physical Review B   68   224305(1-9)   2003年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    The excitation-energy, polarization, and temperature dependeces of Raman scattering spectra have been measured for the RB6 crystals (R=Ca2+, La3+, Ce3+, Pr3+, Sm2.6+, Gd3+, Dy3+, and Yb2+). In the divalent crystals of CaB6 and YbB6, the Eg phonon shows doublet, while the line shape is a broad single peak for the trivalent case. In the trivalent crystals, two kinds of the extra peaks have been clearly observed at around 200 cm-1 and at around 1400 cm-1 in addition to the Raman-active phonons and crystal electric-field (CEF) excitations. The peak intensities at around 200 cm-1 show the anomalous decrease with decreasing<br />
    temperature, and this temperature dependence correlates with that of the mean-square displacement of the R ion. Furthermore, their energies decrease with the increase of the cage space due to B6. These peaks are originated from the second-order excitation of acoustic phonons. The broad peak at around 1400 cm-1 is assigned as the second-order excitation of the T2g phonon. For the T2g phonon, the energy difference and its line-shape change between the trivalent and divalent crystals suggests the existence of electron-phonon interaction. We have observed the CEF excitat

    DOI: 10.1103/PhysRevB.68.224305

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  • Structural modulation induced by the incommensurate antiferromagnetic phase transition in ZnCr2Se4 査読

    M Hidaka, M Yoshimura, N Tokiwa, J Akimitsu, YJ Park, JH Park, SD Ji, KB Lee

    PHYSICA STATUS SOLIDI B-BASIC RESEARCH   236 ( 3 )   570 - 577   2003年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:WILEY-V C H VERLAG GMBH  

    The structural properties of normal-type spinel ZnCr2Se4 have been studied as a function of temperature by means of X-ray diffraction using synchrotron radiation. It is found that a structural phase transition from cubic to orthorhombic symmetry occurs at 22.5 K (T-c). It is also found that, below T-c, Se ions are cooperatively displaced in a pseudo-tetragonal (001) plane. Since the Cr3+ ion has no orbital angular momentum, a CrSe6 octahedron is not distorted by a crystal field produced by surrounding Se2- anions. It is interpreted that the pseudo-tetragonal displacements of Se ions are induced by magnetostriction among Cr3+ ions in CrSe2 chains, in which Cr ions share two Se2- with each other along &lt;110&gt;.

    DOI: 10.1002/pssb.200301551

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  • Structural modulation of spinel ZnCr2Se4 in the vicinity of antifferromagnetic phase transition induced by an applied magnetic field 査読

    M. Hidaka, M. Yoshimura, S. Takahasi, S. Watanabe, J. Akimitsu

    Physica Status Solidi (B) Basic Research   236 ( 1 )   209 - 218   2003年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    Magnetic and structural phase transitions of normal-type spinel ZnCr2Se4 have been studied as functions of applied magnetic field and temperature by means of neutron diffraction. It is found that the spiral spin order of Cr3+ ions is largely affected by the magnetic field, and the satellite-like magnetic reflections are largely decreased in the magnetic field of 0.6 T &lt
    H &lt
    1.2 T and disappear at H ≥ 1.2 T, when the magnetic field was applied vertically to an orthorhombic axis [001]. This means that the spiral configuration of the ferromagnetic CrSe4 chains is perfectly disordered around two-fold screw axes in an orthorhombic symmetry phase (Fddd structure) by the magnetic field H ≥ 1.2 T, though the structural phase transition (Tc) between cubic and orthorhombic symmetry phases simultaneously occurs at the antiferromagnetic phase transition temperature (TN ≈ 21 K) in no magnetic field. It is also found that the main reflections show a magnetic field dependence similar to that of the magnetic reflections. Pseudo-tetragonal displacements of Se ions below TN are modulated by the applied magnetic field. The 3 T magnetic field induces a metastable structural transition at about 15 K, after initially cooling down to about 2 K in no applied magnetic field. The transition is related to the modulation from pseudo-tetragonal to pseudo-cubic displacements of Se ions.

    DOI: 10.1002/pssb.200301556

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  • Tunneling spectroscopy of the superconducting gap in MgB2 査読

    T Ekino, T Takasaki, T Muranaka, J Akimitsu, H Fujii

    PHYSICAL REVIEW B   67 ( 9 )   2003年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    The tunneling technique with a break junction has been applied to investigate the multiple superconducting gaps in MgB2. The observed gap features can be explained by combining inherent and induced order parameters. The largest gap of Delta = 9-10 meV corresponds to a very strong-coupling ratio 2Delta/k(B)T(c) = 5-6. Upon warming, this gap decreases in accordance with the BCS dependence, while the smaller gaps reduce faster than that but persist up to T-c. Therefore, the smallest gap of Delta = 2-2.5 meV gives 2Delta/k(B)T(c) = 1.2-1.5. The magnetic field can distinguish the behavior of the gap. The extrapolated highest gap-closing field for the largest gap agrees with the upper-critical field, thereby indicating that this gap is predominant to MgB2.

    DOI: 10.1103/PhysRevB.67.094504

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  • Correlation between the structural and antiferromagnetic phase transitions in ZnCr2Se4 査読

    M Hidaka, N Tokiwa, M Fujii, S Watanabe, J Akimitsu

    PHYSICA STATUS SOLIDI B-BASIC RESEARCH   236 ( 1 )   9 - 18   2003年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:WILEY-V C H VERLAG GMBH  

    Structural properties of normal-type spinel ZnCr2Se4 have been studied as a function of temperature by means of neutron diffraction. It is found that the structural phase transition (T-C) from a cubic to orthorhombic symmetry simultaneously occurs at the antiferromagnetic phase transition temperature (T-N similar to 21 K), where the cubic and orthorhombic symmetry is O-7(h)-Fd3m and D-24(2h)-Fddd, respectively. The structural phase transition is mainly characterized by the cooperative displacements of Se2- ions, which are induced by a magnetostriction resulting from the magnetic interaction of Cr3+ ions in CrSe4 chains along [110]. The antiferromagnetic magnetic structure below TN is made by a spiral magnetic coupling between the ferromagnetic CrSe4 chains around crystallographic screw axes, which are parallel to an orthorhombic C axis. The spiral long-range order of the spins of Cr3+ is incommensurate and its periodicity of q along a reciprocal lattice axis C* shows temperature dependence from about 0.47 (2.5 K) to 0.43 (18 K). The spiral periodicity of the spin order along the C axis shows a first order transition at about 21 K.

    DOI: 10.1002/pssb.200301502

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  • Suppression of the Charge-Density-Wave State in [Formula presented] by Calcium Doping 査読

    T. Vuletić, B. Korin-Hamzić, S. Tomić, B. Gorshunov, P. Haas, T. Rõõm, M. Dressel, J. Akimitsu, T. Sasaki, T. Nagata

    Physical Review Letters   90 ( 25 )   4   2003年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    The charge response in the spin chain and/or ladder compound [Formula presented] is characterized by dc resistivity, low-frequency dielectric spectroscopy and optical spectroscopy. We identify a phase transition below which a charge-density wave (CDW) develops in the ladder arrays. Calcium doping suppresses this phase with the transition temperature decreasing from 210 K for [Formula presented] to 10 K for [Formula presented], and the CDW gap from 130 meV down to 3 meV, respectively. This suppression is due to the worsened nesting originating from the increase of the interladder tight-binding hopping integrals, as well as from disorder introduced at the Sr sites. These results altogether speak in favor of two-dimensional superconductivity under pressure. © 2003 The American Physical Society.

    DOI: 10.1103/PhysRevLett.90.257002

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  • Electronic Band Structure and Fermi Surface of [Formula presented] Studied by Angle-Resolved Photoemission Spectroscopy 査読

    S. Souma, H. Komatsu, T. Takahashi, R. Kaji, T. Sasaki, Y. Yokoo, J. Akimitsu

    Physical Review Letters   90 ( 2 )   4   2003年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We report high-resolution angle-resolved photoemission spectroscopy (ARPES) on [Formula presented]. The band structure determined by ARPES shows a 1 eV energy gap at the [Formula presented] point between the valence and the conduction bands. We found a small electron pocket at the [Formula presented] point, whose carrier number is estimated to be [Formula presented], in good agreement with the Hall resistivity measurement with the same crystal. The experimental results are discussed in comparison with band structure calculations and theoretical models for the high-temperature ferromagnetism. © 2003 The American Physical Society.

    DOI: 10.1103/PhysRevLett.90.027202

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  • Magnetic penetration depth of a new boride superconductor Re3B 査読

    Takagiwa H, Kawano A, Mizuta Y, Yamamoto T, Yamada M, Ohishi K, Muranaka T, Akimitsu J, Higemoto W, Kadono R

    Physica B-Condensed Matter   326 ( 1-4 )   355 - 358   2003年

  • Resistivity and thermopower of CaB6 single crystal 査読

    K. Yagasaki, S. Notsu, Y. Shimoji, T. Nakama, R. Kaji, T. Yokoo, J. Akimitsu, M. Hedo, Y. Uwatoko

    Physica B: Condensed Matter   329-333 ( II )   1259 - 1260   2003年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:Elsevier  

    The resistivity p and thermopower S of CaB6 single crystal have been measured at temperatures from 2 to 1100 K in magnetic fields up to 15 T. Temperature dependence of ρ is expressed as [ρ(T)-ρ(0)]oc exp[-(T/T)1/2] for T≤90 K and T1.7 for 90 K≤ T≤1100 K, where p0 = 232 μΩ cm. The magnitude of ρ and S is very large suggesting thin concentration of carriers. The temperature dependence of them suggests the conduction is metallic and electron type. ρ and S can be fitted on the basis of free electron model and obtained the Fermi level is situated near the bottom of an electron band with the Fermi energy of 80 K. The experimental and fitting results show this compound is semi-metallic. No scattering anomalies in temperature dependence of ρ were observed and also no sudden change of the Fermi surface accompanied with magnetic transition in S above 100 K including the region where it might be considered to have ferromagnetic transition. © 2003 Elsevier Science B.V. All rights reserved.

    DOI: 10.1016/S0921-4526(02)02217-2

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  • Development of a high-field ESR system under high pressure 査読

    H. Ohta, T. Sakurai, S. Okubo, M. Saruhashi, T. Kunimoto, Y. Uwatoko, J. Akimitsu

    Journal of Physics Condensed Matter   14 ( 44 )   10637 - 10640   2002年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We developed a new high-field ESR system under pressure. A clamped-type pressure cell with sapphire pistons is used to transmit the electromagnetic wave. The first measurements on the spin-Peierls system CuGeO3 under pressure and our development of the new standard of pressure for high-field ESR are presented.

    DOI: 10.1088/0953-8984/14/44/348

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  • Superconducting gap in polycrystalline MgB2 studied by electron tunneling 査読

    T Takasaki, T Ekino, T Muranaka, H Fujii, J Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   378   229 - 233   2002年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Electron-tunneling measurements on the new superconductor MgB2 with T-c = 39 K have been carried out using a break-junction technique. The measurements reveal the largest gap value of 2Delta = 17-19 meV at 4.2 K. This value leads to the ratio 2Delta/k(B)T(c) = 5-5.6, indicating the strong-coupling superconductivity. We have also observed the reproducible gap values of 2Delta congruent to 4-4.5 meV and 7-9 meV. The smallest gap exists even beyond 30 K. We will discuss the implication of the data. (C) 2002 Published by Elsevier Science B.V.

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  • Charge-density wave formation in Sr14-xCaxCu24O41 査読

    B. Gorshunov, P. Haas, T. Rõõm, M. Dressel, T. Vuletić, B. Korin-Hamzić, S. Tomić, J. Akimitsu, T. Nagata

    Physical Review B - Condensed Matter and Materials Physics   66 ( 6 )   605081 - 605084   2002年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    The electrodynamic response of the spin-ladder compound Sr14-xCaxCu24O41(x=0,3,9) has been studied from radio frequencies up to the infrared. At temperatures below 250 K a pronounced absorption peak appears around 12 cm-1 in Sr14Cu24O41 for the radiation polarized along the chains/ladders (E∥c). In addition a strongly temperature-dependent dielectric relaxation is observed in the kHz - MHz range. We explain this behavior by a charge-density wave which develops in the ladders and produces a phason mode pinned at 12 cm-1. With increasing x the mode shifts up in frequency and eventually disappears for x=9 because the dimensionality of the system crosses over from one to two dimensions, giving way to the superconducting ground state under pressure.

    DOI: 10.1103/PhysRevB.66.060508

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  • SEM, STM/STS and heavy ion irradiation studies on magnesium diboride superconductor 査読

    H Narayan, SB Samanta, A Gupta, AV Narlikar, R Kishore, KN Sood, D Kanjilal, T Muranaka, J Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   377 ( 1-2 )   1 - 6   2002年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We have presented here the results of SEM and STM/STS investigations carried out on MgB2 superconductor. The SEM pictures show porous surface and well-linked granular structure in which bigger grains (few micrometer size) seem to be agglomeration of smaller, nearly hexagonal grains (size nearly 100 nm). Hexagonal structure of Mg and B layers have been directly observed in atomically resolved STM images. The lattice constants have been determined to be a(Mg) = 3.1 Angstrom, a(B) = 1.7 Angstrom and c = 3.5 Angstrom. Grain boundaries (GB) of width ranging from 50 to 200 Angstrom have been observed. Absence of weak link effects despite wide GBs has been attributed to the metallic nature of the amorphous region of the GB interior as inferred from STS analysis. Irradiation with 200 MeV Ag-107 ions gives tracks of about 65 Angstrom in the bulk of the sample. This is expected to give higher value of critical current density owing to the flux pinning mechanism, which is related to possibility of wide practical application of this material. (C) 2002 Elsevier Science B.V. All rights reserved.

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  • Evidence for high-frequency phonon mediated S-wave superconductivity: B-11 NMR study of Al-doped MgB2 査読

    H Kotegawa, K Ishida, Y Kitaoka, T Muranaka, N Nakagawa, H Takagiwa, J Akimitsu

    PHYSICAL REVIEW B   66 ( 6 )   2002年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMERICAN PHYSICAL SOC  

    We report B-11 NMR studies that address the underlying two superconducting (SC) gaps model and a possible mechanism for interpreting a high SC transition temperature (T-c) of similar to40 K in a recently discovered MgB2. We reported in a previous paper [Phys. Rev. Lett. 87, 127001 (2001)] that the results of the nuclear-spin-lattice relaxation rate, 1/T-1 in the SC state revealed the existence of a single large SC gap with 2Delta/k(B)T(c)similar to5. This conclusion is corroborated by the calculation that any two-gap models cannot account for the experimental results. In Al-doped MgB2, the result of 1/T-1 in the SC state revealed that a size in the SC gap is not changed by substituting Al for Mg. The reduction in T-c by Al doping is shown to be due to the decrease of the density of states N(E-F) at the Fermi level. According to the McMillan equation, an experimental relation between T-c and the relative change in N(E-F) allowed us to estimate a characteristic phonon frequency omegasimilar to700 K and an electron-phonon coupling constant lambdasimilar to0.87. These results suggest that the high-T-c superconductivity in MgB2 is mediated by the strong electron-phonon coupling with high-frequency phonons.

    DOI: 10.1103/PhysRevB.66.064516

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  • Charge-density wave formation in Sr14-xCaxCu24O41 査読

    B Gorshunov, P Haas, T Room, M Dressel, T Vuletic, B Korin-Hamzic, S Tomic, J Akimitsu, T Nagata

    PHYSICAL REVIEW B   66 ( 6 )   2002年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    The electrodynamic response of the spin-ladder compound Sr14-xCaxCu24O41(x=0,3,9) has been studied from radio frequencies up to the infrared. At temperatures below 250 K a pronounced absorption peak appears around 12 cm(-1) in Sr14Cu24O41 for the radiation polarized along the chains/ladders (Eparallel toc). In addition a strongly temperature-dependent dielectric relaxation is observed in the kHz - MHz range. We explain this behavior by a charge-density wave which develops in the ladders and produces a phason mode pinned at 12 cm(-1). With increasing x the mode shifts up in frequency and eventually disappears for x=9 because the dimensionality of the system crosses over from one to two dimensions, giving way to the superconducting ground state under pressure.

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  • Cu-NMR study on disordered Sr14Cu24O41 査読

    S Ohsugi, S Matsumoto, Y Kitaoka, M Matsuda, M Uehara, T Nagata, J Akimitsu

    ZEITSCHRIFT FUR NATURFORSCHUNG SECTION A-A JOURNAL OF PHYSICAL SCIENCES   57 ( 6-7 )   509 - 512   2002年6月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:VERLAG Z NATURFORSCH  

    The ladder-Cu NMR spectrum of a structural disordered single crystal Sr14Cu24O41 (Sr14-B) under a magnetic field H similar to 11 T gradually splits into two spectra with Curie-like broadening as T decreases from T-sP similar to 150 K. Short-range (SR) staggered polarization (SP) on the ladder planes, originating from single-hole localization, occurs. The separation of the Sr14-B spectrum DeltaH deviates from the Curie-like T dependence below 20 K. This assures that spontaneous moments appear below T-N similar to 20 K in H similar to 11 T.

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  • Ce site substitution effects on the multipolar interactions in CeB6 査読

    M. Sera, F. Iga, S. Kobayashi, H. Sugamoto, K. Nagatomo, K. Umeo, S. Tsuji, K. Kojima, J. Akimitsu, Y. Zenitani

    Journal of the Physical Society of Japan   71 ( Suupl. )   52-55   2002年5月

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)  

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  • Magnetic properties in phase IV of Ce0.8Nd0.2B6 studied by muon spin relaxation 査読

    H. Takagiwa, K. Ohishi, J. Akimitsu, W. Higemoto, R. Kadono, F. Iga, M. Sera

    Journal of the Physical Society of Japan   71 ( Suupl. )   118-120   2002年5月

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)  

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  • Raman scattering investigation of high temperature ferromagnetic crystals 査読

    M. Udagawa, S. Nagai, N. Ogita, F. Iga, R. Kaji, K. Sumida, J. Akimitsu, S. Kunii, K. Suzuki, H. Onodera, Y. Yamaguchiv

    Journal of the Physical Society of Japan   71 ( Suupl. )   2002年5月

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)  

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  • Electronic structure of MgB2: X-ray emission and absorption studies 査読

    EZ Kurmaev, Lyakhovskaya, II, J Kortus, A Moewes, N Miyata, M Demeter, M Neumann, M Yanagihara, M Watanabe, T Muranaka, J Akimitsu

    PHYSICAL REVIEW B   65 ( 13 )   2002年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    Measurements of x-ray emission and absorption spectra of the constituents of MgB2 are presented. The results obtained are in good agreement with calculated x-ray spectra, with dipole matrix elements taken into account. The comparison of x-ray emission spectra of graphite, AlB2, and MgB2 in the binding energy scale supports the idea of charge transfer from sigma to pi bands, which creates holes at the top of the bonding sigma bands and drives the high-T-c superconductivity in MgB2.

    DOI: 10.1103/PhysRevB.65.134509

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  • Nonlocal effects and shrinkage of the vortex core radius in YNi2B2C probed by muon spin rotation 査読

    K Ohishi, K Kakuta, J Akimitsu, W Higemoto, R Kadono, JE Sonier, AN Price, RI Miller, RF Kiefl, M Nohara, H Suzuki, H Takagi

    PHYSICAL REVIEW B   65 ( 14 )   2002年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMERICAN PHYSICAL SOC  

    The magnetic field distribution in the vortex state of YNi2B2C has been probed by muon spin rotation. The analysis based on the London model with nonlocal corrections shows that the vortex lattice has changed from hexagonal to square with increasing magnetic field H. At low fields the vortex core radius, rho(v)(H), decreases with increasing H much steeper than what is expected from the rootH behavior of the Sommerfeld constant gamma(H), strongly suggesting that the anomaly in gamma(H) primarily arises from the quasiparticle excitations outside the vortex cores.

    DOI: 10.1103/PhysRevB.65.140505

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  • Electronic structure of MgB<inf>2</inf>: X-ray emission and absorption studies 査読

    E. Z. Kurmaev, I. I. Lyakhovskaya, J. Kortus, A. Moewes, N. Miyata, M. Demeter, M. Neumann, M. Yanagihara, M. Watanabe, T. Muranaka, J. Akimitsu

    Physical Review B - Condensed Matter and Materials Physics   65 ( 13 )   1345091 - 1345094   2002年4月

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    掲載種別:研究論文(学術雑誌)  

    Measurements of x-ray emission and absorption spectra of the constituents of MgB2 are presented. The results obtained are in good agreement with calculated x-ray spectra, with dipole matrix elements taken into account. The comparison of x-ray emission spectra of graphite, AlB2, and MgB2 in the binding energy scale supports the idea of charge transfer from σ to π bands, which creates holes at the top of the bonding σ bands and drives the high-Tc superconductivity in MgB2.

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  • X-ray emission and absorption spectra and electronic structure of MgB2 査読

    EZ Kurmaev, Lyakhovskaya, II, J Kortus, M Demeter, M Neumann, T Imazono, N Miyata, M Yanagihara, M Watanabe, T Muranaka, J Akimitsu

    SURFACE REVIEW AND LETTERS   9 ( 2 )   1097 - 1102   2002年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:WORLD SCIENTIFIC PUBL CO PTE LTD  

    Measurements of X-ray emission and absorption of the constituents of MgB2 are presented. The results obtained are in good agreement with calculated X-ray spectra, with dipole matrix elements taken into account. The comparison of X-ray emission spectra of graphite, AlB2, and MgB2 in the binding energy scale supports the idea of charge transfer from sigma to pi bands, which creates holes at the top of the bonding sigma bands and drives the high-T-c superconductivity in MgB2.

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  • Manifestation of multiband optical properties of MgB2 査読

    A. B. Kuz'menko, F. P. Mena, H. J.A. Molegraaf, D. Van Ver Marel, B. Gorshunov, M. Dressel, I. I. Mazin, J. Kortus, O. V. Dolgov, T. Muranaka, J. Akimitsu

    Solid State Communications   121 ( 9-10 )   479 - 484   2002年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    The optical conductivity of MgB2 has been determined on a dense polycrystalline sample in the spectral range 6 meV-4.6 eV using a combination of ellipsometric and normal incidence reflectivity measurements. σ1(ω) features a narrow Drude peak with anomalously small plasma frequency (1.4 eV) and a very broad 'dome' structure, which comprises the bulk of the low-energy spectral weight. This fact can be reconciled with the results of band structure calculations by assuming that charge carriers from the 2D σ-bands and the 3D π-bands have principally different impurity scattering rates and negligible interband scattering. This also explains a surprisingly small correlation between the defect concentration and Tc, expected for a two-gap superconductor. The large 3D carrier scattering rate suggests their proximity to the localization limit. © 2002 Elsevier Science Ltd. All rights reserved.

    DOI: 10.1016/S0038-1098(02)00004-2

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  • Field-induced magnetic order in disordered single crystal Sr14Cu24O41: Cu-NMR study 査読

    S Ohsugi, S Matsumoto, Y Kitaoka, M Matsuda, M Uehara, T Nagata, J Akimitsu

    PHYSICA B-CONDENSED MATTER   312   603 - 605   2002年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Cu-NMR results on field-induced magnetic long-range (LR) order in structural disordered single crystal Sr14Cu24O41 (Sr14) are presented. Drastic broadening of both the ladder/Zhang Rice-NMR spectra in Hsimilar to11 T occurs below T-N similar to 20 K. Meanwhile, another evidence of LR ordering is revealed in nuclear relaxation measurement. Staggered polarization originates from single-hole localization on the ladders. The growing interlayer coupling under H, as a result, leads to the appearance of the LR order as well as in the other spin-ladder compounds. (C) 2002 Elsevier Science B.V. All rights reserved.

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  • Vibrational spectroscopy of MgB2 by neutron inelastic scattering 査読

    Takahiro Muranaka, Tetsuya Yokoo, Masatoshi Arai, Erena Margiolaki, Kierin Brigatti, Kosmas Prassides, Oreg Petrenko, Jun Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   71   338 - 340   2002年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    Neutron inelastic scattering measurements have been performed on superconducting MgB2 up to SO and 130 meV over a wide range of scattering angles (3.14 degrees-127,5 degrees) at the temperature range between 23 K and 270 K. In the measurements by E-1 80 meV, we found fine structures in the phonon density of states of MgB2, which has been anticipated by theoretical calculation, especially in the energy range 30 to 40 meV and 45 to 55 meV, Moreover, in the measurements by E-1 = 130 meV, the temperature dependence of the integrated intensity of the generalized phonon density-of-states, where we made multi-phonon corrections, shows a clear anomalous behavior (the integrated intensity increases with decreasing temperature) in the energy range 75 to 80 ineV and 87 to 91 ineV. Since these energy regions are close to the energy of E-2g phonon mode (B in-plane bond stretching) and B1, phonon mode (B out-of-plane bond bending), this behavior can be interpreted as an evidence for a substantial contribution to the electron-phonon coupling from these phonons.

    DOI: 10.1143/JPSJS.71S.338

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  • Electronic structure of (formula presented)X-ray emission and absorption studies 査読

    E. Z. Kurmaev, I. I. Lyakhovskaya, J. Kortus, A. Moewes, N. Miyata, M. Demeter, M. Neumann, M. Yanagihara, M. Watanabe, T. Muranaka, J. Akimitsu

    Physical Review B - Condensed Matter and Materials Physics   65 ( 13 )   1 - 4   2002年

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    掲載種別:研究論文(学術雑誌)  

    Measurements of x-ray emission and absorption spectra of the constituents of (formula presented) are presented. The results obtained are in good agreement with calculated x-ray spectra, with dipole matrix elements taken into account. The comparison of x-ray emission spectra of graphite, (formula presented) and (formula presented) in the binding energy scale supports the idea of charge transfer from (formula presented) to (formula presented) bands, which creates holes at the top of the bonding (formula presented) bands and drives the high-(formula presented) superconductivity in (formula presented). © 2002 The American Physical Society.

    DOI: 10.1103/PhysRevB.65.134509

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  • Grain boundaries as weak links: The case of (formula presented) with reference to (formula presented) 査読

    S. B. Samanta, H. Narayan, A. Gupta, A. V. Narlikar, T. Muranaka, J. Akimitsu

    Physical Review B - Condensed Matter and Materials Physics   65 ( 9 )   1 - 3   2002年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    The grain boundaries (GB’s) in the intermetallic superconductor (formula presented) interestingly, do not show suppression of supercurrent density. This unexpected behavior has been investigated by a scanning tunneling microscopy/spectroscopy technique at atomic resolution. The GB in (formula presented) is seen as an amorphous region extending from 50 to 200 Å and has a metallic character. This observation supports proximity coupling between the grains, which explains why supercurrent density does not degrade in this material. The results for another intermetallic superconductor (formula presented) having GB’s (average width 30 Å) that are quasi-insulating in nature have also been presented and compared with the former. © 2002 The American Physical Society.

    DOI: 10.1103/PhysRevB.65.092510

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  • Evidence for high-frequency phonon mediated S-wave superconductivity: (formula presented) NMR study of Al-doped (formula presented) 査読

    H. Kotegawa, K. Ishida, Y. Kitaoka, T. Muranaka, N. Nakagawa, H. Takagiwa, J. Akimitsu

    Physical Review B - Condensed Matter and Materials Physics   66 ( 6 )   1 - 6   2002年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We report (formula presented) NMR studies that address the underlying two superconducting (SC) gaps model and a possible mechanism for interpreting a high SC transition temperature (formula presented) of (formula presented) in a recently discovered (formula presented) We reported in a previous paper [Phys. Rev. Lett. (formula presented) 127001 (2001)] that the results of the nuclear-spin-lattice relaxation rate, (formula presented) in the SC state revealed the existence of a single large SC gap with (formula presented) This conclusion is corroborated by the calculation that any two-gap models cannot account for the experimental results. In Al-doped (formula presented) the result of (formula presented) in the SC state revealed that a size in the SC gap is not changed by substituting Al for Mg. The reduction in (formula presented) by Al doping is shown to be due to the decrease of the density of states (formula presented) at the Fermi level. According to the McMillan equation, an experimental relation between (formula presented) and the relative change in (formula presented) allowed us to estimate a characteristic phonon frequency (formula presented) and an electron-phonon coupling constant (formula presented) These results suggest that the high- (formula presented) superconductivity in (formula presented) is mediated by the strong electron-phonon coupling with high-frequency phonons. © 2002 The American Physical Society.

    DOI: 10.1103/PhysRevB.66.064516

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  • High-resolution photoemission study of MgB<SUB>2</SUB> 査読

    T. Takahashi, T. Sato, S. Souma, T. Muranaka, J. Akimitsu

    Phys. Rev. Lett.   86   4915 - 4917   2001年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    DOI: 10.1103/PhysRevLett.86.4915

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  • Effect of silver addition on structure and electrical properties of the spin ladder compound La2Cu2O5 single crystals 査読

    C Sekar, T Watanabe, A Matsuda, H Shibata, Y Zenitani, J Akimitsu

    JOURNAL OF CRYSTAL GROWTH   233 ( 3 )   466 - 472   2001年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We have investigated the influence of silver on crystallization and oxygenation or the spin ladder compound La2Cu2O5 (four-leg) single crystals. Large La2Cu2O5 single crystals were successfully grown from charges containing A920 by a self-flux method. Electrical resistivity of the silver grown La2Cu2O5 single crystals (parallel tob-axis) has been found to be about three orders of magnitude lower when compared with the similar crystals grown without silver. Characterizations with optical microscope, XRD, EPMA and HRTEM indicate that the silver does not exist either in free state in the crystal or in the crystal lattice. The results show that the silver facilitates oxygenation in La2Cu2O5 crystals during their growth. (C) 2001 Elsevier Science B.V. All rights reserved.

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  • Crystal growth and transport properties of 4-leg spin-ladder compound La2Cu2O5: occurrence of insulator-metal transition 査読

    C Sekar, T Watanabe, A Matsuda, H Shibata, Y Zenitani, J Akimitsu

    INTERNATIONAL JOURNAL OF INORGANIC MATERIALS   3 ( 8 )   1201 - 1203   2001年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCI LTD  

    A remarkable metal-insulator transition has been observed, for the first time, in hole-doped spin ladder compound La2Cu2O5 (4-leg) single crystals. Hole-doping was carried out by means of high-oxygen-pressure annealing at 600 degreesC under a pressure of 400 atm (20%O-2+80%Ar at a total pressure of 2000 atm using a hot-isostatic-pressing (HIP) furnace, In order to realize metallicity, a prolonged HIP-treatment for about 22 days or more is required. However, this annealing period is reduced to about 5-6 days, when the crystals are grown from the starting charges containing Ag in the form of Ag2O. Even in the as-grown state, the measured room-temperature resistivity is about two to three orders of magnitude lower than that of La2CU2O5 crystals grown from charges without silver. The results show that the silver addition facilitates the oxygenation of the La2CU2O5 crystals. (C) 2001 Published by Elsevier Science Ltd.

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  • Coexistence of ferromagnetism and superconductivity in magnetic superconductor RuSr2YCu2O8 revealed by Cu-63-NMR 査読

    Tokunaga Y, Kotegawa H, Ishida K, Kitaoka Y, Takagiwa H, Akimitsu J

    PHYSICA C   364   379 - 382   2001年11月

  • Evidence for strong-coupling s-wave superconductivity in MgB2: B-11 NMR study 査読

    H Kotegawa, K Ishida, Y Kitaoka, T Muranaka, J Akimitsu

    PHYSICAL REVIEW LETTERS   87 ( 12 )   2001年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMERICAN PHYSICAL SOC  

    We have investigated a gap structure in a newly discovered superconductor, MgB2, through measurement of the B-11 nuclear spin-lattice relaxation rate, (11)(1/T-1). (11) (1/T-1) is proportional to the temperature (T) in the normal state, and decreases exponentially in the superconducting (SC) state, revealing a tiny coherence peak just below T-c. The T dependence of 1/T-1 in the SC state can be accounted for by an s-wave SC model with a large gap size of 2 Delta /k(B)T(c) similar to 5 which suggests it is in a strong-coupling regime.

    DOI: 10.1103/PhysRevLett.87.127001

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  • High-temperature ferromagnetism in CaB2C2 査読

    J. Akimitsu, K. Takenawa, K. Suzuki, H. Harima, Y. Kuramoto

    Science   293 ( 5532 )   1125 - 1127   2001年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We report a high Curie-temperature ferromagnet, CaB2C2. Although the compound has neither transition metal nor rare earth ions, the ferromagnetic transition temperature Tc is about 770 Kelvin. Despite this high Tc, the magnitude of the ordered moment at room temperatures is on the order of 10-4 Bohr magneton per formula unit. These properties are rather similar to those of doped divalent hexaborides, such as Ca1-xLaxB6. The calculated electronic states also show similarity near the Fermi level between CaB2C2 and divalent hexaborides. However, there is an important difference: CaB2C2 crystallizes in a tetragonal structure, and there are no equivalent pockets in the energy bands for electrons and holes - in contrast with CaB6. Thus, the disputed threefold degeneracy, specific to the cubic structure, in the energy bands of divalent hexaborides turns out not to be essential for high-temperature ferromagnetism. It is the peculiar molecular orbitals near the Fermi level that appear to be crucial to the high-Tc ferromagnetism.

    DOI: 10.1126/science.1061501

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  • Crystal growth and transport properties of spin ladder compounds La4+4nCu8+2nO14+8n (n=2, 3) 査読

    T Watanabe, C Sekar, H Shibata, A Matsuda, Y Zenitani, J Akimitsu

    PHYSICA C   357   380 - 383   2001年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Here we summarize recent progress on the study of spin ladder compounds, La2Cu2O5 (four-leg) and La8Cu7O19 (five-leg), n = 2 and 3 members of the homologous series La4+4nCu8+2nO14+8n. Following the successful crystal growth of La2Cu2O5, we grew a pure-phase five-leg system La8Cu7O19 by a modified slow cooling method. The room-temperature resistivity of the as-grown La8Cu7O19 crystal was 1.9 Omega cm, which is much lower than that of the La2Cu2O5 crystals (approximate to1.7 x 10(3) Omega cm). A significant drop in the resistivity (11.0 m Omega cm at 300 K) was observed upon hole doping by means of high oxygen-pressure annealing, although no transition was observed from the semiconducting behavior. In addition, a new crystal growth method to enhance hole doping level of La2Cu2O5 is shown. (C) 2001 Elsevier Science B.V. All rights reserved.

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  • Thermal transport properties of MgB2 査読

    T Muranaka, J Akimitsu, M Sera

    PHYSICAL REVIEW B   64 ( 2 )   2001年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    The thermal transport properties and magnetic susceptibility of MgB2 have been studied. The thermoelectric power with a positive sign shows a nearly T-linear increase up to similar to 150 K but shows a saturated behavior at higher temperatures, which suggests the existence of two types of charge carrier. The magnetic susceptibility above T-c shows a Pauli paramagnetism but shows a substantial T dependence which should include the information of the band structure of MgB2. The thermal conductivity shows a suppression below T-c without a clear anomaly at T-c which is a result of the reduction of the normal charge carrier concentration below T-c.

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  • Novel approaches to crystallize materials with narrow liquidus lines: application to spin ladder compound La4+4nCu8+2nO14+8n (n=2,3) and high-T-c cuprate Bi-2223 査読

    T Watanabe, C Sekar, H Shibata, T Fujii, A Matsuda, Y Zenitani, J Akimitsu

    JOURNAL OF CRYSTAL GROWTH   229 ( 1 )   316 - 320   2001年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We propose two novel approaches, a modified slow cooling (MSC) method and an improved traveling solvent floating zone (I-TSFZ) method, to crystallize materials with narrow liquidus lines. The MSC method uses a flux-poor starting composition to attain sufficient supersaturation, and involves a rapid cooling from high temperature to a proper crystallization temperature to avoid the formation of unwanted phases. The I-TSFZ method uses a very slow growth velocity to grow crystals under small supersaturation, and a steep temperature gradient to precisely control the growth temperature. The MSC method and the I-TSFZ method have been successfully applied to grow spin ladder compounds La4+4nCu8+2nO14+8n (n = 2,3) and a high-T-c, cuprate Bi2Sr2Ca2Cu3O10+delta, respectively, (C) 2001 Elsevier Science B.V. All rights reserved.

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  • Upper critical field of the spin ladder system Sr2.5Ca11.5Cu24O41 査読

    D Braithwaite, T Nagata, Sheikin, I, H Fujino, J Akimitsu, J Flouquet

    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS   226   455 - 456   2001年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We present transport measurements under pressure of the upper critical field of the superconducting spin ladder system Sr2.5Ca11.5Cu24O41. The temperature dependence and anisotropy of H-c2 at low field is compared to results in high-T-c superconductors and in ID organic superconductors, as well as to simple theoretical predictions. (C) 2001 Elsevier Science BN. All rights reserved.

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  • Optical measurements of the superconducting gap in MgB2 査読

    B Gorshunov, CA Kuntscher, P Haas, M Dressel, FP Mena, AB Kuz'menko, D van der Marel, T Muranaka, J Akimitsu

    EUROPEAN PHYSICAL JOURNAL B   21 ( 2 )   159 - 161   2001年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:SPRINGER-VERLAG  

    Far-infrared reflectivity studies on the polycrystalline intermetallic compound MgB2 with a superconducting transition temperature T-c = 39 K were performed at temperatures 20 K to 300 K. We observe a significant raise of the superconducting-to-normal state reflectivity ratio below 70 cm(-1) with a maximum at about 25-30 cm(-1), which gives a lower estimate of the superconducting gap of 2 Delta (0)approximate to 3-4 meV.

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  • Coexistence of superconductivity and (anti-) ferromagnetism in RuSr2YCu2O8 査読

    H Takagiwa, J Akimitsu, H Kawano-Furukawa, H Yoshizawa

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   70 ( 2 )   333 - 336   2001年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    The coexistence of antiferromagnetism and superconductivity in the hybrid ruthenate-cuprate RuSr2YCu2O8 synthesized under high pressure has been confirmed by means of powder neutron diffraction and resistivity measurements. The Ru moments are antiferromagnetically ordered at about 149 K and Ru spins can be aligned by applying a magnetic field, suggesting that the system gradually changes from antiferromagnetism to ferromagnetism. However, the superconductivity in the compound persists even at H = 9 T. This indicates the coexistence of ferromagnetism and superconductivity under a high magnetic field.

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  • Crystal growth, structure, and transport properties of the five-leg spin ladder compound La8Cu7O19 査読

    C Sekar, T Watanabe, A Matsuda, H Shibata, Y Zenitani, J Akimitsu

    JOURNAL OF SOLID STATE CHEMISTRY   156 ( 2 )   422 - 427   2001年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ACADEMIC PRESS INC  

    The crystallization region of La8Cu7O19 exists in a narrow temperature (1045-1050 degreesC) and composition range (1:4.5-1:5.0; La2O3:CuO molar ratio) in the La2O3-CuO system in an air ambient. In this narrow range, we have succeeded in growing large La8Cu7O19 single crystals, for the first time, by a modified slow cooling method. Single-crystal X-ray diffraction analysis reveals that La8Cu7O19 crystallizes in a monoclinic structure with space group C2/c and has the lattice parameters a = 13.82(7) Angstrom, b = 3.734(8) Angstrom, c = 34.77(10) Angstrom, and beta = 99.3(4)degrees. The electrical transport behavior of the as-grown and hole-doped La8Cu7O19 single crystals are semiconducting. A lowest resistivity of 11.0 m Omega -cm is measured at 293 K along the ladder direction (// b-axis) for the hole-doped samples. (C) 2001 Academic Press.

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  • High-field AFMR measurement of Nd2BaNiO5 single crystal in the submillimeter wave region 査読

    S Okubo, T Yamada, H Ohta, T Yokoo, J Akimitsu, H Nojiri, M Motokawa

    PHYSICA B   294   51 - 54   2001年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Millimeter and submillimeter wave ESR measurements of Nd2BaNiO5 have been carried out at 1.8 K using a single crystal that was obtained by the floating-zone method. The measured frequency region is from 50 GHz to 1.2 THz. The measured magnetic field is up to 30 T. Several AFMR modes due to the 16-sublattice magnetic structure below T, are clearly observed. Changes of AFMR modes at B-c1 = 9.6T and B-c2 = 16.2T are observed. AFMR modes with an antiferromagnetic gap of about 240 GHz and 1 THz are observed. (C) 2001 Elsevier Science B.V. All rights reserved.

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  • Antiferromagnetism and spin gap state in the two-leg ladder system 査読

    J Akimitsu

    JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS   62 ( 1-2 )   135 - 140   2001年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PERGAMON-ELSEVIER SCIENCE LTD  

    We report here the magnetic ground state in the hole-doped two-leg ladder system Sr2.5Ca11.5Cu24O41 at ambient pressure. We clearly observed the singlet ground state, of which values are almost unchanged for the different Ca concentration (similar to 32 meV). We also observed the antiferromagnetic long-range order below T-N of which values are between similar to0.6 K (x = 9) and similar to 3.6 K (x = 1.36). A possible magnetic structure of this system, was also discussed. These facts can be interpreted in line with scenario that the singlet state and antiferromagnetic order coexist at ambient pressure and these localized hole pairs begin to move under pressure, resulting in the occurrence of superconductivity, (C) 2000 Elsevier Science Ltd. All rights reserved.

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  • Thermal transport properties of (formula presented) 査読

    T. Muranaka, J. Akimitsu, M. Sera

    Physical Review B - Condensed Matter and Materials Physics   64 ( 2 )   2001年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    The thermal transport properties and magnetic susceptibility of (formula presented) have been studied. The thermoelectric power with a positive sign shows a nearly T-linear increase up to ∼150 K but shows a saturated behavior at higher temperatures, which suggests the existence of two types of charge carrier. The magnetic susceptibility above (formula presented) shows a Pauli paramagnetism but shows a substantial T dependence which should include the information of the band structure of (formula presented) The thermal conductivity shows a suppression below (formula presented) without a clear anomaly at (formula presented) which is a result of the reduction of the normal charge carrier concentration below (formula presented) © 2001 The American Physical Society.

    DOI: 10.1103/PhysRevB.64.020505

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  • Phase inhomogeneities and lattice expansion near T-c in the (MgB2)-B-11 superconductor 査読

    MARGADONNA S, MURANAKA T, PRASSIDES K, MAURIN I, BRIGATTI K, IBBERSON RM, ARAI M, TAKATA M, AKIMITSU J

    JOURNAL OF PHYSICS-CONDENSED MATTER   13 ( 35 )   L795 - L802   2001年

  • CuNMR/NQR studies on magnetism in impurity/hole-doped spin-ladder compounds 査読

    S Ohsugi, Y Kitaoka, M Azuma, Y Fujishiro, M Takano, T Nagata, H Fujino, J Akimitsu

    HYPERFINE INTERACTIONS   133 ( 1-4 )   157 - 162   2001年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:KLUWER ACADEMIC PUBL  

    Cu nuclear magnetic resonance (NMR) spectra of impurities (Zn, Ni, and La)-doped spin-1/2 Heisenberg ladder compounds SrCu2O3 (Sr123) are broadened with Curie-like temperature (T) dependence. The spectra have been successfully fit by using a quasi-one-dimensional (Q1D) staggered polarization (SP) model. Such a SP has also revealed in Cu NMR measurements of Sr14-xCaxCu24O41 (Cax) with hole-doped ladders. The origin of possible 3D antiferromagnetic (A-F) long-range ordering in (Zn and Ni)-doped Sr123 and Cax around x = 12 at low T is considered to be similar. Once unpaired spins S-0's are induced and 3D interlayer interaction occurs, the localized spins couple in the whole system.

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  • Coexistence of antiferromagnetic order and superconductivity in the spin ladder system Sr2.5Ca11.5Cu24O41 査読

    D Braithwaite, J Thomasson, B Salce, T Nagata, Sheikin, I, H Fujino, J Akimitsu, J Flouquet

    FRONTIERS OF HIGH PRESSURE RESEARCH II: APPLICATION OF HIGH PRESSURE TO LOW-DIMENSIONAL NOVEL ELECTRONIC MATERIALS   48   383 - 396   2001年

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    記述言語:英語   掲載種別:研究論文(国際会議プロシーディングス)   出版者・発行元:SPRINGER  

    The crucial question of whether the superconducting phase found in the spin ladder compound Sr14-xCaxCu24O41 is due to the natural pair forming mechanism predicted for isolated ladders is still open. A recent experimental breakthrough enables us to perform calorimetric measurements under hydrostatic pressure in a diamond anvil cell. We report here measurements on the compound Sr2.5Ca11.5Cu24O41 which show that the superconductivity is a bulk property and that antiferromagnetic order and superconductivity coexist over a wide pressure range. At the pressure where the sample becomes superconducting, the variation of the Neel temperature is continuous, However at high pressure there is a sharp increase of T-N, and T-N and T-C become comparable. The possible nature of the superconducting phase is discussed in the light of these results and previous measurements of the upper critical field.

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  • High-frequency ESR measurements using pulsed magnetic fields in Kobe 査読

    H. Ohta, S. Okubo, T. Sakurai, T. Goto, K. Kirita, K. Ueda, Y. Uwatoko, T. Saito, M. Azuma, M. Takano, J. Akimitsu

    Physica B: Condensed Matter   294-295   624 - 629   2001年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Elsevier Science Publishers B.V.  

    Recent development of the high-field facility in Kobe University is presented. We installed a 3 kV, 100 kJ capacitor bank, this is four times larger in energy than the previous one, and we achieved 42 T recently using a Cu-Ag pulsed magnet. Recent high-field ESR studies of quantum spin systems such as high pressure phase (VO)2P2O7 will be presented. The high-pressure phase of (VO)2P2O7 is the S = 1/2 bond alternating antiferromagnet
    we found an anomaly in the ESR mode of the high-pressure phase (VO)2P2O7 single crystal at 20 T by observing the frequency dependence at 4.2 K. Finally, we refer to our new development of the high-field ESR system under high pressure. We developed a compact pressure cell with sapphire window which fits into our pulsed magnet. The ESR result using this pressure cell will be presented.

    DOI: 10.1016/S0921-4526(00)00731-6

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  • Lattice dynamics of CuGeO3 by inelastic neutron scattering 査読

    M. Nishi, K. Kakurai, Y. Fujii, S. Katano, J. Akimitsu

    Journal of Physics and Chemistry of Solids   62 ( 1-2 )   355 - 356   2001年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    Inelastic neutron scattering study has revealed that along the a*-axis the energies of the longitudinal acoustic and the transverse acoustic phonon with polarized along the b*-axis are nearly equal. The similar relationship has also been found for the phonon branches propagating along the b*-axis. This strange feature may originate from the special structure of CuGeO3 that has CuO2 and GeO chains aligned along the c-axis and interacting very weakly with each other.

    DOI: 10.1016/S0022-3697(00)00163-3

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  • Hole distribution in (Sr, Ca, Y, La) 14Cu24O41 ladder compounds studied by x-ray absorption spectroscopy 査読

    N. Nücker, M. Merz, C. A. Kuntscher, S. Gerhold, S. Schuppler, R. Neudert, M. S. Golden, J. Fink, D. Schild, S. Stadler, V. Chakarian, J. Freeland, Y. U. Idzerda, K. Conder, M. Uehara, T. Nagata, J. Goto, J. Akimitsu, N. Motoyama, H. Eisaki, S. Uchida, U. Ammerahl, A. Revcolevschi

    Physical Review B - Condensed Matter and Materials Physics   62 ( 21 )   14384 - 14392   2000年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    The unoccupied electronic structure for the Sr14Cu24O41 family of two-leg ladder compounds was investigated for different partial substitutions of Sr2+ by Ca2+, leaving the nominal hole count constant, and by Y3+ or La3+, reducing the nominal hole count from its full value of 6 per formula unit. Using polarization-dependent x-ray absorption spectroscopy on single crystals, hole states on both the chain and ladder sites could be studied. While for intermediate hole counts all holes reside on O sites of the chains, a partial hole occupation on the ladder sites in orbitals oriented along the legs is observed for the fully doped compound Sr14Cu24O41. On substitution of-Ca for Sr orbitals within the ladder planes but perpendicular to the legs receive some hole occupation as well.

    DOI: 10.1103/PhysRevB.62.14384

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  • Hole distribution in (Sr,Ca,Y,La)(14)Cu24O41 ladder compounds studied by x-ray absorption spectroscopy 査読

    N Nucker, M Merz, CA Kuntscher, S Gerhold, S Schuppler, R Neudert, MS Golden, J Fink, D Schild, S Stadler, Chakarian, V, J Freeland, YU Idzerda, K Conder, M Uehara, T Nagata, J Goto, J Akimitsu, N Motoyama, H Eisaki, S Uchida, U Ammerahl, A Revcolevschi

    PHYSICAL REVIEW B   62 ( 21 )   14384 - 14392   2000年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    The unoccupied electronic structure for the Sr14Cu24O41 family of two-leg ladder compounds was investigated for different partial substitutions of Sr2+ by Ca2+, leaving the nominal hole count constant, and by Y3+ or La3+, reducing the nominal hole count from its full value of 6 per formula unit. Using polarization-dependent x-ray absorption spectroscopy on single crystals, hole states on both the chain and ladder sites could be studied. While for intermediate hole counts all holes reside on O sites of the chains, a partial hole occupation on the ladder sites in orbitals oriented along the legs is observed for the fully doped compound Sr14Cu24O41. On Substitution of Ca for Sr orbitals within the ladder planes but perpendicular to the legs receive some hole occupation as well.

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  • Trial of hole-doping to 4-and 5-leg ladder compounds 査読

    Y Zenitani, N Watanabe, J Akimitsu

    PHYSICA C   341   355 - 358   2000年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    The electrical resistivity and magnetic susceptibility measurements have been made for the spin-1/2 4- and 5-leg ladder compounds, La6Cu8O20 and La8Cu7O19, respectively Hole-doping has been attempted by means of high oxygen pressure annealing and substitution of Li+1 for Cu+2,La8Cu8-xLixO20 For La8Cu8O20 and La8Cu7O19, the electrical resistivities exhibit semiconductive behaviors, and the magnetic susceptibilities have a broad peak at around 150K. For the annealed samples under high oxygen pressure, the electrical resistivity becomes more conductive than that for as-sintered samples. The Li-substituted samples La8Cu8-xLixO20 with x=0.4, 0.8 and 1.2 also shows the semiconductive behavior, but it becomes more conductive with increasing the Li concentration x. The magnetic susceptibility shows the spin-glass like behavior was observed at around 10K. This transition is shifted to low temperature with increasing Li concentration.

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  • Role of structural modulation in the composite crystal (Sr2Cu2O3)(0.70)CuO2, "Sr14Cu24O41" with two-legged spin ladder 査読

    Y Gotoh, Y Takahashi, J Akimoto, S Mizuta, M Onoda, H Fujino, T Nagata, J Akimitsu

    PHYSICA C   341   469 - 470   2000年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Modulated structure analysis of the mutually incommensurate composite crystal (Sr2Cu2O3)(0.70)CuO2 with two-legged spin ladder has been performed on the basis of (3+1)-dimensional superspace group approach. It has been found that the symmetry of (Sr2Cu2O3)(0.70)CuO2 Should be described by a centrosymmetric superspace group. In the modulated composite structure of (Sr2Cu2O3)(0.70)CuO2, displacive modulation of O atoms in the CuO2 chain is fairly large and plays an important role. Structural interaction between the CuO2 chain and the Cu2O3 ladder has been extensively investigated.

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  • Spin dynamics of 4f electrons in CeB6 studied by muon spin relaxation 査読

    R Kadono, W Higemoto, A Koda, K Kakuta, K Ohishi, H Takagiwa, T Yokoo, J Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   69 ( 10 )   3189 - 3192   2000年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    The fluctuation rate of Ce-4f moments in the paramagnetic phase of CeB6 near zero field was determined by longitudinal muon spin relaxation. The rate scales perfectly with resistivity in the entire paramagnetic phase, indicating that the fluctuation is predominantly determined by the Kondo effect. The absence of magnetic order in the presumed antiferro-quadrupolar (AFQ) phase was also confirmed, while no evidence for coupling between the AFQ order parameter and electric field gradient at the B site was found near zero field by the muon nuclear level-crossing resonance technique.

    DOI: 10.1143/JPSJ.69.3189

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  • Raman scattering study of Sr14-xCaxCu24O41

    N Ogita, Y Fujita, Y Sakaguchi, Y Fujino, T Nagata, J Akimitsu, M Udagawa

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   69 ( 8 )   2684 - 2690   2000年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    Polarized Raman scattering spectra of Sr14-xCaxC24O41 have been measured for x = 0 and 11.5 at ambient pressure. In the spectra, the intra molecular vibrations due to the unit structure of (Sr/Ca)(2)Cu2O3 and CuO2 have been clearly observed below 700 cm(-1). A comparison of the number of the observed peaks with the results of a Factor group analysis indicates that the crystallographic symmetry of the ladder is F-222 for x = 0 and 11.5, instead of centro-symmetric F-mmm, while that of the chain is A(mma) for both compositions. However the symmetry of the chain for x = 11.5 is close to F-222 or F-mmm, That is, the structure of the chains changes From an incomplete staggered structure to almost complete staggered one with increasing x. fn the (c.c) and (a; a) spectra, the broad two-magnon peaks have been observed at around 3000 cm(-1) From the analysis of the recent theory of the magnetic scattering in the 2-Ieg spin system done by Natsume ei al., the following magnetic paraIneters for a: = 0 are obtained: J(leg) = 151meV, J(rung) = 91 meV, and Delta(ladder) = 44 meV. The values agree with those reported by neutron scattering.

    DOI: 10.1143/JPSJ.69.2684

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  • Antiferromagnet (Ln,Ca)CuO2.5 near the quantum critical point (Ln = Pr,Nd) 査読

    K Kakuta, K Ohishi, S Furuya, W Higemoto, R Kadono, J Akimitsu

    PHYSICA B   289   168 - 171   2000年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    The Neel temperatures T-N of two-leg ladder compounds LnCuO(2.5) (Ln: Pr, Nd) kin to LaCuO2.5 were determined by zero-field muon spin precession. It was found that T-N = 17 K in PrCuO2.5 and 1.8 K in NdCuO2.5, which are both considerably reduced from T-N = 125 K in LaCuO2.5. No spin precession signal was observed in Nd0.68Ca0.35CuO2.5. These results demonstrate that the spin-ladder system approaches the quantum critical point by substituting Pr or Nd for La, and that the Neel order of these compounds is completely destroyed upon Ca doping. (C) 2000 Elsevier Science B.V. All rights reserved.

    DOI: 10.1016/S0921-4526(00)00352-5

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    その他リンク: http://orcid.org/0000-0003-1494-6502

  • NMR and neutron scattering studies of quasi one-dimensional magnet CuV2O6 査読

    J Kikuchi, K Ishiguchi, K Motoya, M Itoh, K Inari, N Eguchi, J Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   69 ( 8 )   2660 - 2668   2000年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    Magnetic properties of a quasi one-dimensional magnet CuV2O6 have been studied by means of susceptibility measurement, NMR and powder neutron diffraction. Cu2+ spins (S = 1/2) order antiferromagnetically below 22.6K, and the saturation moment is about 0.70 mu(B) at 0K. The magnetic structure determined in the present study indicates that in CuV2O6 one-dimensional antiferromagnetic spin chains are along [110], suggesting a dominant antiferromagnetic exchange interaction between the next-nearest-neighbor spins rather than the nearest-neighbor ones.

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  • Spatially modulated antiferromagnetism in Sr2.5Ca11.5Cu24O41 probed by Muon spin relaxation 査読

    K Ohishi, T Yokoo, K Kakuta, H Fujino, T Nagata, J Akimitsu, R Kadono

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   69 ( 8 )   2427 - 2430   2000年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    A strongly modulated Neel order has been inferred from muon spin relaxation (mu SR) measurement in Sr2.5Ca11.5Cu24O41 below T-N = 2.2 +/- 0.1 K in conjunction with the previous result of neutron scattering. This ii; consistent with the scenario of staggered moments induced by the partial destruction of the singlet ground state by localized holes. Comparison of the mu SR result with that in an isostructural compound La3Sr3Ca8Cu24O41 indicates that the static magnetic order is induced by appropriate hole carriers at both chain and ladder layers.

    DOI: 10.1143/JPSJ.69.2427

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  • The magnetic ordering in the spin ladder compound Sr2.5Ca11.5Cu24O41 査読

    J Akimitsu, K Ohishi, T Yokoo, K Kakuta, H Fujino, T Nagata, R Kadono, M Nishi, K Kakurai, S Katano

    PHYSICA B   289   157 - 160   2000年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    mu SR and neutron experiments on Sr2.5Ca11.5Cu24O41 single crystal have been performed. The time-dependent muon polarization shows a significant change at around T-N similar or equal to 2.1 K indicating the onset of magnetic long-range order. Considering both the mu SR and neutron scattering data, a possible magnetic structure on the chain and ladder sites has been proposed. However. the lack of coherent mu SR precession signal strongly suggests that the distribution of the internal field is not homogeneous. This can be interpreted in line with the scenario that the singlet state and antiferromagnetic order coexist at ambient pressure and these localized hole pairs begin to move under pressure, resulting in the occurrence of superconductivity. (C) 2000 Elsevier Science B.V. All rights reserved.

    DOI: 10.1016/S0921-4526(00)00339-2

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  • Anomalous local magnetic shielding effect at muon site in Sr2.5Ca11.5Cu24O41 and Ce0.99Cu2.02Si2 査読

    R Kadono, A Koda, W Higemoto, K Ohishi, K Kakuta, J Akimitsu, K Ishida, Y Kitaoka, C Geibel, F Steglich

    PHYSICA B   289   322 - 325   2000年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We present that the time structure of ZF- and LF-mu SR spectra in the magnetic phase of Sr2.5Ca11.5Cu24O41 and in the A-phase of Ce0.99Cu2.02Si2 shares a number of common features. In particular, the magnitude of local magnetic field at the muon site estimated from ZF-mu SR is seemingly far smaller than that estimated by LF-quenching experiment. As a microscopic model of such an anomalous local magnetic shielding, we discuss the possibility of muon-induced local magnetism due to the magnetic instability of frustrated spin systems near the quantum critical point. (C) 2000 Elsevier Science B.V. All rights reserved.

    DOI: 10.1016/S0921-4526(00)00402-6

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  • Appearance of magnetic long-range order in the spin ladder compound La1-xSrxCuO2.5 査読

    K Ishii, A Yamashita, K Ohishi, J Akimitsu, R Kadono, N Kobayashi, M Takano, Z Hiroi

    PHYSICA B   289   165 - 167   2000年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    The magnetic behavior of La1-xSrxCuO2.5 has been investigated by means of muon spin relaxation (mu SR). The Neel temperatures T-N for several Sr concentrations (0.0 less than or equal to x less than or equal to 0.20) have been measured. We found that T-N is about 125 K in the insulating region, and 10-15 K in the metallic region, suggesting that the hole carriers are localized in the insulating region and delocalized in the metallic region. (C) 2000 Elsevier Science B.V. All rights reserved.

    DOI: 10.1016/S0921-4526(00)00351-3

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  • Anomalous field dependence of the vortex-core radius and magnetic penetration depth in YNi2B2C probed by mu SR 査読

    K Ohishi, J Akimitsu, W Higemoto, R Kadono, RI Miller, A Price, RF Kiefl, JE Sonier, M Nohara, H Suzuki, H Takagi

    PHYSICA B   289   377 - 380   2000年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Muon spin rotation measurements were performed to deduce the magnetic penetratioin depth lambda and vortex core radius rho(v) in the mixed state of YNi2B2C for H-c1 much less than H less than or equal to 0.7 H-c2. A preliminary analysis based on the modified London model shows that rho(v) decreases steeply with increasing H, while lambda increases linearly in H. (C) 2000 Elsevier Science B.V. All rights reserved.

    DOI: 10.1016/S0921-4526(00)00415-4

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  • High-field magnetization measurement of three-dimensional antiferromagnet Nd2BaNiO5 single crystal 査読

    S Okubo, H Ohta, T Tanaka, T Yokoo, J Akimitsu

    PHYSICA B-CONDENSED MATTER   284   1475 - 1476   2000年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    High-field magnetization measurements have been performed on Nd2BaNiO5 single crystal using a pulsed magnetic field up to 24 T at 4.2 K. Two anomalies were observed at 9.6 and 16.2 T for H\\c. As we used the single crystal, we confirmed that they were metamagnetic transitions. The origin of the metamagnetic transitions is discussed. (C) 2000 Elsevier Science B.V. All rights reserved.

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  • Cu NMR and NQR studies on the magnetic order in Sr2.5Ca11.5Cu24O41 査読

    S Ohsugi, Y Kitaoka, K Magishi, T Nagata, H Fujino, J Akimitsu

    PHYSICA B   284   1591 - 1592   2000年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    The magnetic order in a single-crystal Sr2.5Ca11.5Cu24O41 (Ca11.5) was investigated by Cu nuclear magnetic and quadrupole resonance (NMR\NQR) techniques. The magnetic order was found to be three-dimensional (3D). Small moments similar to 0.02 mu(B), art: spontaneously ordered on the ladders. Dissociation of hole pairs into single holes which localize below T-L similar to 60 K is responsible for the manifestation of the spin degree of freedom on the ladders. (C) 2000 Elsevier Science B.V. All rights reserved.

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  • Upper critical field of spin-ladder Sr2Ca12Cu24O41 査読

    T Nakanishi, H Takahashi, N Takeshita, N Mori, N Motoyama, H Eisaki, S Uchida, H Fujino, T Nagata, J Akimitsu

    PHYSICA B   281   957 - 958   2000年6月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We report the upper critical magnetic field, H-c2(T), of the spin-ladder Sr14-xCaxCU24O41 (X = 12) Single crystal, which becomes superconducting above similar to 3.0GPa. From the measurements of the temperature dependence of AC susceptibility at 4.0 GPa in magnetic fields applied along the a-axis (perpendicular to ladder), the b-axis (perpendicular to ladder-plane) and the c-axis (parallel to ladder), we have determined H-c2(T) for these three directions down to about 1.6 K. A clear difference is observed among these three curves, indicating that this system has an anisotropic superconducting ground state. (C) 2000 Elsevier Science B.V. All rights reserved.

    DOI: 10.1016/s0921-4526(99)00966-7

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  • Magnetic excitations in the haldane system R2BaNiO5 査読

    T Yokoo, J Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   69   95 - 101   2000年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    Magnetic excitations in 1-dimensional (1D) antiferromagnet R2BaNiO5 (R=Nd and Y1-xNdx) has been investigated by means of both polarized and unpolarized neutron scattering technique. Observed spin gap at 1D zone center above Neel temperature was confirmed to be originated by the same as Haldane gap obtained in the pure Haldane system Y2BaNiO5. In the 3D ordered region where the magnetic Bragg peaks clearly develop, the gap energy shows the sharp upturn at around T-N and increases linearly with decreasing temperature. Polarized neutron experiment reveals the coexistence of both quantum 1D Haldane and 3D long range antiferromagnetic (AF) phases.

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  • Role of single-ion excitations in the mixed-spin quasi-one-dimensional quantum antiferromagnet Nd2BaNiO5 査読

    A Zheludev, S Maslov, T Yokoo, J Akimitsu, S Raymond, SE Nagler, K Hirota

    PHYSICAL REVIEW B   61 ( 17 )   11601 - 11612   2000年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    Inelastic neutron scattering experiments on Nd2BaNiO5 single crystals and powder samples are used to study the dynamic coupling of one-dimensional Haldane-gap excitations in the S=1 Ni2+-chains to local crystal-field transitions, associated with the rare earth ions. Substantial interference between the two types of excitations is observed even in the one-dimensional paramagnetic phase. Despite that, the results provide solid justification for the previously proposed "static staggered field" model for R2BaNiO5 nickelates. The observed behavior is qualitatively explained by a simple chain-random-phase-approximation model.

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  • Hole migration in the spin-ladder, dimer chain material Sr2.5Ca11.5Cu24O41 査読

    R Bewley, RS Eccleston, T Nagata, J Akimitsu

    PHYSICA B   276   662 - 663   2000年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    The structure of the (SrxCa1-x)(14)Cu24O41 series contains both Cu2O3 spin-ladder layers and CuO2 spin chains. Previous measurements on the undoped sample (x = 1) showed all the holes were located on the chains, with the magnetic Cu2+ ions on the chain forming a well-defined dimer configuration. We have performed inelastic neutron scattering on a single crystal of the Ca-doped sample Sr2.5Ca11.5Cu24O41. The results show that two magnetic states now coexist on the chain, with one hole per formula unit migrating to the ladders (C) 2000 Elsevier Science B.V. AII rights resented.

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  • Angle-Resolved Photoemission Study of 1D Chain and Two-Leg Ladder 査読

    T. Sato, H. Fujisawa, T. Takahashi, M. Uehara, T. Nagata, J. Akimitsu, S. Miyasaka, M. Kibune, H. Takagi

    Stripes and Related Phenomena   8   263   2000年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

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  • Upper critical field of the spin ladder system Sr2.5Ca11.5Cu24O41 査読

    D Braithwaite, T Nagata, Sheikin, I, H Fujino, J Akimitsu, J Flouquet

    SOLID STATE COMMUNICATIONS   114 ( 10 )   533 - 536   2000年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PERGAMON-ELSEVIER SCIENCE LTD  

    We present transport measurements under pressure of the anisotropic upper critical field of the superconducting spin ladder compound Sr2.5Ca11.5CU24O41. When the field is applied perpendicular to the ladder plane, the temperature dependence of H-c2 is anomalous and shows a strong positive curvature. H-c2 for fields applied in the ladder plane is only weakly anisotropic between the a and c directions, however the initial slope is a factor of 6 larger than for H \\ b. We compare these measurements of H-c2( T) with results in the high-T-c cuprates and in the quasi-1D organic superconductors. (C) 2000 Elsevier Science Ltd. All rights reserved.

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  • Submillimeter wave ESR study of spin gap excitations in CuGeO3 査読

    H Nojiri, H Ohta, S Okubo, O Fujita, J Akimitsu, M Motokawa

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   68 ( 10 )   3417 - 3423   1999年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOCIETY JAPAN  

    Transitions between the ground singlet state and the excited triplet state have been observed in CuGeO3 by means of submillimeter wave electron spin resonance. The energy gap at zero field is evaluated to be 570 GHz (2.36 meV) and this value is nearly identical to the gap at the zone center observed by inelastic neutron scattering. The absorption intensity shows strong field orientation dependence, but it shows no significant dependence on magnetic field intensity. These features have been explained by considering the existence of Dzyaroshinsky-Moriya (DM) antisymmetric exchange interaction. The doping effect on this singlet-triplet excitation has been also studied. A drastic broadening of the absorption line is observed by the doping of only 0.5 % of Si.

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  • Magnetic order in the hole-doped two-leg ladder compound Sr2.5Ca11.5Cu24O41: Evidence from Cu-NMR and -NQR studies on a single crystal 査読

    S Ohsugi, K Magishi, S Matsumoto, Y Kitaoka, T Nagata, J Akimitsu

    PHYSICAL REVIEW LETTERS   82 ( 23 )   4715 - 4718   1999年6月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMERICAN PHYSICAL SOC  

    We report from Cu-NQR and -NMR experiments that hole-doped two-leg ladder compound Sr-2.5- Ca11.5Cu24O41 comprising Cu2O3 ladders and CuO2 chains reveals three-dimensional (3D) magnetic order below T-M = 2.2K. We found that small moments similar to 0.02 mu(B) are spontaneously ordered on the ladders, whereas large moments similar to 0.56 mu(B) are on the magnetic Cu sites occupying about one-fourth of the chain Cu sites. It is suggested that this 3D magnetic order is a cooperative phenomenon driven by the formation of pinned charge density wave-like state on the ladders which the optical conductivity measurement has clarified. [S0031-9007(99)09206-6].

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  • Polarized-neutron observation of longitudinal haldane-gap excitations in Nd2BaNiO5 査読

    S Raymond, T Yokoo, A Zheludev, SE Nagler, A Wildes, J Akimitsu

    PHYSICAL REVIEW LETTERS   82 ( 11 )   2382 - 2385   1999年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    Polarized and unpolarized inelastic neutron scattering is used to study Haldane-gap excitations in the mixed-spin linear-chain antiferromagnet Nd2BaNiO5. The longitudinal mode, polarized along the direction of ordered moments, is observed for the first time. The model of isolated Haldane chains in a static staggered exchange field, that is known to work very well for static properties and transverse spin excitations in R2BaNiO5 compounds, fails to explain new results for the longitudinal spin gap.

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  • Superconductivity and magnetic properties in the spin ladder compound (Sr,Ca)(14)Cu24O41 査読

    J Akimitsu

    PHYSICA B-CONDENSED MATTER   259-61   1030 - 1031   1999年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We review our recent experimental results on the tao-leg ladder compound Sr14-xCaxCu24O41, mainly focused an the superconductivity under pressure and coexistence between the spin gap and antiferromagnetic order induced by appropriate hole doping at ambiant pressure. (C) 1999 Elsevier Science B.V. All rights reserved.

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  • Coexistence of the spin liquid state and the magnetic ordering in the spin ladder compound; Sr14-xCaxCu24O41 査読

    Nagata T, Fujino H, Ohishi K, Akimitsu J, Katano S, Nishi M, Kakurai K

    Journal of Physics and Chemistry of Solids   60 ( 8 )   1039 - 1043   1999年

  • ESR study of the Haldane gap system Y<SUB>2</SUB>BaNiO<SUB>5</SUB> in high magnetic fields 査読

    Y. Imanaka, N. Miura, H. Nojiri, S. Luther, M. von Ortenberg, T. Yokoo, J. Akimitsu

    Physica B   246-247   561 - 564   1998年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    DOI: 10.1016/S0921-4526(97)00987-3

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  • Angle-resolved photoemission study of superconducting ladder compound Sr<SUB>14-x</SUB>Ca<SUB>x</SUB>Cu<SUB>24</SUB>O<SUB>41</SUB> 査読

    T. Sato, T. Yokoya, T. Takahashi, M. Uehara, T. Nagata, J. Goto, J. Akimitsu

    J. Phys. Chem. Solids   59   1912 - 1916   1998年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    DOI: 10.1016/S0022-3697(98)00148-6

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  • Magnetic ordering, spin waves, and Haldane-gap excitations in (NdxY1-x)(2)BaNiO5 linear-chain mixed-spin antiferromagnets 査読

    T Yokoo, S Raymond, A Zheludev, S Maslov, E Ressouche, Zaliznyak, I, R Erwin, M Nakamura, J Akimitsu

    PHYSICAL REVIEW B   58 ( 21 )   14424 - 14435   1998年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    Linear-chain nickelates with the composition (NdxY1-x)(2)BaNiO5 (x = 1, x = 0.75, x = 0.5, and x = 0.25) are studied in a series of neutron-scattering experiments. Powder diffraction is used to determine the temperature dependence of the magnetic structure in all four systems. Single-crystal inelastic neutron scattering is employed to investigate the temperature dependence of the Haldane-gap excitations and low-energy spin waves in the x= 1 compound Nd2BaNiO5. The results of these experiments are discussed in the context of the "Haldane chain in a staggered field" model for R2BaNiO5 systems, and quantitative agreement with theory is obtained. [SO163-1829(98)07945-4].

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  • Dopant-bound spinons in Cu1-cusive Greek chiZncusive Greek chiGeO3 査読

    G. Els, G. S. Uhrig, P. Lemmens, H. Vonberg, P. H.M. Van Loosdrecht, G. Güntherodt, O. Fujita, J. Akimitsu, G. Dhalenne, A. Revcolevschi

    Europhysics Letters   43 ( 4 )   463 - 468   1998年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    Polarized inelastic light scattering experiments on Cu1-cusive Greek chiZncusive Greek chiGeO3 (0 ≤ cusive Greek chi ≤ 0.045) single crystals show for cusive Greek chi ≠ 0 a new distinct mode at nearly half the energy of the singlet response below the spin-Peierls transition. The temperature, magnetic field, polarization, and doping dependences of this mode are similar to those of the singlet bound state. The data are interpreted in terms of a spinon-assisted light scattering process. Position and form of the peak provide strong evidence for the presence of dopant-bound spinons in Cu1-cusive Greek chiZncusive Greek chiGeO3.

    DOI: 10.1209/epl/i1998-00382-3

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  • Pressure-induced dimensional crossover and superconductivity in the hole-doped two-leg ladder compound Sr14-xCaxCu24O41 査読

    T Nagata, M Uehara, J Goto, J Akimitsu, N Motoyama, H Eisaki, S Uchida, H Takahashi, T Nakanishi, N Mori

    PHYSICAL REVIEW LETTERS   81 ( 5 )   1090 - 1093   1998年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMERICAN PHYSICAL SOC  

    Anisotropic electrical resistivity under high pressure was measured for a single crystal of Sr2.5Ca11.5Cu24O41, a hole-doped two-leg ladder compound. Superconductivity was observed between 3.5 and 8 GPa, accompanied by metallic resistivity between the ladders. which indicates semiconducting behavior at ambient pressure. This in turn strongly suggests that the application of pressure brings about a dimensional crossover from one to two, and that superconductivity in this system is a consequence of an insulator to superconductor transition in the anisotropic two-dimensional system.

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  • A new superconductor with 1222 structure (Cu1-xMx)Sr-2(Y1-yCey)(2)Cu2O delta(M = Ti, V, Cr, Fe, Co, Ga, Ge, Mo, Ce, W and Re) 査読

    M Tamura, M Sato, T Den, J Akimitsu

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   303 ( 1-2 )   1 - 10   1998年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We systematically synthesized (Cu, M)-1222 cuprates, (Cu1-xMx)Sr-2(Y1 -yCey)(2)Cu2Odelta (M = Ti, V, Cr, Fe, Co, Ga, Ce, Mo, Ce, W and Re), and succeeded for the first time in synthesizing new 1222 compounds with M = Ti, V, Cr, Co, Ge, W and Re. All samples except for the compound with Fe show superconductivity, giving the highest T-c similar to 43 K for annealed M = Re sample ((Cu0.85M0.15)Sr-2(Y0.75Ce0.25)(2)Cu2Oepsilon). We compared the T-c's with the crystal structures and investigated the difference between annealed and as-sintered samples. We point out here that T-c is related to the valencies of the M-elements and the O-Cu-O angle in the CuO2 planes. (C) 1998 Elsevier Science B.V. All rights reserved.

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  • Low-temperature specific heat of the electron-doped superconductor Pr2-xCexCuO4-partial derivative 査読

    TE Hargreaves, H Noma, J Akimitsu, DF Brewer, NE Hussey, AL Thomson

    PHYSICA C   303 ( 1-2 )   33 - 40   1998年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We report measurements of the low-temperature specific heat of superconducting and non-superconducting Pr1.85Ce0.15CuO4 and non-superconducting Pr1.875Ce0.125CuO4 between 0.2 K and 4 K in magnetic fields up to 4 testa. For all three samples, the zero-field specific heat C(T) consists of three contributions, a nuclear (T-2) term, a phononic (T-3) term and a residual low-power term proportional to T-n where n &lt; 1. The residual term follows an unusual temperature dependence in a magnetic field, similar to that found in hole-doped cuprate superconductors, suggesting a common origin for this residual term in all doped cuprates. We discuss the possibility that this low-power term arises from disorder-induced localization of normal carriers at low temperatures. (C) 1998 Elsevier Science B.V. All rights reserved.

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  • S=1/2 degrees of freedom in haldane system Y2BaNiO5 observed by submillimeter wave ESR 査読

    S Kimura, H Ohta, M Motokawa, T Yokoo, J Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   67 ( 7 )   2514 - 2521   1998年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We have performed submillimeter wave ESR measurement of Haldane system Y2BaNiO5 in the frequency region from 50 to 383 GHz using the pulsed magnetic field up to 16 T. Due to the excess oxygens, a felv percent of Ni2+ ions of Y2BaNiO5, which we used, are substituted by the Ni3+ ion of which spin is S = 1/2. The ESR signals coming from the S = 1/2 degrees of freedom originated by the Ni3+ ion at the end of the finite chain, are observed at 4.2 K. The frequency-field relations of these ESR signals are analyzed by the model based on the VBS model, which is used in the case of Cu2+ doped NENP. The signs of the single ion anisotropies D and E are determined from the analysis.

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  • Superconductivity and spin gap state in the two-leg ladder material Sr2.5Ca11.5Cu24O41 査読

    J Akimitsu

    JOURNAL OF THE KOREAN PHYSICAL SOCIETY   33   S6 - S10   1998年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:KOREAN PHYSICAL SOC  

    We review our recent experimental results, mainly focused on the superconductivity under pressure and the spin gap state for various Ca concentration on the two-leg ladder material Sr14-xCaxCu24O41.

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  • High-pressure transport properties of the superconducting spin-ladder system Sr14-xCaxCu24O41+delta 査読

    T Nakanishi, N Mori, C Murayama, H Takahashi, T Nagata, M Uehara, J Akimitsu, K Kinoshita, N Motoyama, H Eisaki, S Uchida

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   67 ( 7 )   2408 - 2414   1998年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    We report the electrical resistivity, the Hall coefficient, Rw, and the magnetoresistance, MR, at pressures up to 2.0 GPa, of the spin-ladder compound Sr14-xCaxCu24O41+delta, on both a polycrystal (x = 13.6) and single crystals (x = 11 and 12), in which superconductivity has been observed only in a range of both Ca content (similar to 11 &lt; x &lt; 14) and pressure (similar to 1.5 &lt; P &lt; similar to 8 GPa). The pressure dependence of the R-H, with positive sign, indicates that carrier density increases with increasing pressure and localization of hole carriers at low temperature is substantially suppressed by applying pressure. We have also found that a positive MR, which violates Kohler's rule, seems to appear below the temperature at which a spin gap begins to open and strongly depends on the temperature as well as both on Ca content and applied pressure, suggesting that this MR is closely related to the mobile holes in the spin singlet state.

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  • Spin gap and dynamics inSr14−xCaxCu24O41comprising hole-doped two-leg spin ladders: Cu NMR study on single crystals

    K. Magishi, S. Matsumoto, Y. Kitaoka, K. Ishida, K. Asayama, M. Uehara, T. Nagata, J. Akimitsu

    Physical Review B   57 ( 18 )   11533 - 11544   1998年5月

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    掲載種別:研究論文(学術雑誌)   出版者・発行元:American Physical Society (APS)  

    DOI: 10.1103/physrevb.57.11533

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    その他リンク: http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.57.11533/fulltext

  • Spin gap and dynamics in Sr14-xCaxCu24O41 comprising hole-doped two-leg spin ladders: Cu NMR study on single crystals 査読

    K Magishi, S Matsumoto, Y Kitaoka, K Ishida, K Asayama, M Uehara, T Nagata, J Akimitsu

    PHYSICAL REVIEW B   57 ( 18 )   11533 - 11544   1998年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    We report comprehensive Cu NMR studies on single crystals of Sr14-xCaxCu24O41, which contain simple CuO2 chains and two-leg Cu2O3 ladders. From measurements of the Cu-63 NMR shift, it is clear that the spin gap in the ladders decreases with isovalent Ca substitution from Delta = 550+/-30 K for Sr14Cu24O41 (Sr14) to 350+/-30 K, 280+/-30 K, and 270+/-30 K for x=6 (Ca6), x=9 (Ca9), and x=11.5 (Ca11.5), respectively. The exponential decrease of the nuclear spin-lattice relaxation rate 1/T-1 below similar to 130 K is consistent with the presence of the spin gap in the spin excitation spectrum. In the T range higher than similar to 200 K, we observed the following dependences: 1/T-1=const and the square of Gaussian spin-echo decay time, T-2G(2) proportional to T which are consistent with the scaling theory for the S=1/2 one-dimensional (1D) Heisenberg model. The value of T-2G/T-1 root T is compatible with the theoretical prediction of an exchange constant along the leg J(parallel to)similar to 1800 K for Ca6 and J(parallel to)similar to 1500 K for Ca9 and Ca11.5. A notable finding is that the magnitude of the spin gap remains nearly constant and characteristics of novel 1D-Like spin dynamics are maintained in the content varying from Ca9 to Ca11.5. On the other hand, the charge transport changes with increasing Ca content so that the more conductive Ca11.5 exhibits pressure-induced superconductivity exceeding 3.5 GPa. We have found that T-2G(2), which is proportional to the inverse spin correlation length xi(-1), deviates from a linear T dependence upon cooling and is described by A + BT exp(-Delta/T), regardless of the Ca substitution; We point out that the value of T-2G(2)(T=0)=A is proportional to the finite value of xi(0)(-1)=Delta/c(1D), where c(1D)=(pi/2)J(parallel to)is the spin-wave velocity. From the result that the values of A(-1)similar to xi(eff) for Ca6, Ca9, and Ca11.5 are significantly reduced compared to that for Sr14, it is suggested that xi(eff) is dominated at low T by an average distance d among mobile holes obeying the relation xi(eff)similar to d=xi(h). From an estimate of xi(0)/a similar to 5.2 for Sr14, where a is the Cu-Cu distance along the leg, xi h/a is obtained as similar to 3.5, 2.3, and 2.0, and hole content x as similar to 0.14, 0.22, and 0.25 per Cu2O3 ladder for Ca6, Ca9, and Ca11.5, respectively. These values were consistent with x= 0.14, 0.2, and 0.22 for Ca6, Ca9, and Call estimated from the optical conductivity experiment by Osafune et al. [Phys. Rev. Lett. 78, 1980 (1997)]. The Sr14-xCaxCu24O41 compounds are thus hole-doped two-leg spin-ladder systems which reveal the metallic behavior dominated by the 1D-like spin dynamics at high T and accompanied by the spin gap formation at low T. For Ca11.5, as the spin gap is formed upon cooling below similar to 180 K, the resistivity increases in the direction perpendicular to the ladder, whereas the conductivity along the ladder remains metallic, followed by the localization of mobile holes in both directions below T-L similar to 60 K. We point out that preformed pairs are confined in each ladder and localized below similar to 60 K at an ambient pressure.

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  • Unusual magnetic properties of the D+AF and M+AF phases in CuGe1-xSixO3 査読

    M Sera, M Hiroi, K Yamamoto, N Kobayashi, O Fujita, A Ogiwara, J Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   67 ( 5 )   1544 - 1547   1998年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    The thermal expansion, specific heat and magnetization of CuGe1-xSixO3 have been studied. Although the Neel temperature T-N Of the D + AF phase, where the antiferromagnetic (AF) phase coexists with the dimerized (D) one, is proportional to x up to 0.02 in H = 0 as in Cu1-xZnxGeO3, T-N Of the M + AF phase, where the AF phase coexists with the magnetic (M) one above 10 T, suddenly increases with x between x = 0.005 and 0.008. The AF properties in the D + AF phase are quite unusual in contrast with the normal ones in the AF state for x = 0.02. This is because the AF properties in the D + AF phase are largely influenced by the existence of dimerization as a background, which exists below T-SP, and the quantum effect plays an important role in this phenomenon.

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  • Experimental measurement of the staggered magnetization curve for a Haldane spin chain 査読

    A Zheludev, E Ressouche, S Maslov, T Yokoo, S Raymond, J Akimitsu

    PHYSICAL REVIEW LETTERS   80 ( 16 )   3630 - 3633   1998年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    Long-range magnetic ordering in R2BaNiO5 (R = magnetic rare earth) quasi-one-dimensional mixed-spin antiferromagnets is described by a simple mean-field model that is based on the intrinsic staggered magnetization function of isolated Haldane spin chains for the Ni subsystem, and single-ion magnetization functions for the rare earth ions. The model is applied to new experimental results obtained in powder diffraction experiments on Nd2BaNiO5 and NdYBaNiO5, and to previously published diffraction data for Er2BaNiO5. From this analysis we extract the bare staggered magnetization curve for Haldane spin chains in these compounds.

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  • Magnetic excitations in the Si doped spin-Peierls compound CuGe1-xSixO3 査読

    Hirota K, Hase M, Akimitsu J, Masuda T, Uchinokura K, Shirane G

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   67 ( 2 )   645 - 650   1998年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    The magnetic excitations of CuGe 1- x Si x O 3 single crystals ( x =0.011 and 0.03) have been studied by inelastic neutron-scattering techniques. We confirm the coexistence of the antiferromagnetic and spin-Peierls excitations as well as a splitting of the antiferromagnetic excitation peak at the magnetic zone center ( q =0) due to orthorhombic anisotropy. The q dependence of the antiferromagnetic spin-wave width Γ scales well with q 2 for small q ( q &lt;0.1 Å -1 ). We also find that the spin-wave velocity increases as the size of induced magnetic moment becomes larger. These experimental results strongly support recent theoretical predictions by Saito and Fukuyama.

    DOI: 10.1143/JPSJ.67.645

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  • ESR study of Haldane system Y2BaNiO5 in submillimeter wave region 査読

    S Kimura, H Ohta, M Motokawa, T Yokoo, J Akimitsu

    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS   177   624 - 625   1998年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Submillimeter wave ESR of Haldane system Y2BaNiO5 has been performed. The ESR absorption lines, which were observed in the temperature range above 15 K, are attributed to the transition between the excited triplet states. Frequency-field relations of these absorption lines are analyzed by the theory which explained those observed in NENP as the transitions between the excited triplet states near k = pi. (C) 1998 Elsevier Science B.V. All rights reserved.

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  • Systematic NMR studies of high-T-c and two-leg spin-ladder systems 査読

    Y Kitaoka, K Magishi, S Matsumoto, K Ishida, S Ohsugi, K Asayama, M Uehara, T Nagata, J Akimitsu

    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS   177   487 - 492   1998年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We review systematic NMR studies on underdoped high-T-c cuprates and two-leg spin-ladder systems, focusing on their spin gap and dynamics. The pressure-induced superconductivity was discovered in Sr14-xCaxCu24O41 comprising the quasi-one-dimensional S = 1/2 two-leg spin-ladder Cu2O3 planes. Remarkably, the new parent cuprate is the Mott insulator with the spin gap in contrast to the antiferromagnetic (AF) insulator for the high-T-c cuprates. From the Cu-NMR results in the ladder Cu2O3 plane on single crystals Sr14-xCaxCu24O41 with x = 9 (hereafter denoted as Ca9) and x = 11.5 (Ca11.5), it is shown that the spin gap, Delta = 510 K in the Sr14 is significantly reduced upon isovalent Ca substitution, but its magnitude remains nearly constant with Delta = 270 and 280 K for the Ca9 and Ca11.5, respectively. In T range of T greater than or equal to d, the spin dynamics in carrier-doped two-leg spin-ladder systems are characterized by the behaviour similar to the S = 1/2 1D Heisenberg system. By contrast, the transport property shares a common feature, i.e, due to the spin-gap opening the charge transport is confined in the 2D CuO2 plane for underdoped high-T-c cuprates and in the quasi-1D ladder for spin-ladder compounds. (C) 1998 Elsevier Science B.V. All rights reserved.

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  • Spin dynamics of the spin-ladder dimer-chain material Sr14Cu24O41 査読

    Roger S. Eccleston, Masatomo Uehara, Jun Akimitsu, Hiroshi Eisaki, Naoki Motoyama, Shin-Ichi Uchida

    Physical Review Letters   81 ( 8 )   1702 - 1705   1998年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We have performed inelastic neutron scattering on a single crystal sample of Sr14Cu24O41 to study the spin dynamics of the Cu2O3 spin ladder layers and CuO2 chains. Our data are best fitted with a dispersion which has a spin gap of 32.5±0.1meV and a maximum of 193.5±2.4meV, consistent with a coupling along the ladders, J∥=130 meV and a rung coupling J⊥ = 72 meV. Excitations with an energy transfer of approximately 11.5 meV arise from a dimer chain with an antiferromagnetic intradimer coupling J1=11.2meV between next-nearest-neighbor Cu ions, and a ferromagnetic interdimer coupling J2=−1.1meV. The dimer chains have a periodicity of five units. © 1998 The American Physical Society.

    DOI: 10.1103/PhysRevLett.81.1702

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  • High-Pressure Studies on the Superconducting Spin-Ladder Compound Sr14-xCaxCu24O41+δ 査読

    H. Takahashi, H. Takahashi, N. Môri, T. Nakanishi, T. Nagata, M. Uehara, J. Akimitsu, K. Kinoshita, N. Motoyama, H. Eisaki, S. Uchida

    Review of High Pressure Science and Technology/Koatsuryoku No Kagaku To Gijutsu   7   388 - 393   1998年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    Superconductivity has been observed in a spin-ladder compound Ca13.6Sr0.4Cu24O41+δ having Cu2O3 ladder-layer. Electrical resistivity measurements show Tc appears at 7 K at 1.5 GPa and disappears above 6 GPa after having maximum Tc of 12 K at 3 GPa. The bulk superconductivity was confirmed by AC susceptibility measurements. The electrical resistivity measurements for single crystals are also performed. The anisotropy of the electrical resistivity between a-axis and c-axis are reduced under pressure. [superconductivity, spin-ladder material, electrical resistivity, magnetic susceptivility, Hall coefficient]. © 1998, The Japan Society of High Pressure Science and Technology. All rights reserved.

    DOI: 10.4131/jshpreview.7.388

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  • Magnetic phase diagram of the spin-Peierls and antiferromagnetic system 査読

    S. Katano, O. Fujita, J. Akimitsu

    Physical Review B - Condensed Matter and Materials Physics   57 ( 17 )   10280 - 10283   1998年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    Neutron-diffraction experiments have been performed for (Formula presented) single crystals over a wide range of the Si concentration (0.4, 1.0, 1.7, 1.8, 2.1, 2.6, and 5.1 at. %) to study relations between the original spin-Peierls (SP) state and the antiferromagnetic (AF) long-range ordering induced by the Si doping. The SP state is strongly suppressed with the appearance of the AF state with the small amount of Si, and disappears at the concentration of around 2.5% at low temperatures. The AF phase persists throughout the concentration range studied here. Hence, the coexistence of these two states occurs in the concentration range up to around 2.5 at. % Si. The magnitudes of the AF magnetic moment and the lattice dimerization are given as a function of (Formula presented) The magnetic phase diagram obtained from these experiments has been compared with the previous reports on the Si and also Zn doping. © 1998 The American Physical Society.

    DOI: 10.1103/PhysRevB.57.10280

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  • ESR study of the field-induced magnetic phase of spin-Peierls CuGeO3 and MEM(TCNQ)2 査読

    H. Nojiri, T. Hamamoto, O. Fujita, J. Akimitsu, S. Takagi, M. Motokawa

    Journal of Magnetism and Magnetic Materials   177-181 ( 1 )   687 - 688   1998年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Elsevier  

    ESR measurements have been performed in high magnetic fields up to 30 T on the spin-Peierls compounds CuGeO3 and MEM(TCNQ)2 to examine the magnetic excitation in the magnetic phase (M phase) induced by external fields. Discontinuous changes in the resonance fields have been observed around the phase boundary between the dimerized spin-Peierls phase and the M phase for both CuGeO3 and MEM(TCNQ)2. Anisotropic AFMR modes are observed in the M phase Of CuGeO3. © 1998 Elsevier Science B.V. All rights reserved.

    DOI: 10.1016/S0304-8853(97)00390-9

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  • Unusual antiferromagnetic state in Si-doped CuGeO<SUB>3</SUB> single crystals studied by specific-heat measurements in magnetic fields 査読

    M. Hiroi, T. Hamamoto, M. Sera, H. Nojiri, N. Kobayashi, M. Motokawa, O. Fujita, A. Ogiwara, J. Akimitsu

    Phys. Rev. B   55   R6125 - 6128   1997年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    DOI: 10.1103/PhysRevB.55.R6125

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  • Angle-resolved photoemission study of ladder compound Sr<SUB>14</SUB>Cu<SUB>24</SUB>O<SUB>41</SUB> 査読

    T. Takahashi, T. Yokoya, A. Ashihara, O. Akaki, H. Fujisawa, A. Chainani, M. Uehara, T. Nagata, J. Akimitsu, H. Tsunetsugu

    Physical Review B   56   7870 - 7873   1997年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    DOI: 10.1103/PhysRevB.56.7870

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  • High-resolution angle-resolved photoemission study of Sr<SUB>14-x</SUB>Ca<SUB>x</SUB>Cu<SUB>24</SUB>O<SUB>41</SUB>(x=O and 6) 査読

    T. Yokoya, H.-D. Kim, A. Ashihara, H. Kumigashira, H. Fujisawa, T. Takahashi, M. Uehara, T. Nagata, J. Akimitsu

    Physica C   282-287   997 - 998   1997年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    DOI: 10.1016/S0921-4534(97)00632-1

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  • Electron Density in YTiO3 査読

    J. R. Hester, K. Tomimoto, H. Noma, F. P. Okamura, J. Akimitsu

    Acta Crystallographica Section B: Structural Science   53 ( 5 )   739 - 744   1997年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Blackwell Munksgaard  

    Accurate electron density analyses have been performed on the title compound, yttrium titanate, at room and low temperature using WKα radiation. Room- and low-temperature Δρ maps are generally consistent. Difference electron densities near the two symmetrically independent O atoms differ markedly. These observations may have significance for understanding the role of the O atom in mediating d-electron interactions between neighbouring Ti coordination polyhedra. Features having lower symmetry than that of the approximately octahedral local environment are observed within the Ti coordination octahedron. Crystal data at room temperature: YTiO3, Mr = 184.79, T = 293 K, Pnma, a = 5.6901(4), b = 7.6130(7), c = 5.3381(6) Å, V = 231.24(4)Å3, Z = 4, Dx = 5.308 Mg m3, μ(WKα) = 1.02mm-1, R = 0.024, wR = 0.017, S = 1.64(2) for 2996 independent reflections. Low temperature: T = 127K, Pnma, a = 5690(4), b = 7.583(6), c = 5.318(5)Å, V = 229.5(3)Å3, Dx = 5.35Mgm3, μ(WKα) = 1.02mm-1, R = 0.021, wR = 0.015, S = 1.80(2) for 2968 independent reflections.

    DOI: 10.1107/S010876819700551X

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  • Pressure-induced superconductivity in quantum spin-ladder compound Sr14-xCaxCu24O41+delta 査読

    N Mori, T Nakanishi, H Takahashi, T Nagata, M Uehara, J Akimitsu, N Motoyama, H Eisaki, S Uchida

    PHYSICA B   239 ( 1-2 )   137 - 140   1997年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Superconductivity in a spin-ladder system was discovered only under high pressure on the compound Sr14-xCaxCu24O41+delta having Cu2O3 ladder-layer with x from 11.5 to 13.6. Superconducting transition temperature (T-c) was found to appear suddenly under high pressure. For the single crystalline sample with x = 11.5 the T-c has a maximum temperature of about 9.5 K at around 4 GPa and decreases with pressure above it. A substantial decrease of resistivity along the a-axis was revealed by applying pressure. This result suggests that the superconductivity occurs just after the crossover from one-to two-dimensional properties takes place.

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  • Superconductivity in the ladder compound Sr2.5Ca11.5Cu24O41 (single crystal) 査読

    T Nagata, M Uehara, J Goto, N Komiya, J Akimitsu, N Motoyama, H Eisaki, S Uchida, H Takahashi, T Nakanishi, N Mori

    PHYSICA C   282   153 - 156   1997年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We report the superconductivity in the single crystal of Sr2.5Ca11.5Cu24O41 at T-c(onset) = 6.5 K under an optimal pressure of about 4.5 GPa. Normal resistivity along ladder direction rho(c), gradually changes from T-linear to T-2-dependence with increasing an applied pressure. The normal resistivity ratio rho(alpha)/rho(c) in the ladder plane indicates the crossover from 1D to 2D with increasing an applied pressure.

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  • Cu-63 NMR study on spin ladder compounds Sr14-xCaxCu24O41 (x=9, 11.5) 査読

    K Magishi, S Matsumoto, K Ishida, Y Kitaoka, K Asayama, M Uehara, T Nagata, J Akimitsu

    PHYSICA C   282   1115 - 1116   1997年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Cu-63 NMR measurements for the ladder sites on single crystals Sr14-xCaxCu24O41 (x = 9, 11.5), which consist of simple chains and 2-leg ladders of Cu2+ ions, have been carried out at f = 125.1 MHz for H parallel to b-axis. From respective measurements of the Knight shift and the nuclear spin-lattice relaxation rate, 1/T-1, the spin gap in the ladder has been found to be as large as Delta = 250 K (260 K) and 340 K (360 K) for x = 11.5 (x = 9), respectively.

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  • Superconductivity in quantum spin-ladder compound Ca13.6Sr0.4Cu24O41+delta at high pressure 査読

    H Takahashi, N Mori, T Nakanishi, T Nagata, M Uehara, J Akimitsu, K Kinoshita

    PHYSICA B-CONDENSED MATTER   237   112 - 114   1997年7月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Superconductivity has been observed in a spin-ladder compound Ca13.6Sr0.4Cu24O41+delta having Cu2O3 ladder layer. Electrical resistivity measurements show that T-C appears at 7 K at 1.5 GPa and disappears above 6 GPa after having the maximum T-C of 12 K at 3 GPa. The bulk superconductivity was suggested in this system from AC susceptibility measurements, the volume fraction of which was estimated to be about 5% at 4.2 K at 3.5 GPa. In contrast with the oxide superconductor having CuO2-layer, this system shows superconductivity in the very narrow pressure range.

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  • Ni-chain gap excitations in (NdxY1-x)(2)BaNiO5: One-dimensional to three-dimensional crossover 査読

    T Yokoo, A Zheludev, M Nakamura, J Akimitsu

    PHYSICAL REVIEW B   55 ( 17 )   11516 - 11520   1997年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    Linear-chain mixed-spin antiferromagnets (NdxY1-x)(2)BaNiO5 were investigated by powder inelastic neutron scattering for x=0.25, 0.5, and x=0.75. Long-range magnetic ordering was observed in all samples. The temperature dependence of the Ni-chain spin-gap modes was measured and compared to previous results for the x=0 and x=1 materials. In the magnetically disordered phase the spin dynamics was shown to have a composition-independent character. The results suggest that one-dimensional gap excitations seen in the paramagnetic and Neel phases of Nd2BaNiO5 are inherently related to Haldane spin excitations in the quantum-disordered system Y2BaNiO5.

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  • The observation of g-shifts in spin-Peierls material CuGeO3 by submillimeter wave ESR 査読

    Y Yamamoto, H Ohta, M Motokawa, O Fujita, J Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   66 ( 4 )   1115 - 1123   1997年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    The g-shifts in spin-Peierls (SP) and magnetic (M) phases of the inorganic spin-Peierls material CuGeO3 have been observed by submillimeter wave ESR. The temperature dependence of the g-shifts from U phase to SP or M phase showed a continuous change. Since such g-shifts were also observed in the organic spin-Peierls material, for example TTF-AuBDT, such g-shifts can be considered the universal phenomena due to the lattice distortion in the spin-peierls materials. But, in the case of CuGeO3, the g-shifts are so large that we cannot explain them in the framework of the point charge model with considering the continuous lattice distortion.

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  • Magnetic property of CeNiC2 studied by macroscopic and microscopic measurements 査読

    K Motoya, K Nakaguchi, N Kayama, K Inari, J Akimitsu, K Izawa, T Fujita

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   66 ( 4 )   1124 - 1129   1997年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    Magnetic susceptibility, specific heat, nuclear magnetic resonance and neutron diffraction measurements have been made to investigate the magnetic property of a ternary compound CeNiC2. Temperature variations of magnetic susceptibility and specific heat suggest that magnetic transitions take place at T = 20 K, 10 K and at 2.2 K. NMR line width of C-13 nuclei increases remarkably below T = 20 K indicating the increase of transferred hyperfine held due to magnetic order. In the neutron diffraction pattern below T = 20 K, additional peaks appear which indicate the presence of long-range magnetic order. Based on these measurements, a magnetic structure model of CeNiC2 which involves a transition from an incommensurate to a commensurate antiferromagnetic order is presented.

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  • Raman scattering investigation of Y2BaNiO5 査読

    N Ogita, Y Tsunezumi, T Yokoo, J Akimitsu, M Udagawa

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   66 ( 3 )   873 - 876   1997年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    Raman scattering spectra of Y2BaNiO5 have been measured in the temperature region between 7 K and 530 K, and also an infrared absorption spectrum has been measured at room temperature. We have observed the A(g) phonon modes at 244 and 532 cm(-1), B-2g at 171 and 322 cm(-1), and B-3g at 178, 257 and 435 cm(-1), respectively. For only (a, a) spectrum among the A(g) spectra, a strong doublet peak at about 760 cm(-1) and a weak triplet peak at about 1500 cm(-1) have been found. With the increase of temperature, the doublet peak becomes single peak and the triplet peak almost disappears in the higher temperature region than 400 K, where magnetic susceptibility shows the maximum. This suggests that these peaks can be related to the magnetic excitations.

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  • Magnetic phase diagram and antiferromagnetic resonance in CuGe1-ySiyO3 査読

    H. Nojiri, T. Hamamoto, Z. J. Wang, S. Mitsudo, M. Motokawa, S. Kimura, H. Ohta, A. Ogiwara, O. Fujita, J. Akimitsu

    Journal of Physics Condensed Matter   9 ( 6 )   1331 - 1338   1997年2月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    Magnetization and ESR measurements have been performed on Si-doped CuGe1-ySiyO3 (y = 0.01 and 0.02) single crystals. The y = 0.01 sample shows a spin-Peierls transition at TsP = 11 K as well as undoped material and additionally show an antiferromagnetic state below TN = 2.8 K. When an external field is applied to the c-axis below TN, two magnetization jumps due to the spin-flop and the transition from dimerized spin-Peierls phase to magnetic phase are observed at 0.98 T and 11.5 T, respectively, which means the coexistence of spin-Peierls and antiferromagnetic states. On the other hand, the y = 0.02 sample shows only an antiferromagnetic transition at TN = 4.8 K.

    DOI: 10.1088/0953-8984/9/6/016

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  • Observation of three-magnon light scattering in CuGeO_3 査読

    G. Els, P. H.M. Van Loosdrecht, P. Lemmens, H. Vonberg, G. Güntherodt, G. S. Uhrig, O. Fujita, J. Akimitsu, G. Dhalenne, A. Revcolevschi

    Physical Review Letters   79 ( 25 )   5138 - 5141   1997年1月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    Temperature and magnetic field dependent polarized Brillouin spectra of CuGeO_3 are reported. In addition to a bound singlet state at 30 cm^(-1), a new feature has been observed at 18 cm^(-1). This feature is interpreted in terms of a novel three-magnon light scattering process between excited triplet states. © 1997 The American Physical Society.

    DOI: 10.1103/PhysRevLett.79.5138

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  • Appearance of magnetic long range order in the spin ladder compound LaCuO2.5 査読

    J Akimitsu, H Okajima, A Yamashita, K Ishii, T Yokoo, T Nagata, M Uehara, R Kadono, K Nagamine, N Kobayashi, Z Hiroi, M Takano, H Takahashi, N Mori, K Kinoshita

    HYPERFINE INTERACTIONS   104 ( 1-4 )   43 - 48   1997年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:SPRINGER  

    We report here the two current topics in the ladder compounds.
    1. The new ladder compound LaCuO2.5 recently discovered by Hiroi and Takano shows a clear sign of magnetic long range order at around 125 K observed by mu(+)SR technique. This result demonstrates that the ground state of LaCuO2.5 is not a spin liquid state, but a magnetically ordered state.
    2. Superconductivity has been found in the ladder material (Sr,Ca)(14)Cu24O41 under high pressure between 3 GPa and 4.5 GPa. The superconducting transition temperatures T-c (onset) are 12 K and 9 K at 3 GPa and 4.5 GPa, respectively.

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  • Ni-chain gap excitations: One-dimensional to three-dimensional crossover 査読

    T. Yokoo, A. Zheludev, M. Nakamura, J. Akimitsu

    Physical Review B - Condensed Matter and Materials Physics   55 ( 17 )   11516 - 11520   1997年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    Linear-chain mixed-spin antiferromagnets ((Formula presented)(Formula presented)(Formula presented)(Formula presented) were investigated by powder inelastic neutron scattering for x=0.25, 0.5, and x=0.75. Long-range magnetic ordering was observed in all samples. The temperature dependence of the Ni-chain spin-gap modes was measured and compared to previous results for the x=0 and x=1 materials. In the magnetically disordered phase the spin dynamics was shown to have a composition-independent character. The results suggest that one-dimensional gap excitations seen in the paramagnetic and Néel phases of (Formula presented)(Formula presented) are inherently related to Haldane spin excitations in the quantum-disordered system (Formula presented)(Formula presented). © 1997 The American Physical Society.

    DOI: 10.1103/PhysRevB.55.11516

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  • Magnetic excitation spectrum from spin-Peierls system, CuGeO3 査読

    M. Fujita, M. Arai, M. Motokawa, O. Fujita, J. Akimitsu, S. M. Bennington, C. D. Frost

    Physica B   234-236   536 - 538   1997年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    DOI: 10.1016/S0921-4526(96)01059-9

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  • Unusual antiferromagnetic state in the dimerized phase in studied by lattice distortion 査読

    M. Sera, K. Yamamoto, M. Hiroi, N. Kobayashi, O. Fujita, A. Ogiwara, J. Akimitsu

    Physical Review B - Condensed Matter and Materials Physics   56 ( 22 )   14771 - 14775   1997年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    We have studied the thermal expansion and magnetostriction of (Formula presented) ((Formula presented) 0.01, 0.015, and 0.02) single crystals under magnetic fields up to 15 T. The lattice distortion in the antiferromagnetic (AF) state, which is opposite to that in the dimerized (Formula presented) phase, exhibits unusual magnetic-field dependence for (Formula presented) where the AF state coexists with the (Formula presented) state. The lattice distortion due to AF ordering is rapidly suppressed and in its place, the lattice distortion due to the dimerization is enhanced by magnetic field up to (Formula presented) Anomalies in the thermal expansion and specific heat at (Formula presented) are very small at (Formula presented) in contrast with their clear anomalies in lower and higher magnetic fields. These indicate that the nature of the AF state is largely changed by the magnetic field and that the electronic state below and above (Formula presented) is not so different at (Formula presented) Such unusual behavior in a magnetic field is not observed in the AF state for (Formula presented) where there exists only AF ordering. © 1997 The American Physical Society.

    DOI: 10.1103/PhysRevB.56.14771

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  • Magnetic excitations of dimerized CuGeO3: continuum scattering and interchain modulation 査読

    D. Welz, M. Nishi, M. Fujita, K. Kakurai, M. Arai, J. Akimitsu, Y. Fujii

    Physica B   241-243   540 - 542   1997年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    DOI: 10.1016/S0921-4526(97)00856-9

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  • Spin dynamics of the spin-Peierls system CuGeO3 at various temperatures 査読

    M. Fujita, C. D. Frost, M. Arai, M. Motokawa, O. Fujita, J. Akimitsu, S. M. Benningto

    Physica B   241-243   528 - 530   1997年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    DOI: 10.1016/S0921-4526(97)00634-0

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  • Study on quantum magnetic excitation of CuGeO3 using neutron scattering 査読

    M. Fujita, M. Arai, M. Motokawa, O. Fujita, J. Akimitsu, S. M. Bennington, C. D. Frost

    Physica B   237-238   132 - 134   1997年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    DOI: 10.1016/S0921-4526(97)00073-2

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  • Effect of high pressure on T-c of a homologous series of oxycarbonate superconductors Sr-2(Ca,Sr)(n-1)Cu-n(CO3)(1-x)(BO3)(x)O-y (n=1,2) and Ba2Ca2Cu3(CO3)(1-x)(BO3)(x)O-y 査読

    M Sato, J Akimitsu, H Takahashi, N Mori

    PHYSICA C   271 ( 1-2 )   79 - 82   1996年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    The superconducting transition temperatures T-c of Sr-2(Ca,Sr)(n-1)Cu-n(CO3)(1-x)(BO3)(x)O-y (Sr n = 1, 2) and Ba2Can-1Cun(CO3)(1-x)(BO3)(x)O-y (Ba n = 3) have been measured under high pressure up to 8 GPa. The T-c values increase monotonically with respect to pressure for all samples. The maximum T-c obtained was as high as 128 K at 8 GPa in Ba2Ca2Cu3(CO3)(1-x)(BO3)(x)O-y.

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  • Long-range magnetic ordering in the spin ladder compound LaCuO2.5 probed by muon-spin relaxation 査読

    R Kadono, H Okajima, A Yamashita, K Ishii, T Yokoo, J Akimitsu, N Kobayashi, Z Hiroi, M Takano, K Nagamine

    PHYSICAL REVIEW B   54 ( 14 )   R9628 - R9630   1996年10月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMER PHYSICAL SOC  

    A clear sign of long-range magnetic ordering has been observed by muon-spin rotation/relaxation in the two-leg spin-ladder compound LaCuO0.25, below 125 K with a relatively large hyperfine field (similar to 0.1 T). This result demonstrates that the ground state of the undoped LaCuO2.5 is a magnetically ordered state, rather than the predicted quantum spin-liquid state.

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  • Neutron scattering study of magnetic excitations in the spin S=1 one-dimensional Heisenberg antiferromagnet Y2BaNiO5 査読

    T Sakaguchi, K Kakurai, T Yokoo, J Akimitsu

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   65 ( 9 )   3025 - 3031   1996年9月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:PHYSICAL SOC JAPAN  

    Magnetic excitations of the ideal one-dimensional, spin S = 1, antiferromagnet Y2BaNiO5 are studied by means of inelastic neutron scattering on single crystal sample. Our experimental results at low temperature indicate that Y2BaNiO5 can be regarded as the best isotropic Haldane-system sofar studied with the intrachain exchange energy J/k(B) = -24.1 meV and the averaged gap Delta(T = 7 K) = 8.5 meV. Y2BaNiO5 exhibits a good magnetic one-dimensionality with the inter-/intrachain exchange ratio of \J'/J\ less than or equal to 10(-4) and has only very small easy-axis and in-plane single ion anisotropy of \D/J\ = 0.03 and \E/J\ similar to 0.01. In addition the temperature dependence of the Haldane gap Delta(T) and its damping rate was determined.

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  • The change of the symmetry of the soft-mode spin wave at the spin-flop transition as observed in MnTiO3 using inelastic polarized neutron scattering 査読

    M Steiner, K Kakurai, B Dorner, R Pynn, J Akimitsu

    JOURNAL OF PHYSICS-CONDENSED MATTER   8 ( 32 )   5905 - 5914   1996年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:IOP PUBLISHING LTD  

    The energy and the eigenvector of the spin waves in MnTiO3 have been studied as functions of the applied magnetic field using inelastic neutron scattering with and without polarization analysis. It is found that the behaviour depends on the distance q from the antiferromagnetic zone centre: for q congruent to 0 the mode softens completely at the spin-flop field H-sf, but it has finite energy for q &gt; 0; the eigenvector retains its Ising character for small q up to the highest fields measured (5 T = 0.86H(sf), assuming H-sf = 5.8 T), but it changes continuously with held from Ising to Heisenberg character for large q. A very strong, temperature-dependent line width was found at H = 6 T, close to H-sf which is interpreted as further evidence for the soft-mode instability at the spin-flop transition.

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  • Temperature dependence of the spin-Peierls energy gap and anomalous line shapes in CuGeO3 査読

    MC Martin, G Shirane, C Fujii, M Nishi, O Fujita, J Akimitsu, M Hase, K Uchinokura

    PHYSICAL REVIEW B   53 ( 22 )   14713 - 14716   1996年6月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMERICAN PHYSICAL SOC  

    Neutron-scattering measurements on a large single crystal of CuGeO3 have been used to determine the temperature dependence of the spin-Peierls energy gap. While the power-law behavior of the intensity of structural superlattice peaks is well fit by I(T)proportional to(T-c - T)(2 beta) with an exponent of beta=0.33, the exponent for the temperature dependence of the energy gap is significantly smaller than expected for conventional spin-Peierls materials. Usual scaling relations relate the energy gap to the superlattice reflection intensity as Delta(T)proportional to I-a with a = 1/3; the present results suggest an exponent of a approximate to 1/6 for CuGeO3. An additional scattering cross section is observed in constant-q and constant-E scans creating a long tail extending to higher energies relating to a proposed scattering continuum.

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  • The phase diagram of the new superconductor Sr2CuO2(CO3)(1-x)(BO3)(x) 査読

    H Nakata, H Okajima, T Yokoo, A Yamashita, J Akimitsu, S Katano

    PHYSICA C   263 ( 1-4 )   344 - 348   1996年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    A phase diagram for the boron-doped new superconductor Sr2CuO2(CO3)(1-x)(BO3)(x) has been systematically investigated The parent compound Sr2CuO2(CO3) is an antiferromagnetic (AF) insulator with a Neel temperature of T-N similar to 285 K. T-N of Sr2CuO2(CO3)(1-x)(BO3)(x) changes drastically with increasing boron content x. In the lightly doped AF state (0 less than or equal to x less than or equal to 0.04), the strong superexchange interaction within the CuO2-layers still yields a long-range AF ordering. The superconductivity is confined to the narrow compositional range 0.04 less than or equal to x less than or equal to 0.15 and its highest T-c is 32 K at x = 0.15.

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  • Metal-insulator transition in the S=1/2 spin ladder system (Sr0.4Ca0.6)(14)Cu24-xCoxO(41-delta) 査読

    J Akimitsu, M Uehara, T Nagata, S Matsumoto, Y Kitaoka, H Takahashi, N Mori

    PHYSICA C   263 ( 1-4 )   475 - 481   1996年5月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We have confirmed that the (Sr0.4Ca0.6)(14)Cu24-xCoxO41-delta system with x = 0 has a spin gap of 735 K from NMR measurement. This system shows a metal-insulator transition at Co content x between 2.9 and 4.5 after high-pressure O-2 annealing (650 degrees C, 8 MPa, 20 h). The behavior of the electrical resistivity for the high-pressure O-2 annealed sample with x = 4.8 can be characterized by the one-dimensional variable range hopping scheme at low temperature. These facts can be interpreted as the trapped hole carriers in the 1D CuO2 chain being released into the ladder by Co doping x above 2.9 under high-pressure O-2 annealing.

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  • Raman scattering of CuGeO3 査読

    N Ogita, Y Tsunezumi, H Aoki, M Udagawa, O Fujita, A Ogihara, J Akimitsu

    PHYSICA B-CONDENSED MATTER   219-20   107 - 109   1996年4月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Detailed temperature dependence of Raman scattering spectra of CuGeO3 has been measured in the temperature region between 4.2 and 30 K, to clarify the anomaly of phonons and magnetic excitations in the vicinity of spin-Peierls (SP) transition at 14 K. Among the five Raman peaks, which appear in the SP state, only a peak at 230 cm(-1) remains above SP transition temperature. Judging from the line shape, this peak can be assigned to be an ordinary two-magnon excitation of antiferromagnets.

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  • Tunneling spectroscopy of the superconducting energy gap in RNi(2)B(2)C (R=Y and Lu) 査読

    T Ekino, H Fujii, M Kosugi, Y Zenitani, J Akimitsu

    PHYSICAL REVIEW B   53 ( 9 )   5640 - 5649   1996年3月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMERICAN PHYSICAL SOC  

    Superconducting energy gap in RNi(2)B(2)C (R=Y and Lu) has been investigated using a break-junction tunneling technique. The observed differential conductance dI/dV is well described by the BCS density of states with a small lifetime broadening parameter. We have found small gap values for the local low-T-c phases in addition to the bulk T-c of 15.6-16.6 K. The gap value of 2 Delta = 4.5 meV with T-c = 15 K gives the BCS ratio of 2 Delta/k(B)T(c) = 3.5, which is kept constant down to the observed lowest T-c of 4.6 K. These results give clear evidence for the BCS mechanism of superconductivity for these compounds.

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  • New superconductor with apical bromine (Ca, A)2CuO2Br2: A=Na, K 査読

    Yuji Zenitani, Shuji Sahoda, Jun Akimitsu, Nobuhiko Kubota, Muneo Ayabe

    Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy   43 ( 9 )   1087 - 1089   1996年

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    記述言語:日本語   掲載種別:研究論文(学術雑誌)   出版者・発行元:Funtai Funamtsu Yakin Kyokai/Japan Soc. of Powder Metallurgy  

    We report a new superconductor, (Ca,A)2CuO2+yBr2-y,:A=(Na,K) (Tc=24 K) with apical bromines synthesized under high pressure . X-ray powder diffraction shows that the superconducting phase has a tetragonal structure with a=3.858 Å, c=17.257 Å. This structure is so called La2CuO4 (214) type with an apical site of octahedral CuO6 being almost occupied by the bromine atoms.

    DOI: 10.2497/jjspm.43.1087

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  • Tunneling spectroscopy of the superconducting energy gap 査読

    Toshikazu Ekino, Hironobu Fujii, Makoto Kosugi, Yuji Zenitani, Jun Akimitsu

    Physical Review B - Condensed Matter and Materials Physics   53 ( 9 )   5640 - 5649   1996年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    Superconducting energy gap in (Formula presented) has been investigated using a break-junction tunneling technique. The observed differential conductance (Formula presented) is well described by the BCS density of states with a small lifetime broadening parameter. We have found small gap values for the local low-(Formula presented) phases in addition to the bulk (Formula presented) of 15.6-16.6 K. The gap value of (Formula presented) meV with (Formula presented) K gives the BCS ratio of (Formula presented), which is kept constant down to the observed lowest (Formula presented) of 4.6 K. These results give clear evidence for the BCS mechanism of superconductivity for these compounds. © 1996 The American Physical Society.

    DOI: 10.1103/PhysRevB.53.5640

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  • 31a-PS-73 Bi_2Sr_2CaCu_2O_yのエネルギーギャップの異方性(低温(高温超伝導)) 査読

    村中 隆弘, 甲斐田 陽一, 小杉 真, 秋光 純, 清水 勝, 津田 惟雄, 岸尾 光二

    日本物理学会講演概要集. 年会   51 ( 0 )   358 - 358   1996年

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    記述言語:日本語   出版者・発行元:一般社団法人 日本物理学会  

    DOI: 10.11316/jpsgaiyog.51.3.0_358_1

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  • Long-range magnetic ordering in the spin ladder compound LaC probed by muon-spin relaxation 査読

    R. Kadono, H. Okajima, A. Yamashita, K. Ishii, T. Yokoo, J. Akimitsu

    Physical Review B - Condensed Matter and Materials Physics   54 ( 14 )   R9628 - R9630   1996年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    A clear sign of long-range magnetic ordering has been observed by muon-spin rotation/relaxation in the two-leg spin-ladder compound LaCu(Formula presented) below 125 K with a relatively large hyperfine field (∼0.1 T). This result demonstrates that the ground state of the undoped LaCu(Formula presented) is a magnetically ordered state, rather than the predicted quantum spin-liquid state. © 1996 The American Physical Society.

    DOI: 10.1103/PhysRevB.54.R9628

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  • Lattice anomaly and dynamics of CuGeO3 査読

    M. Fujita, K. Ubukata, M. Arai, T. Tonegawa, M. Mino, M. Motokawa, K. Knight, B. Forsyth, S. M. Bennington, J. Akimitsu, O. Fujita

    Physica B   219-220   95 - 97   1996年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    DOI: 10.1016/0921-4526(95)00661-3

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  • Quantum spin excitations in the spin-Peierls system CuGeO3 査読

    M. Arai, M. Fujita, M. Motokawa, J. Akimitsu, S. M. Benningto

    Phys. Rev. Lett.   77   3649 - 3652   1996年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    DOI: 10.1103/PhysRevLett.77.3649

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  • Quantum effect on magnetic excitation 査読

    M. Fujita, M. Arai, M. Motokawa, O. Fujita, J. Akimitsu, S. M. Benningto

    Czech. J. Phys.   46   1965 - 1966   1996年

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    記述言語:英語   掲載種別:研究論文(学術雑誌)  

    DOI: 10.1007/BF02570974

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  • STRUCTURAL PHASE-TRANSITION OF SR2CUO2(CO3) 査読

    H NAKATA, J AKIMITSU, S KATANO, T MINAMI, N OGITA, M UDAGAWA

    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS   255 ( 1-2 )   157 - 166   1995年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    The structural phase transition of Sr2CuO2(CO3) has been studied by X-ray and neutron powder diffraction and phonon Raman scattering. The crystal structure at room temperature is tetragonal with space group of &lt;I(4)over bar&gt; (S-4(2)) and the lattice parameters a = 7.8045(1) Angstrom and c = 14.993(1) Angstrom. The structural phase-transition temperature determined by X-ray powder diffraction is found at 490 K. The structure above this transition temperature is tetragonal P42(1)2 (D-2(4)), and the lattice parameters are a = 5.54364(8) Angstrom and c = 7.53823(11) Angstrom at 573 K. The neutron-diffraction data are refined by a Rietveld analysis and these structural parameters for below and above the transition temperature are presented. Raman spectra of Sr2CuO2(CO3) have been measured in the temperature region between 5 K and 593 K. In the temperature region between 5 K and 483 K, a new extra mode was not observed, but the peak intensity of the strongest mode at 480 cm(-1) was decreased from room temperature to 373 K. Above 490 K, a new peak has been observed at 160 cm(-1) and the weak peaks which are observed at room temperature disappear. Judging from the facts that the spectral shape and line width originating from the CO3 slab change at the transition temperature but a soft mode has not been observed, the type of this phase transition is order-disorder due to the change of the CO3 alignments.

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  • Metal-insulator transition in the S=1/2 spin ladder system (Sr0.4Ca0.6)(14)Cu24-xCoxO41-delta 査読

    M Uehara, M Ogawa, J Akimitsu

    PHYSICA C   255 ( 3-4 )   193 - 203   1995年12月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    We have found that the (Sr0.4Ca0.6)(14)Cu24-xCoxO41-delta system shows a metal-insulator transition at Co content x between 4.0 and 4.5 after high-pressure O-2 annealing. For the as-sintered samples the T dependence of the electrical resistivities is semiconducting over the whole x range and the associated activation energies systematically increase with increasing x. For the high-pressure O-2 annealed samples the T dependence of the electrical resistivities is semiconducting, similar to the as-sintered ones at x below 3.8, whereas the electrical resistivities scarcely depend on the temperature variation with two types of small activation energies, similar to 59 meV (200-100 K) and similar to 2 meV (100-4.2 K) at x = 4.8. The behaviors of the. electrical resistivities are characterized by the one-dimensional variable-range hopping scheme at a low-temperature region. These facts can be interpreted: the hole carriers beyond the capacity of the 1D CuO2 chain for trapping holes are released into the ladder by Co doping x above 2.9 under high-pressure O-2 annealing (650 degrees C, 8 MPa, 20 h).

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  • ELECTRONIC-STRUCTURE OF PRNIO3 STUDIED BY PHOTOEMISSION AND X-RAY-ABSORPTION SPECTROSCOPY - BAND-GAP AND ORBITAL ORDERING 査読

    T MIZOKAWA, A FUJIMORI, T ARIMA, Y TOKURA, N MORI, J AKIMITSU

    PHYSICAL REVIEW B   52 ( 19 )   13865 - 13873   1995年11月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:AMERICAN PHYSICAL SOC  

    The electronic structure of PrNiO3 has been studied by photoemission and x-ray-absorption spectroscopy. By analyzing the spectra using configuration-interaction calculations on a NiO6 cluster model, it has been found that the charge-transfer energy Delta is similar to 1 eV and the Ni 3d and O 2p orbitals are strongly hybridized in the ground state. From the cluster-model calculation, the magnetic moment of Ni 3d is estimated to be similar to 0.9 mu(B), which is close to the ionic value of Ni3+ and in good agreement with that obtained from neutron-diffraction experiments. Using the electronic-structure parameters deduced from the cluster-model analysis, we have performed unrestricted Hartree-Fock calculations on a Ni 3d-O 2p perovskite-type lattice model in order to study the effect of GdFeO3-type distortion on the orbital polarization and band gap.

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  • 2-DIMENSIONAL DIFFRACTION PATTERNS OF ERFE2O4-DELTA 査読

    S KATANO, H KITO, J AKIMITSU, K SIRATORI, S FUNAHASHI, HR CHILD

    PHYSICA B   213   212 - 214   1995年8月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)   出版者・発行元:ELSEVIER SCIENCE BV  

    Diffraction patterns in the (h h l) plane of ErFe2O4-delta, measured as a function of temperature, show the two-dimensional spin (ferrimagnetic)-charge ordering of Fe2+ and Fe3+ with weak correlations between the planes. A model recently proposed for this spin-charge ordering has been examined by measurements of the intensity for three different crystal directions in the c-plane, since the model leads to the existence of three kinds of domain with quite different volumes. In contrast to this expectation, the results show no appreciable intensity differences.