Faculty Profiles - YOKOYA Takayoshi

Papers - YOKOYA Takayoshi

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Local-structure insight into the improved superconducting properties of Pb-substituted La(O, F)BiS2: a photoelectron holography study

YaJun Li, Yusuke Hashimoto, Noriyuki Kataoka, Zexu Sun, Sota Kawamura, Hiroto Tomita, Taro Setoguchi, Soichiro Takeuchi, Shunjo Koga, Kohei Yamagami, Yoshinori Kotani, Satoshi Demura, Kanako Noguchi, Hideaki Sakata, Tomohiro Matsushita, Takanori Wakita, Yuji Muraoka, Takayoshi Yokoya

Scientific Reports 15 ( 1 ) 2025.3

Publishing type：Research paper (scientific journal) Publisher：Springer Science and Business Media LLC

Abstract

Pb-substituted La(O, F)BiS₂ (Pb-LaOFBiS₂) exhibits improved superconducting properties and a resistivity anomaly around 100 K that is attributed to a structural transition. We have performed temperature(T)-dependent photoelectron holography (PEH) to study dopant incorporation sites and the local structure change across the anomaly. The PEH study of Pb-LaOFBiS₂ provided evidence for the dominant incorporation sites of Pb and F: Pb atoms are incorporated into the Bi sites and F atoms are incorporated into the O site. No remarkable difference in the local structures around Pb and Bi atoms was observed. Across the temperature of the resistivity anomaly (T ^*), photoelectron holograms of Bi 4f changed. Comparisons of holograms with those of non-substituted LaOFBiS₂ sample, as well as simulated holograms, suggested that (1), above T ^*, the tetragonal structure of Pb-LaOFBiS₂ is different from the tetragonal structure of LaOFBiS₂ and (2), below T ^*, the tetragonal structure still remains in Pb-LaOFBiS₂. We discuss a possible origin of the difference in the structure above T ^* and the implication of the result below T ^*, which are necessary ingredients to understand the physical properties of Pb-LaOFBiS₂.

DOI： 10.1038/s41598-025-86233-2

Other Link： https://www.nature.com/articles/s41598-025-86233-2

Effects of thermal energy on the formation of lattice strain in VO2 thin films grown on TiO2(001) Reviewed

Reki Nakamoto, Hiroyuki Okazaki, Takanori Wakita, Takayoshi Yokoya, Yuji Muraoka

Langmuir 40 ( 46 ) 24447 - 24454 2024.11

First-principles calculations concerning ferromagnetism in Q-carbon Reviewed

Qiang Yue, Takayoshi Yokoya, Yuji Muraoka

Diamond and Related Materials 111807 - 111807 2024.11

Structural and Superconducting Properties of Bi₂Rh₃(Se_1–xS_x)₂ (x = 0–1.0) Reviewed

Zhiyan Zhang, Wanli Pan, Masaki Utsumi, Yuki Yamamoto, Hidenori Goto, Ryusuke Kondo, Takayoshi Yokoya, Ryota Goban, Tomoki Higashikawa, Ritsuko Eguchi, Yasuhiro Takabayashi, Koichi Hayashi, Hirofumi Ishii, Tatsuo C. Kobayashi, Yoshihiro Kubozono

Inorganic Chemistry 63 ( 45 ) 21531 - 21540 2024.10

First-principles molecular dynamics simulations for the properties of boron-doped tetrahedral amorphous carbon Reviewed

Qiang Yue, Takayoshi Yokoya, Yuji Muraoka

Diamond and Related Materials 143 110894 2024.2

Molecular dynamics simulation of deposition of amorphous carbon films on sapphire surfaces Reviewed

Qiang Yue, Takayoshi Yokoya, Yuji Muraoka

Diamond and Related Materials 140 110514 - 110514 2023.12

Photoelectron holographic evidence for the incorporation site of Se and suppressed atomic displacement of the conducting layer of La(O,F)BiSSe Reviewed

YaJun Li, ZeXu Sun, Noriyuki Kataoka, Taro Setoguchi, Yusuke Hashimoto, Soichiro Takeuchi, Shunjo Koga, Kazuhisa Hoshi, Yoshikazu Mizuguchi, Tomohiro Matsushita, Takanori Wakita, Yuji Muraoka, Takayoshi Yokoya

Japanese Journal of Applied Physics 62 ( 12 ) 125001 - 125001 2023.11

Authorship：Corresponding author Publishing type：Research paper (scientific journal) Publisher：IOP Publishing

Abstract

La(O,F)BiS_2-xSe_x is a layered material that is considered to be a candidate exotic superconductor as well as a promising thermoelectrical material. We performed soft X-ray photoelectron holography to study the Se incorporation site and the local atomic arrangement of the conducting layer. A comparison of the experimental holograms with the simulated holograms indicates that Se atoms preferentially occupy the S sites in the conducting Bi–S plane of La(O,F)BiS₂. A comparison between the state-of-the-art holographic reconstructions of La(O,F)BiSSe and La(O,F)BiS₂ suggests that Se substitution suppresses the displacement of S atoms in La(O,F)BiS₂. These results provide photoelectron holographic evidence for the Se incorporation site and the Se-induced suppression of in-plane disorder.

DOI： 10.35848/1347-4065/ad079b

Other Link： https://iopscience.iop.org/article/10.35848/1347-4065/ad079b/pdf

Controlling of Localization by Elemental-substitution Effect in Layered BiCh₂-based Compounds LaO₁₋_xF_xBiS₂₋_ySe_y Reviewed

Kazuhisa Hoshi, Hiroto Arima, Noriyuki Kataoka, Masayuki Ochi, Aichi Yamashita, Anne de Visser, Takayoshi Yokoya, Kazuhiko Kuroki, Yoshikazu Mizuguchi

Journal of the Physical Society of Japan 92 ( 5 ) 2023.5

Photoelectron Holography Study of La(O,F)BiS₂ Reviewed

YaJun Li, ZeXu Sun, Noriyuki Kataoka, Taro Setoguchi, Yusuke Hashimoto, Soichiro Takeuchi, Shunjo Koga, Satoshi Demura, Kanako Noguchi, Hideaki Sakata, Yoshikazu Mizuguchi, Tomohiro Matsushita, Takanori Wakita, Yuji Muraoka, Takayoshi Yokoya

Journal of the Physical Society of Japan 92 ( 4 ) 2023.4

Synthesis and Characterization of a Trigonal Layered Compound AgInS₂ Reviewed

Takahiro Sawahara, Ryo Matsumoto, Yuki Nakahira, Hidetomo Usui, Noriyuki Kataoka, Ryusei Saitou, Takanori Wakita, Takayoshi Yokoya, Aichi Yamashita, Yosuke Goto, Yoshihiko Takano, Akira Miura, Yoshikazu Mizuguchi

ACS Omega 8 ( 12 ) 11288 - 11292 2023.3

Self-organized vertical multilayer structures in spinodally decomposed TiO2-VO2 films on glass substrates Reviewed

Yuji Muraoka, Kazutada Takeda, Yoshito Takemoto, Takanori Wakita, Takayoshi Yokoya

Thin Solid Films 769 139749 - 139749 2023.3

Temperature dependent local inhomogeneity and magnetic moments of (Li_1−xFe_x)OHFeSe superconductors Reviewed

G. Tomassucci, L. Tortora, G. M. Pugliese, F. Stramaglia, L. Simonelli, C. Marini, K. Terashima, T. Wakita, S. Ayukawa, T. Yokoya, K. Kudo, M. Nohara, T. Mizokawa, N. L. Saini

Physical Chemistry Chemical Physics 2023

Photoelectron Diffraction and Holography Studies on Dopant Local Structures Invited Reviewed

Takayoshi Yokoya

Journal of the Physical Society of Japan 91 ( 9 ) 2022.9

Modification of the synthesis of layered titanium chloride nitride Reviewed

Masashi Tanaka, Noriyuki Kataoka, Haruya Kobayashi, Masaya Fujioka, Masaru Oda, Aishi Yamamoto, Kensei Terashima, Junji Nishii, Hiromi Tanaka, Takayoshi Yokoya

Materials Research Bulletin 153 111896 - 111896 2022.9

Anomalously large spin-dependent electron correlation in the nearly half-metallic ferromagnet <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>CoS</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math> Reviewed

Hirokazu Fujiwara, Kensei Terashima, Junya Otsuki, Nayuta Takemori, Harald O. Jeschke, Takanori Wakita, Yuko Yano, Wataru Hosoda, Noriyuki Kataoka, Atsushi Teruya, Masashi Kakihana, Masato Hedo, Takao Nakama, Yoshichika Ōnuki, Koichiro Yaji, Ayumi Harasawa, Kenta Kuroda, Shik Shin, Koji Horiba, Hiroshi Kumigashira, Yuji Muraoka, Takayoshi Yokoya

Physical Review B 106 ( 8 ) 2022.8

Superconductivity in the α-Form Layer Structured Metal Nitride Halide Reviewed

Masashi Tanaka, Noriyuki Kataoka, Takayoshi Yokoya

Condensed Matter 7 ( 2 ) 33 - 33 2022.4

Publishing type：Research paper (scientific journal) Publisher：MDPI AG

Layered metal nitride halides MNX (M = Ti, Zr, Hf; X = Cl, Br, I) have two polymorphs, including α- and β-forms, which have the FeOCl and SmSI structures, respectively. These compounds are band insulators and become metals and show superconductivity after electron doping by intercalating alkali metals between the layers. The superconductivity of β-form had been extensively characterized from decades ago, but it is not easy to consistently interpret all experimental results using conventional phonon-mediated Bardeen–Cooper–Schriefer mechanisms. The titanium compound TiNCl crystallizes only in the α-form structure. TiNCl also exhibits superconductivity as high as ~16 K after electron doping by intercalating metals and/or organic basis. It is important to compare the superconductivity of different M–N networks. However, α-form compounds are vulnerable to moisture, unlike β-form ones. The intercalation compounds are even more sensitive to humid air. Thus, there are few experimental studies on the superconducting mechanism of α-form, although it has been discussed for exotic Cooper-pairing mechanisms. This short review gathers the recent progress in experimental studies of TiNCl.

DOI： 10.3390/condmat7020033

Incorporation Site and Valence State of Sn Atoms in Sn-Substituted La(O,F)BiS2 Superconductor Reviewed

YaJun Li, ZeXu Sun, Noriyuki Kataoka, Taro Setoguchi, Yusuke Hashimoto, Soichiro Takeuchi, Shunjo Koga, Takayuki Muro, Satoshi Demura, Kanako Noguchi, Hideaki Sakata, Tomohiro Matsushita, Ikuto Kawasaki, Shin-ichi Fujimori, Takanori Wakita, Yuji Muraoka, Takayoshi Yokoya

J. Phys. Soc. Jpn. 91 ( 5 ) 054602 2022.4

Strain relaxation and spinodal decomposition in composition adjusted TiO2-VO2 films on TiO2(100) substrates Reviewed

Takahiro Fukuda, Yoshito Takemoto, Takanori Wakita, Takayoshi Yokoya, Yuji Muraoka

Thin Solid Films 751 139210 2022.4

The Local Structure of the BiS2 Layer in RE(O,F)BiS2 Determined by In-Plane Polarized X-ray Absorption Measurements Reviewed

G. M. Pugliese, L. Tortora, E. Paris, T. Wakita, K. Terashima, A. Puri, M. Nagao, R. Higashinaka, T. D. Matsuda, Y. Aoki, T. Yokoya, T. Mizokawa, N. L. Saini

Physchem 1 ( 3 ) 250 - 258 2021.11

Publishing type：Research paper (scientific journal) Publisher：MDPI AG

We have investigated the local structure of BiS2-based layered materials by Bi L3-edge extended X-ray absorption fine structure (EXAFS) measurements performed on single crystal samples with polarization of the X-ray beam parallel to the BiS2 plane. The results confirm highly instable nature of BiS2 layer, characterized by ferroelectric like distortions. The distortion amplitude, determined by the separation between the two in-plane (Bi-S1) bonds, is found to be highest in LaO0.77F0.23BiS2 with ΔR∼0.26 Å and lowest in NdO0.71F0.29BiS2 with ΔR∼0.13 Å. Among the systems with intrinsic doping, CeOBiS2 shows smaller distortion (ΔR∼0.15 Å) than PrOBiS2 (ΔR∼0.18 Å) while the highest distortion appears for EuFBiS2 revealing ΔR∼0.22 Å. It appears that the distortion amplitude is controlled by the nature of the RE(O,F) spacer layer in the RE(O,F)BiS2 structure. The X-ray absorption near edge structure (XANES) spectra, probing the local geometry, shows a spectral weight transfer that evolves systematically with the distortion amplitude in the BiS2-layer. The results provide a quantitative measurements of the local distortions in the instable BiS2-layer with direct implication on the physical properties of these materials.

DOI： 10.3390/physchem1030019

Photoinduced oxygen transport in cobalt double-perovskite crystal EuBaCo2O5.39 Reviewed

M. Hada, O. S. Ohmura, T. Ishikawa, M. Saigo, N. Keio, W. Yajima, T. Suzuku, D. Urushihara, K. Takubo, Y. Masaki, M. Kuwahara, K. Tsuruta, Y. Hayashi, J. Matsuno, T. Yokooya, K. Onda, F. Shimojo, M. Mase, S. Ishihara, T. Asaka, N. Abe

Applied Materials Today 24 101167 - 101167 2021.9

Soft x-ray irradiation induced metallization of layered TiNCl Reviewed

Noriyuki Kataoka, Masashi Tanaka, Wataru Hosoda, Takumi Taniguchi, Shin-ichi Fujimori, Takanori Wakita, Yuji Muraoka, Takayoshi Yokoya

Journal of Physics: Condensed Matter 33 ( 3 ) 035501 - 035501 2021.1

Authorship：Last author,　Corresponding author Publishing type：Research paper (scientific journal) Publisher：IOP Publishing

Abstract

We have performed soft x-ray spectroscopy in order to study the photoirradiation time dependence of the valence band structure and chemical states of layered transition metal nitride chloride TiNCl. Under the soft x-ray irradiation, the intensities of the states near the Fermi level (E_F) and the Ti³⁺ component increased, while the Cl 2p intensity decreased. Ti 2p–3d resonance photoemission spectroscopy confirmed a distinctive Fermi edge with Ti 3d character. These results indicate the photo-induced metallization originates from deintercalation due to Cl desorption, and thus provide a new carrier doping method that controls the conducting properties of TiNCl.

DOI： 10.1088/1361-648x/abbbc3

Other Link： https://iopscience.iop.org/article/10.1088/1361-648X/abbbc3/pdf

Momentum Dependent Band Renormalization and Surface Aging Effect on a Zone Center Electron Pocket in NaSn2As2 Revealed by Angle-Resolved Photoemission Spectroscopy Reviewed

Yuka Akabane, Taiki Shimaiwa, Yosuke Goto, Yoshikazu Mizuguchi, Takayoshi Yokoya, Masashi Arita, Shiv Kumar, Eike F. Schwier, Kenya Shimada, Naurang L. Saini, Takashi Mizokawa

Journal of the Physical Society of Japan 89 ( 11 ) 114707 - 114707 2020.11

The local structure of self-doped BiS2-based layered systems as a function of temperature Reviewed

G. M. Pugliese, E. Paris, F. G. Capone, F. Stramaglia, T. Wakita, K. Terashima, T. Yokoya, T. Mizokawa, Y. Mizuguchi, N. L. Saini

Physical Chemistry Chemical Physics 22 ( 39 ) 22217 - 22225 2020.10

Formation of Q-carbon by adjusting sp3 content in diamond-like carbon films and laser energy density of pulsed laser annealing Reviewed

Hiroki Yoshinaka, Seiko Inubushi, Takanori Wakita, Takayoshi Yokoya, Yuji Muraoka

Carbon 167 504 - 511 2020.10

Theory for High‐Angular‐Resolution Photoelectron Holograms Considering the Inelastic Mean Free Path and the Formation Mechanism of Quasi‐Kikuchi Band Reviewed

Tomohiro Matsushita, Takayuki Muro, Takayoshi Yokoya, Kensei Terashima, Yukako Kato, Hirosuke Matsui, Naoyuki Maejima, Yusuke Hashimoto, Fumihiko Matsui

physica status solidi (b) 2000117 - 2000117 2020.7

Skewed electronic band structure induced by electric polarization in ferroelectric BaTiO<inf>3</inf> Reviewed

Norihiro Oshime, Jun Kano, Eiji Ikenaga, Shintaro Yasui, Yosuke Hamasaki, Sou Yasuhara, Satoshi Hinokuma, Naoshi Ikeda, Pierre Eymeric Janolin, Jean Michel Kiat, Mitsuru Itoh, Takayoshi Yokoya, Tatsuo Fujii, Akira Yasui, Hitoshi Osawa

Scientific Reports 10 ( 1 ) 2020.7

Publishing type：Research paper (scientific journal)

© 2020, The Author(s). Skewed band structures have been empirically described in ferroelectric materials to explain the functioning of recently developed ferroelectric tunneling junction (FTJs). Nonvolatile ferroelectric random access memory (FeRAM) and the artificial neural network device based on the FTJ system are rapidly developing. However, because the actual ferroelectric band structure has not been elucidated, precise designing of devices has to be advanced through appropriate heuristics. Here, we perform angle-resolved hard X-ray photoemission spectroscopy of ferroelectric BaTiO3 thin films for the direct observation of ferroelectric band skewing structure as the depth profiles of atomic orbitals. The depth-resolved electronic band structure consists of three depth regions: a potential slope along the electric polarization in the core, the surface and interface exhibiting slight changes. We also demonstrate that the direction of the energy shift is controlled by the polarization reversal. In the ferroelectric skewed band structure, we found that the difference in energy shifts of the atomic orbitals is correlated with the atomic configuration of the soft phonon mode reflecting the Born effective charges. These findings lead to a better understanding of the origin of electric polarization.

DOI： 10.1038/s41598-020-67651-w

High-pressure x-ray absorption and diffraction study of the self-doped superconductor EuFBiS2 Reviewed

E. Paris, B. Joseph, C. Marini, K. Terashima, T. Wakita, T. Yokoya, Y. Mizuguchi, T. Mizokawa, N. L. Saini

Physical Review B 101 ( 21 ) 2020.6

Change in the electronic structure of the bismuth chalcogenide superconductor CsBi4−x Pb x Te6 by dissociation of the bismuth dimers Reviewed

Hiroyuki Okazaki, Kensei Terashima, David Billington, Keiji Iwata, Takanori Wakita, Masashi Tanaka, Yoshihiko Takano, Yuji Muraoka, Takayoshi Yokoya

Journal of Physics: Condensed Matter 32 ( 14 ) 145501 - 145501 2020.4

Change in the work function of VO2 thin film across the metal-insulator transition in VO2/TiO2:Nb(001) heterostructures Reviewed

Yuji Muraoka, Yuki Suga, Hayato Nakahara, Takayoshi Yokoya

JPS Conf. Proc. 30 011071 - 011071 2020.3

Language：English Publishing type：Research paper (international conference proceedings) Publisher：IOP Publishing

Abstract

We have performed soft x-ray spectroscopy in order to study the photoirradiation time dependence of the valence band structure and chemical states of layered transition metal nitride chloride TiNCl. Under the soft x-ray irradiation, the intensities of the states near the Fermi level (E_F) and the Ti³⁺ component increased, while the Cl 2p intensity decreased. Ti 2p–3d resonance photoemission spectroscopy confirmed a distinctive Fermi edge with Ti 3d character. These results indicate the photo-induced metallization originates from deintercalation due to Cl desorption, and thus provide a new carrier doping method that controls the conducting properties of TiNCl.

DOI： 10.1088/1361-648x/abbbc3

Other Link： https://iopscience.iop.org/article/10.1088/1361-648X/abbbc3/pdf

Strain effects on spinodal decomposition in TiO2-VO2 films on TiO2(100) substrates Reviewed

Yuji Muraoka, Fumiya Yoshii, Takahiro Fukuda, Yuji Manabe, Mikiko Yasuno, Yoshito Takemoto, Kensei Terashima, Takanori Wakita, Takayoshi Yokoya

Thin Solid Films 698 137854 2020.2

Data-driven exploration for pressure-induced superconductors using diamond anvil cell with boron-doped diamond electrodes and undoped diamond insulating layer Reviewed

Ryo Matsumoto, Zhufeng Hou, Shintaro Adachi, Masanori Nagao, Sayaka Yamamoto, Peng Song, Noriyuki Kataoka, Pedro Baptista de Castro, Kensei Terashima, Hiroyuki Takeya, Hiromi Tanaka, Takayoshi Yokoya, Tetsuo Irifune, Kiyoyuki Terakura, Yoshihiko Takano

HIGH PRESSURE RESEARCH 40 ( 1 ) 22 - 34 2020.1

Temperature dependent local atomic displacements in NaSn2As2 system Reviewed

Pugliese G. M, Stramaglia F, Goto Y, Terashima K, Simonelli L, Fujiwara H, Puri A, Marini C, Hacisalihoglu M. Y, d'Acapito F, Yokoya T, Mizokawa T, Mizuguchi Y, Saini N. L

JOURNAL OF PHYSICS-CONDENSED MATTER 31 ( 42 ) 2019.10

mu-PES Studies on TiNCl and Quasi-two-dimensional Superconductor Na-intercalated TiNCl Reviewed

Kataoka Noriyuki, Terashima Kensei, Tanaka Masashi, Hosoda Wataru, Taniguchi Takumi, Wakita Takanori, Muraoka Yuji, Yokoya Takayoshi

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 88 ( 10 ) 104709 - 104709 2019.10

Pressure-induced superconductivity in B i2-x S bx T e3-y S ey Reviewed

Tong He, Xiaofan Yang, Tomoya Taguchi, Teppei Ueno, Kaya Kobayashi, Jun Akimitsu, Hitoshi Yamaoka, Hirofumi Ishii, Yen Fa Liao, Hiromi Ota, Hidenori Goto, Ritsuko Eguchi, Kensei Terashima, Takayoshi Yokoya, Harald O. Jeschke, Xianxin Wu, Yoshihiro Kubozono

Physical Review B 100 ( 9 ) 2019.9

Publishing type：Research paper (scientific journal)

© 2019 American Physical Society. We systematically investigated the pressure dependence of electrical transport and the crystal structure of topological insulator, Bi2-xSbxTe3-ySey, which showed no superconductivity down to 2.0 K at ambient pressure. The Bi2-xSbxTe3-ySey crystal showed two structural phase transitions under pressure, from rhombohedral structure (space group No. 166, R3m, termed phase I) to monoclinic structure (space group No. 12, C2/m, termed phase II), and from phase II to another monoclinic structure (space group No. 12, C2/m, termed phase III). Superconductivity appeared when applying pressure; actually the superconductivity of all Bi2-xSbxTe3-ySey samples emerged in phase I. The superconducting transition temperature, Tc, increased against pressure in a pressure range of 0-15 GPa for all Bi2-xSbxTe3-ySey samples, and the maximum Tc was 5.45 K, recorded at 13.5 GPa in Bi2-xSbxTe3-ySey at x=0 and y=1.0. The magnetic field (H) dependence of the R-T plot for Bi2-xSbxTe3-ySey was measured to characterize the superconducting pairing mechanism of pressure-induced superconducting phase.

DOI： 10.1103/PhysRevB.100.094525

Asymmetric Phosphorus Incorporation in Homoepitaxial P-Doped (111) Diamond Revealed by Photoelectron Holography Reviewed

T. Yokoya, K. Terashima, A. Takeda, T. Fukura, H. Fujiwara, T. Muro, T. Kinoshita, H. Kato, S. Yamasaki, T. Oguchi, T. Wakita, Y. Muraoka, T. Matsushita

Nano Letters 19 ( 9 ) 5915 - 5919 2019.9

Selective Reduction Mechanism of Graphene Oxide Driven by the Photon Mode versus the Thermal Mode Reviewed

Masaki Hada, Kiyoshi Miyata, Satoshi Ohmura, Yusuke Arashida, Kohei Ichiyanagi, Ikufumi Katayama, Takayuki Suzuki, Wang Chen, Shota Mizote, Takayoshi Sawa, Takayoshi Yokoya, Toshio Seki, Jiro Matsuo, Tomoharu Tokunaga, Chihiro Itoh, Kenji Tsuruta, Ryo Fukaya, Shunsuke Nozawa, Shin-ichi Adachi, Jun Takeda, Ken Onda, Shin-ya Koshihara, Yasuhiko Hayashi, Yuta Nishina

ACS NANO 13 ( 9 ) 10103 - 10112 2019.9

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC

A two-dimensional nanocarbon, graphene, has attracted substantial interest due to its excellent properties. The reduction of graphene oxide (GO) has been investigated for the mass production of graphene used in practical applications. Different reduction processes produce different properties in graphene, affecting the performance of the final materials or devices. Therefore, an understanding of the mechanisms of GO reduction is important for controlling the properties of functional two-dimensional systems. Here, we determined the average structure of reduced GO prepared via heating and photoexcitation and clearly distinguished their reduction mechanisms using Graphene oxide ultrafast time-resolved electron diffraction, time-resolved infrared vibrational spectroscopy, and time-dependent density functional theory calculations. The oxygen atoms of epoxy groups are selectively removed from the basal plane of GO by photoexcitation (photon mode), in stark contrast to the behavior observed for the thermal reduction of hydroxyl and epoxy groups (thermal mode). The difference originates from the selective excitation of epoxy bonds via an electronic transition due to their antibonding character. This work will enable the preparation of the optimum GO for the intended applications and expands the application scope of two-dimensional systems.

DOI： 10.1021/acsnano.9b03060

The local structure of the Ca0.9Pr0.1Fe2As2 superconductor as a function of temperature Reviewed

Paris E, Wakita T, Simonelli L, Marini C, Olszewski W, Terashima K, Stramaglia F, Pugliese G. M, Mizokawa T, Loka S, Kudo K, Nohara M, Yokoya T, Saini N. L

SUPERCONDUCTOR SCIENCE & TECHNOLOGY 32 ( 9 ) 2019.9

Inhomogeneous charge distribution in a self-doped EuFBiS2 superconductor Reviewed

Sugimoto T, Paris E, Terashima K, Barinov A, Giampietri A, Wakita T, Yokoya T, Kajitani J, Higashinaka R, Matsuda T, Aoki Y, Mizokawa T, Saini N

PHYSICAL REVIEW B 100 ( 6 ) 2019.8

Three-dimensional dopant imaging in semiconductor crystals using photoelectron holography with chemical state identification

Tomohiro Matsushita, Takayuki Muro, Kazuo Tsutsui, Takayoshi Yokoya

19th International Workshop on Junction Technology, IWJT 2019 2019.6

Language：English Publishing type：Research paper (international conference proceedings) Publisher：Institute of Electrical and Electronics Engineers Inc.

Doping is an important technology for modern science. For example, to create a semiconductor device, a circuit is formed by controlling carriers by doping. It is important to search for appropriate conditions since the carrier emission from dopant differs depending on the doping conditions. The atomic arrangement around the dopant differs depending on the conditions. Therefore, it has been desired to observe the atomic arrangement around the dopant, but it has been difficult with conventional measurement methods. The atomic resolution holography such as photoelectron holography, X-ray fluorescence holography, neutron holography, which are methods that can measure the three-dimensional (3D) atomic arrangement of the dopant. Among them, photoelectron holography can measure the atomic structure of each dopant depending on the chemical state. We have built photoelectron holography apparatuses at BL25SU in SPring-8. We also developed a software platform 3D-AIR-IMAGE for data processing, simulation of photoelectron holograms, and 3D atomic image reconstruction.

DOI： 10.23919/IWJT.2019.8802898

Recent Photoemission Studies on BiCh(2)-Based Compounds Invited Reviewed

Terashimas Kensei, Wakita Takanori, Yokoya Takayoshi

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 88 ( 4 ) 2019.4

Enhanced thermoelectricity by controlled local structure in bismuth-chalcogenides Reviewed

Terashima K, Yano Y, Paris E, Goto Y, Mizuguchi Y, Kamihara Y, Wakita T, Muraoka Y, Saini N. L, Yokoya T

JOURNAL OF APPLIED PHYSICS 125 ( 14 ) 2019.4

Fermi level tuning of Ag-doped Bi2Se3 topological insulator Reviewed

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

SCIENTIFIC REPORTS 9 ( 1 ) 2019.3

Physics of heavily doped diamond: Electronic states and superconductivity

Takanori Wakita, Kensei Terashima, Takayoshi Yokoya

Physics and Chemistry of Carbon-Based Materials: Basics and Applications 65 - 96 2019.3

Language：English Publishing type：Part of collection (book) Publisher：Springer Singapore

Diamond is a popular material, not only as a gem but also for basic and applied research owing to its extraordinary properties. The discovery of superconductivity in heavily doped diamond has increased its importance. Although the highest superconducting critical temperature (Tc) is 10K, it is regarded as a superconductor with a potentially high Tc, owing to the strong covalent bonding between carbon atoms and the light atomic mass of carbon. In this chapter, after reviewing the fundamentals of the electronic states of pristine and doped diamond, we describe the electronic states, together with the vibrational properties, of borondoped superconductive diamond, which have been revealed by photoemission spectroscopy and other experimental techniques.

DOI： 10.1007/978-981-13-3417-7_3

Origins of Thermal Spin Depolarization in Half-Metallic Ferromagnet CrO2 Reviewed

Fujiwara Hirokazu, Terashima Kensei, Sunagawa Masanori, Yano Yuko, Nagayama Takanobu, Fukura Tetsushi, Yoshii Fumiya, Matsuura Yuka, Ogata Makoto, Wakita Takanori, Yaji Koichiro, Harasawa Ayumi, Kuroda Kenta, Shin Shik, Horiba Koji, Kumigashira Hiroshi, Muraoka Yuji, Yokoya Takayoshi

PHYSICAL REVIEW LETTERS 121 ( 25 ) 2018.12

Authorship：Last author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

Using high-resolution spin-resolved photoemission spectroscopy, we observe a thermal spin depolarization to which all spin-polarized electrons contribute. Furthermore, we observe a distinct minority spin state near the Fermi level and a corresponding depolarization that seldom contributes to demagnetization. The origin of this depolarization has been identified as the many-body effect characteristic of half-metallic ferromagnets. Our investigation opens an experimental field of itinerant ferromagnetic physics focusing on phenomena with sub-meV energy scale.

DOI： 10.1103/PhysRevLett.121.257201

Fermi surface topology in a metallic phase of VO2 thin films grown on TiO2(001) substrates Reviewed

Muraoka Yuji, Nagao Hiroki, Yao Yuichiro, Wakita Takanori, Terashima Kensei, Yokoya Takayoshi, Kumigashira Hiroshi, Oshima Masaharu

SCIENTIFIC REPORTS 8 2018.12

Multilayer formation via spinodal decomposition in TiO2-VO2 epitaxial films on sapphire substrates Reviewed

Matsuura Yuka, Yoshii Fumiya, Otsuka Tsubasa, Kadowaki Kenji, Ijiri Masataka, Takemoto Yoshito, Terashima Kensei, Wakita Takanori, Yokoya Takayoshi, Muraoka Yuji

JOURNAL OF THE EUROPEAN CERAMIC SOCIETY 38 ( 15 ) 5043 - 5050 2018.12

Suppression of structural instability in LaOBiS2-xSex by Se substitution Reviewed

Paris E, Mizuguchi Y, Wakita T, Terashima K, Yokoya T, Mizokawa T, Saini N. L

JOURNAL OF PHYSICS-CONDENSED MATTER 30 ( 45 ) 2018.11

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

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

PHYSICAL CHEMISTRY CHEMICAL PHYSICS 20 ( 36 ) 23783 - 23788 2018.9

Language：English Publishing type：Research paper (scientific journal) Publisher：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

Temperature-dependent local structure and superconductivity of BaPd2As2 and SrPd2As2 Reviewed

Terashima K, Paris E, Simonelli L, Salas-Colera E, Puri A, Wakita T, Yamada Y, Nakano S, Idei H, Kudo K, Nohara M, Muraoka Y, Mizokawa T, Yokoya T, Saini N. L

PHYSICAL REVIEW B 98 ( 9 ) 2018.9

Change of bent band structure on ferroelectric BaTiO₃ by electric polarization reversal

Oshime Norihiro, Kano Jun, Ikenaga Eiji, Yasui Shintaro, Hinokuma Satoshi, Ikeda Naoshi, Hamasaki Yosuke, Yasuhara Sou, Yokoya Takayoshi, Itoh Mitsuru, Fujii Tatsuo, Yasui Akira, Osawa Hitoshi

JSAP Annual Meetings Extended Abstracts 2018.2 1334 - 1334 2018.9

Direct observation of bent band structure on ferroelectric BaTiO₃ II

Oshime Norihiro, Kano Jun, Ikenaga Eiji, Yasui Shintaro, Hinokuma Satoshi, Ikeda Naoshi, Hamasaki Yosuke, Yasuhara Sou, Yokoya Takayoshi, Itoh Mitsuru, Fujii Tatsuo, Yasui Akira

JSAP Annual Meetings Extended Abstracts 2018.1 1434 - 1434 2018.3

Metallic phase in stoichiometric CeOBiS2 revealed by space-resolved ARPES

T. Sugimoto, E. Paris, T. Wakita, K. Terashima, T. Yokoya, A. Barinov, J. Kajitani, R. Higashinaka, T. D. Matsuda, Y. Aoki, T. Mizokawa, N. L. Saini

Scientific Reports 8 ( 1 ) 2018.1

Direct observation of double valence-band extrema and anisotropic effective masses of the thermoelectric material SnSe Reviewed

Takanobu Nagayama, Kensei Terashima, Takanori Wakita, Hirokazu Fujiwara, Tetsushi Fukura, Yuko Yano, Kanta Ono, Hiroshi Kumigashira, Osamu Ogiso, Aichi Yamashita, Yoshihiko Takano, Hitoshi Mori, Hidetomo Usui, Masayuki Ochi, Kazuhiko Kuroki, Yuji Muraoka, Takayoshi Yokoya

JAPANESE JOURNAL OF APPLIED PHYSICS 57 ( 1 ) 2018.1

Authorship：Last author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

Synchrotron-based angle-resolved photoemission spectroscopy is used to determine the electronic structure of layered SnSe, which was recently turned out to be a potential thermoelectric material. We observe that the top of the valence band consists of two nearly independent hole bands, whose tops differ by similar to 20meV in energy, indicating the necessity of a multivalley model to describe the thermoelectric properties. The estimated effective masses are anisotropic, with in-plane values of 0.16-0.39 m(0) and an out-of-plane value of 0.71 m(0), where m(0) is the rest electron mass. Information of the electronic structure is essential to further enhance the thermoelectric performance of hole-doped SnSe. (C) 2018 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.57.010301

Effect of molecular intercalation on the local structure of superconducting Na-x(NH3)(y)MoSe2 system Reviewed

L. Simonelli, E. Paris, T. Wakita, C. Marini, K. Terashima, X. Miao, W. Olszewski, N. Ramanan, D. Heinis, Y. Kubozono, T. Yokoya, N. L. Saini

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 111 70 - 74 2017.12

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

We have studied the local structure of layered Na-x(NH3)(y)MoSe2 system by Mo K-edge extended X-ray absorption fine structure (EXAFS) measurements performed as a function of temperature. We find that molecular intercalation in MoSe2 largely affects the Mo-Se network while Mo-Mo seems to sustain small changes. The Einstein temperature (Theta(E)) of Mo-Mo distance hardly changes (similar to 264 K) indicating that bond strength of this distance remains unaffected by intercalation. On the other hand, Mo-Se distance suffers a softening, revealed by the decrease of (Theta(E)) from similar to 364 K to similar to 350 K. The results indicate that Na+ ion transported by NH3 molecules may enter between the two MoSe-layers resulting reduced Se-Se coupling. Therefore, increased hybridization between Se 4p and Mo 4d orbitals due to inter-layer disorder is the likely reason of metallicity in intercalated MoSe2 and superconductivity at low temperature.

DOI： 10.1016/j.jpcs.2017.07.011

Distinct local structure of superconducting Ca10M4As8(Fe2As)(5) (M = Pt, Ir) Reviewed

E. Paris, T. Wakita, O. Proux, T. Yokoya, K. Kudo, D. Mitsuoka, T. Kimura, K. Fujimura, N. Nishimoto, S. Ioka, M. Nohara, T. Mizokawa, N. L. Saini

PHYSICAL REVIEW B 96 ( 22 ) 2017.12

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We have studied the local structure of superconducting Ca10Pt4As8(Fe2As2)(5) (Pt10418) and Ca10Ir4As8(Fe2As2)(5) (Ir10418) iron arsenides, showing different transition temperatures (T-c = 38 and 16 K, respectively), by polarized Fe K-edge extended x-ray absorption fine-structure measurements. Despite the similar average crystal structures, the local structures of the FeAs4 tetrahedra in the two compounds are found to be very different. The FeAs4 in Pt10418 is close to a regular tetrahedron, while it deviates largely in Ir10418. The Fe-Fe correlations in the two compounds are characterized by similar bond-length characteristics; however, the static disorder in Pt10418 is significantly lower than that in Ir10418. The results suggest that the optimized local structure and reduced disorder are the reasons for higher Tc and well-defined electronic states in Pt10418 unlike Ir10418 showing the coexistence of glassy and normal electrons at the Fermi surface, and hence provide direct evidence of the local-structure-driven optimization of the electronic structure and superconductivity in iron arsenides.

DOI： 10.1103/PhysRevB.96.224507

The electronic structure of Ag1-xSn1+xSe2 (x=0.0, 0.1, 0.2, 0.25 and 1.0) Reviewed

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

PHYSICAL CHEMISTRY CHEMICAL PHYSICS 19 ( 39 ) 26672 - 26678 2017.10

Language：English Publishing type：Research paper (scientific journal) Publisher：ROYAL SOC 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-delta 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

Observation of intrinsic half-metallic behavior of CrO2 (100) epitaxial films by bulk-sensitive spin-resolved PES Reviewed

Hirokazu Fujiwara, Masanori Sunagawa, Kensei Terashima, Tomoko Kittaka, Takanori Wakita, Yuji Muraoka, Takayoshi Yokoya

JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 220 46 - 49 2017.10

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We have investigated the electronic states and spin polarization of half-metallic ferromagnet CrO2 (100) epitaxial films by bulk-sensitive spin-resolved photoemission spectroscopy with a focus on non-quasiparticle (NQP) states derived from electron-magnon interactions. We found that the averaged values of the spin polarization are approximately 100% and 40% at 40 K and 300 K, respectively. This is consistent with the previously reported result [H. Fujiwara et al., Appl. Phys. Lett. 106, 202404 (2015).]. At 100 K, peculiar spin depolarization was observed at the Fermi level (E-F), which is supported by theoretical calculations predicting NQP states. This suggests the possible appearance of NQP states in CrO2. We also compare the temperature dependence of our spin polarizations with that of the magnetization. (C) 2017 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2017.02.003

Inhomogeneous electronic states associated with charge-orbital order/disorder in BaV10O15 probed by photoemission spectromicroscopy Reviewed

T. Yoshino, K. Wakita, E. Paris, A. Barinov, T. Kajita, T. Katsufuji, V. Kandyba, T. Sugimoto, T. Yokoya, N. L. Saini, T. Mizokawa

PHYSICAL REVIEW B 96 ( 11 ) 2017.9

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We have performed scanning photoemission spectromicroscopy of BaV10O15 across the metal-insulator transition at 123 K, which is accompanied by V 3d charge/orbital order and V trimerization. Nucleation of metallic domains is observed at the cleaved surface of BaV10O15 single crystals, similar to Cr-doped V2O3 in which electronic configurations of Cr3+ and V3+ are the same as those of V2+ and V3+ in BaV10O15. Typical domain size is similar to 5- 10 mu m at 150 K, just above the transition temperature. The metallic domains continuously grow up to 240 K, well above the transition temperature. The temperature evolution of the metallic phase in BaV10O15 is different from that of Cr-doped V2O3, probably due to the charge degrees of freedom in BaV10O15.

DOI： 10.1103/PhysRevB.96.115161

Direct observation of bent band structure on ferroelectric BaTiO₃

Oshime Norihiro, Kano Jun, Ikenaga Eiji, Yasui Shintaro, Hinokuma Satoshi, Ikeda Naoshi, Hamasaki Yosuke, Yasuhara Sou, Yokoya Takayoshi, Itoh Mitsuru, Fujii Tatsuo, Yasui Akira

JSAP Annual Meetings Extended Abstracts 2017.2 1242 - 1242 2017.8

Effect of aliovalent dopants on the kinetics of spinodal decomposition in rutile-type TiO2-VO2 Reviewed

Makoto Ogata, Kenji Kadowaki, Masataka Ijiri, Yoshito Takemoto, Kensei Terashima, Takanori Wakita, Takayoshi Yokoya, Yuji Muraoka

JOURNAL OF THE EUROPEAN CERAMIC SOCIETY 37 ( 9 ) 3177 - 3183 2017.8

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCI LTD

The effects of aliovalent dopants on the kinetics of spinodal decomposition in a rutile-type TiO2-VO2 system were studied by means of x-ray diffraction and transmission electron microscopy. In this study, as prepared polycrystalline samples were annealed at 673 k inside the spinodal. For undoped samples, phase decomposition occurred along the [001] direction, and a lamellar structure formed on the nanometer length scale. The phase decomposition kinetics in this system were sensitive to the type of aliovalent dopant; the decomposition rate in a sample doped with Al2O3 was more than one order of magnitude higher than a sample doped with . These observations could be explained on the basis of the premise that cation interstitial mobility is greater than cation vacancy mobility. The obtained results are consistent with those for a TiO2-SnO2 system, indicating that phase separation in both rutile-type oxide systems can be interpreted through a common framework. (C) 2017 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jeurceramsoc.2017.03.039

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

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 ) 2017.7

Language：English Publishing type：Research paper (scientific journal) Publisher：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

Unusual upper critical field behavior in Nb-doped bismuth selenides Reviewed

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

Physical Review B 95 ( 18 ) 2017.5

Language：English Publishing type：Research paper (scientific journal) Publisher：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

Electronic structure of K-doped picene film on HOPG Reviewed

T. Wakita, H. Okazaki, T. Jabuchi, H. Hamada, Y. Muraoka, T. Yokoya

JOURNAL OF PHYSICS-CONDENSED MATTER 29 ( 6 ) 2017.2

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

We have performed potassium (K)-doping dependent photoemission (PES) measurements of picene film on highly oriented pyrolytic graphite (HOPG). K-doping dependent valence band PES data exhibits a shift of the valence band to a higher binding energy, indicative of the charge transfer from the K atoms to the picene film. K-doping dependent PES spectra near the Fermi level (EF) show the appearance and disappearance of a Fermi edge, indicating the metallic properties of the film at certain K concentrations. High-resolution PES spectrum at the lowest measured temperature does not show an opening of the superconducting gap, requesting further studies to explore the superconducting properties of the K-doped picene film. The results will be discussed by comparison with previous and recent spectroscopic studies.

DOI： 10.1088/1361-648X/29/6/064001

Evolution of Eu valence and superconductivity in layered Eu0.5La0.5FBiS2-xSex system Reviewed

Y. Mizuguchi, E. Paris, T. Wakita, G. Jinno, A. Puri, K. Terashima, B. Joseph, O. Miura, T. Yokoya, N. L. Saini

PHYSICAL REVIEW B 95 ( 6 ) 2017.2

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We have studied the effect of Se substitution on Eu valence in a layered Eu0.5La0.5FBiS2-x Se-x superconductor using a combined analysis of x-ray absorption near-edge structure (XANES)and x-ray photoelectron spectroscopy (XPS) measurements. Eu L-3-edge X Lambda NES spectra reveal that Eu is in the mixed valence state with coexisting Eu2+ and Eu3+. The average Eu valence decreases sharply from similar to 2.3 for x = 0.0 to similar to 2.1 for x = 0.4. Consistently, Eu 3d XPS shows a clear decrease in the average valence by Se substitution. Bi 4f XPS indicates that effective charge carriers in the BiCh(2) (Ch = S, Se) layers are slightly increased by Se substitution. On the basis of the present results it has been discussed that the metallic character induced by Se substitution in Eu0.5La0.5FBiS2- x Se-x is likely to be due to increased in-plane orbital overlap driven by reduced in-plane disorder that affects the carrier mobility.

DOI： 10.1103/PhysRevB.95.064515

Electronic structure of self-doped layered Eu3 F4Bi2 S4 material revealed by x-ray absorption spectroscopy and photoelectron spectromicroscopy

E. Paris, T. Sugimoto, T. Sugimoto, T. Sugimoto, T. Wakita, A. Barinov, K. Terashima, V. Kandyba, O. Proux, J. Kajitani, R. Higashinaka, T. D. Matsuda, Y. Aoki, T. Yokoya, T. Mizokawa, N. L. Saini

Physical Review B - Condensed Matter and Materials Physics 95 2017.1

© 2017 American Physical Society.We have studied the electronic structure of Eu3F4Bi2S4 using a combination of Eu L3-edge x-ray absorption spectroscopy (XAS) and space-resolved angle-resolved photoemission spectroscopy (ARPES). From the Eu L3-edge XAS, we have found that the Eu in this system is in mixed valence state with coexistence of Eu2+/Eu3+. The bulk charge doping was estimated to be ∼0.3 per Bi site in Eu3F4Bi2S4, which corresponds to the nominal x in a typical REO1-xFxBiS2 system (RE: rare-earth elements). From the space-resolved ARPES, we have ruled out the possibility of any microscale phase separation of Eu valence in the system. Using a microfocused beam we have observed the band structure as well as the Fermi surface that appeared similar to other compounds of this family with disconnected rectangular electronlike pockets around the X point. The Luttinger volume analysis gives the effective carrier to be 0.23 electrons per Bi site in Eu3F4Bi2S4, indicating that the system is likely to be in the underdoped region of its superconducting phase diagram.

DOI： 10.1103/PhysRevB.95.035152

Electronic structure of self-doped layered Eu3F4Bi2S4 material revealed by x-ray absorption spectroscopy and photoelectron spectromicroscopy Reviewed

E. Paris, T. Sugimoto, T. Wakita, A. Barinov, K. Terashima, V. Kandyba, O. Proux, J. Kajitani, R. Higashinaka, T. D. Matsuda, Y. Aoki, T. Yokoya, T. Mizokawa, N. L. Saini

PHYSICAL REVIEW B 95 ( 3 ) 2017.1

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We have studied the electronic structure of Eu3F4Bi2S4 using a combination of Eu L-3-edge x-ray absorption spectroscopy (XAS) and space-resolved angle-resolved photoemission spectroscopy (ARPES). From the Eu L-3-edge XAS, we have found that the Eu in this system is in mixed valence state with coexistence of Eu2+/Eu3+. The bulk charge doping was estimated to be similar to 0.3 per Bi site in Eu3F4Bi2S4, which corresponds to the nominal chi in a typical REO1-chi F chi BiS2 system (RE: rare-earth elements). From the space-resolved ARPES, we have ruled out the possibility of any microscale phase separation of Eu valence in the system. Using a microfocused beam we have observed the band structure as well as the Fermi surface that appeared similar to other compounds of this family with disconnected rectangular electronlike pockets around the X point. The Luttinger volume analysis gives the effective carrier to be 0.23 electrons per Bi site in Eu3F4Bi2S4, indicating that the system is likely to be in the underdoped region of its superconducting phase diagram.

DOI： 10.1103/PhysRevB.95.035152

Ce 4f electronic states of CeO1-xFxBiS2 studied by soft x-ray photoemission spectroscopy Reviewed

Wakita, Takanori, Terashima, Kensei, Hamada, Takahiro, Fujiwara, Hirokazu, Minohara, Makoto, Kobayashi, Masaki, Horiba, Koji, Kumigashira, Hiroshi, Kutluk, Galif, Nagao, Masanori, Watauchi, Satoshi, Tanaka, Isao, Demura, Satoshi, Okazaki, Hiroyuki, Takano, Yoshihiko, Mizuguchi, Yoshikazu, Miura, Osuke, Okada, Kozo, Muraoka, Yuji, Yokoya, Takayoshi

Physical Review B 95 ( 8 ) 2017

Determination of the local structure of CsBi4-xPbxTe6 (x=0, 0.5) by X-ray absorption spectroscopy Reviewed

Takanori Wakita, Eugenio Paris, Takashi Mizokawa, Muammer Yasin Hacisalihoglu, Kensei Terashima, Hiroyuki Okazaki, Olivier Proux, Isabelle Kieffer, Eric Lahera, William Del Net, Luca Olivi, Yoshihiko Takano, Yuji Muraoka, Takayoshi Yokoya, Naurang L. Saini

PHYSICAL CHEMISTRY CHEMICAL PHYSICS 18 ( 36 ) 25136 - 25142 2016.9

Language：English Publishing type：Research paper (scientific journal) Publisher：ROYAL SOC CHEMISTRY

We have studied the local structure and valence electronic unoccupied states of thermoelectric CsBi4Te6 and superconducting CsBi3.5Pb0.5Te6 (T-c similar to 3 K) by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) measurements. The Bi-L-3 edge EXAFS reveals wide Bi-Te distance distribution for both compounds indicating complex atomic arrangements in the studied system. The mean square relative displacements (MSRDs) of the Bi-Te bond distances appear largely increased in Pb substituted system due to larger overall local disorder, however, one of the Bi-Te bonds shows a reduced disorder. On the other hand, the Bi-L-3 edge XANES is hardly affected by Pb substitution while the Te-L-1 edge XANES reveals increased density of unoccupied Te 5p states. This suggests that the carriers introduced by the Pb substitution in CsBi4-xPbxTe6 preferentially goes on Te sites. Similarly, the Cs-L-3 edge XANES also shows small changes due to Pb-substitution and reduced local disorder indicated by the reduced width of the Cs-L3 edge white line. We have also shown that the X-ray photoemission spectroscopy (XPS) measurements on various electronic core levels are in a qualitative agreement with the XANES results. These findings are consistent with carrier doping and a reduced disorder in one direction to be likely factors to drive the thermoelectric CsBi4Te6 into a bulk superconductor by Pb-substitution in CsBi4-xPbxTe6.

DOI： 10.1039/c6cp04949d

Emergence of superconductivity in (NH3)(y)MxMoSe2 (M: Li, Na and K) Reviewed

Xiao Miao, Saki Nishiyama, Lu Zheng, Hidenori Goto, Ritsuko Eguchi, Hiromi Ota, Takashi Kambe, Kensei Terashima, Takayoshi Yokoya, Huyen T. L. Nguyen, Tomoko Kagayama, Naohisa Hirao, Yasuo Ohishi, Hirofumi Ishii, Yen-Fa Liao, Yoshihiro Kubozono

SCIENTIFIC REPORTS 6 2016.7

Language：English Publishing type：Research paper (scientific journal) Publisher：NATURE PUBLISHING GROUP

We report syntheses of new superconducting metal-doped MoSe2 materials (MxMoSe2). The superconducting MxMoSe2 samples were prepared using a liquid NH3 technique, and can be represented as '(NH3)(y)MxMoSe2'. The T(c)s of these materials were approximately 5.0 K, independent of x and the specific metal atom. X-ray diffraction patterns of (NH3)(y)NaxMoSe2 were recorded using polycrystalline powders. An increase in lattice constant c showed that the Na atom was intercalated between MoSe2 layers. The x-independence of c was observed in (NH3)(y)NaxMoSe2, indicating the formation of a stoichiometric compound in the entire x range, which is consistent with the x-independence of T-c. A metallic edge of the Fermi level was observed in the photoemission spectrum at 30 K, demonstrating its metallic character in the normal state. Doping of MoSe2 with Li and K also yielded superconductivity. Thus, MoSe2 is a promising material for designing new superconductors, as are other transition metal dichalcogenides.

DOI： 10.1038/srep29292

Observation of a Hidden Hole-Like Band Approaching the Fermi Level in K-Doped Iron Selenide Superconductor Reviewed

Masanori Sunagawa, Kensei Terashima, Takahiro Hamada, Hirokazu Fujiwara, Tetsushi Fukura, Aya Takeda, Masashi Tanaka, Hiroyuki Takeya, Yoshihiko Takano, Masashi Arita, Kenya Shimada, Hirofumi Namatame, Masaki Taniguchi, Katsuhiro Suzuki, Hidetomo Usui, Kazuhiko Kuroki, Takanori Wakita, Yuji Muraoka, Takayoshi Yokoya

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 85 ( 7 ) 2016.7

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

One of the ultimate goals of the study of iron-based superconductors is to identify the common feature that produces the high critical temperature (T-c). In the early days, based on a weak-coupling viewpoint, the nesting between hole-and electron-like Fermi surfaces (FSs) leading to the so-called s +/- state was considered to be one such key feature. However, this theory has faced a serious challenge ever since the discovery of alkali-metal-doped FeSe (AFS) superconductors, in which only electron-like FSs with a nodeless superconducting gap are observed. Several theories have been proposed, but a consistent understanding is yet to be achieved. Here we show experimentally that a hole-like band exists in KxFe2-ySe2, which presumably forms a hole-like Fermi surface. The present study suggests that AFS can be categorized in the same group as iron arsenides with both hole- and electron-like FSs present. This result provides a foundation for a comprehensive understanding of the superconductivity in iron-based superconductors.

DOI： 10.7566/JPSJ.85.073704

Temperature dependent local atomic displacements in ammonia intercalated iron selenide superconductor Reviewed

E. Paris, L. Simonelli, T. Wakita, C. Marini, J. -H. Lee, W. Olszewski, K. Terashima, T. Kakuto, N. Nishimoto, T. Kimura, K. Kudo, T. Kambe, M. Nohara, T. Yokoya, N. L. Saini

SCIENTIFIC REPORTS 6 2016.6

Language：English Publishing type：Research paper (scientific journal) Publisher：NATURE PUBLISHING GROUP

Recently, ammonia-thermal reaction has been used for molecular intercalation in layered FeSe, resulting a new Li-x(NH3)(y)Fe2Se2 superconductor with T-c similar to 45 K. Here, we have used temperature dependent extended x-ray absorption fine structure (EXAFS) to investigate local atomic displacements in single crystals of this new superconductor. Using polarized EXAFS at Fe K-edge we have obtained direct information on the local Fe-Se and Fe-Fe bondlengths and corresponding mean square relative displacements (MSRD). We find that the Se-height in the intercalated system is lower than the one in the binary FeSe, suggesting compressed FeSe4 tetrahedron in the title system. Incidentally, there is hardly any effect of the intercalation on the bondlengths characteristics, revealed by the Einstein temperatures, that are similar to those found in the binary FeSe. Therefore, the molecular intercalation induces an effective compression and decouples the FeSe slabs. Furthermore, the results reveal an anomalous change in the atomic correlations across T-c, appearing as a clear decrease in the MSRD, indicating hardening of the local lattice mode. Similar response of the local lattice has been found in other families of superconductors, e.g., A15-type and cuprates superconductors. This observation suggests that local atomic correlations should have some direct correlation with the superconductivity.

DOI： 10.1038/srep27646

Local electronic state in the half-metallic ferromagnet CrO2 investigated by site-selective Cr-53 NMR measurements Reviewed

Hikaru Takeda, Yasuhiro Shimizu, Yoshiaki Kobayashi, Masayuki Itoh, Takaaki Jin-no, Masahiko Isobe, Yutaka Ueda, Sho Yoshida, Yuji Muraoka, Takayoshi Yokoya

PHYSICAL REVIEW B 93 ( 23 ) 2016.6

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We have made Cr-53 NMR measurements on polycrystalline and single-crystalline film samples to study the local electronic state of a half-metallic ferromagnet CrO2 which has the rutile structure with one crystallographically equivalent Cr site. We observe two kinds of Cr-53 NMR spectra in the ferromagnetic state of both the samples. An analysis on the hyperfine field shows the presence of two Cr sites with different orbital occupancies, suggesting that a local orbital order takes place with breaking the local symmetry in the rutile structure. This may be ascribed to the negative charge transfer between chromium and oxygen ions.

DOI： 10.1103/PhysRevB.93.235129

Preparation of TaO2 thin films using NbO2 template layers by a pulsed laser deposition technique Reviewed

Y. Muraoka, Y. Fujimoto, M. Kameoka, Y. Matsuura, M. Sunagawa, K. Terashima, T. Wakita, T. Yokoya

THIN SOLID FILMS 599 125 - 132 2016.1

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE SA

TaO2 thin films were prepared using rutile-type NbO2 template layers on Al2O3 (0001) substrates by a pulsed laser deposition technique. The structural and electronic properties of the films were characterized by performing four-circle X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS) measurements. Four-circle XRD measurements revealed the formation and epitaxial growth of (100)-oriented tetragonal TaO2 on the NbO2 template layers. On the other hand, XPS measurements showed two kinds of valence states, namely Ta4+ and Ta5+ in the surface region of the film. The results from the XRD and XPS measurements suggest that the film is composed of epitaxially grown TaO2 with an amorphous Ta2O5 surface layer. It is found from control experiments that the NbO2 template layers play an important role in inducing the crystallization of the TaO2 films and promoting the formation of Ta4+ in the films deposited thereon. The results indicate the effectiveness of the NbO2 template layers for the formation of TaO2 thin films. Our work presents a promising method for the preparation of TaO2 thin films and provides a starting point for further research on TaO2. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tsf.2015.12.066

Comparative ARPES studies of LaOxF1-xBiS2 (x=0.23 and 0.46) Reviewed

K. Terashima, T. Wakita, M. Sunagawa, H. Fujiwara, T. Nagayama, K. Ono, H. Kumigashira, M. Nagao, S. Watauchi, I. Tanaka, H. Okazaki, Y. Takano, Y. Mizuguchi, H. Usui, K. Kuroki, Y. Muraoka, T. Yokoya

TMU INTERNATIONAL SYMPOSIUM ON NEW QUANTUM PHASES EMERGING FROM NOVEL CRYSTAL STRUCTURE 683 2016

Language：English Publishing type：Research paper (international conference proceedings) Publisher：IOP PUBLISHING LTD

BiS2-based layered superconductors are new superconductors, having been paid special attention due to the layered crystal structure and the characteristic electronic structure that can lead to exotic mechanism of superconductivity as well as cooperation of superconductivity and ordered phases. In this article, we report valence band dispersions and Fermi surface topology of LaO1-xFxBiS2 (x = 0.23, T-c < 2 K) studied by angle-resolved photoemission spectroscopy, and compared them with those of LaO1-xFxBiS2 (x = 0.46, T-c = 3.1 K). We discuss experimental x dependent electronic structures and its relation to T-c. We also point to anomalous spectroscopic features in LaO1-xFxBiS2 (x = 0.46) that can be explained by band structure calculations considering distorted lattice, suggesting role of dynamical or static lattice distortion to the physical properties.

DOI： 10.1088/1742-6596/683/1/012002

Bulk sensitive angle-resolved photoelectron spectroscopy on Nd(O,F)BiS2 Reviewed

K. Terashima, J. Sonoyama, M. Sunagawa, H. Fujiwara, T. Nagayama, T. Muro, M. Nagao, S. Watauchi, I. Tanaka, H. Okazaki, Y. Takano, Y. Mizuguchi, H. Usui, K. Suzuki, K. Kuroki, T. Wakita, Y. Muraoka, T. Yokoya

TMU INTERNATIONAL SYMPOSIUM ON NEW QUANTUM PHASES EMERGING FROM NOVEL CRYSTAL STRUCTURE 683 2016

Language：English Publishing type：Research paper (international conference proceedings) Publisher：IOP PUBLISHING LTD

Bulk electronic structure of novel layered superconductor Nd(O,F)BiS2 was studied by using soft x-ray angle-resolved photoelectron spectroscopy (ARPES). Electron-like Fermi surface centered at the X(R) point was observed, consistent with earlier ARPES reports on surface-sensitive VUV light source. Based on the comparison of the electronic structure between Nd(O,F)BiS2 and La(O,F)BiS2, we discuss possible important factors for the superconductivity in this series of material.

DOI： 10.1088/1742-6596/683/1/012003

19aBP-1 ARPES study on thermoelectric material SnSe

Nagayama T., Terashima K., Tanaka M., Ogiso O., Takano Y., Mori H., Usui H., Kuroki K., Wakita T., Muraoka Y., Yokoya T.

Meeting Abstracts of the Physical Society of Japan 71 1441 - 1441 2016

19pAJ-1 Surface metallicity of the half-metal CrO_2(001) films

Fujiwara H., Sunagawa M., Terashima K., Kittaka T., Ogata M., Ideta S., Tanaka K., Wakita T., Muraoka Y., Yokoya T.

Meeting Abstracts of the Physical Society of Japan 71 2492 - 2492 2016

21pBN-6 Angle-resolved photoemission study on Sr_<2-x>La_xIrO_4

Terashima K., Hamada T., Sunagawa M., Fujiwara H., Fujii M., Horigane K., Kobayashi K., Horie R., Akimitsu J., Muraoka Y., Yokoya T.

Meeting Abstracts of the Physical Society of Japan 71 2315 - 2315 2016

Superconductivity in aromatic hydrocarbons Reviewed

Yoshihiro Kubozono, Hidenori Goto, Taihei Jabuchi, Takayoshi Yokoya, Takashi Kambe, Yusuke Sakai, Masanari Izumi, Lu Zheng, Shino Hamao, Huyen L. T. Nguyen, Masafumi Sakata, Tomoko Kagayama, Katsuya Shimizu

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 514 199 - 205 2015.7

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

'Aromatic hydrocarbon' implies an organic molecule that satisfies the (4n + 2) pi-electron rule and consists of benzene rings. Doping solid aromatic hydrocarbons with metals provides the superconductivity. The first discovery of such superconductivity was made for K-doped picene (K(x)picene, five benzene rings). Its superconducting transition temperatures (T-c's) were 7 and 18 K. Recently, we found a new superconducting K(x)picene phase with a T-c as high as 14 K, so we now know that K(x)picene possesses multiple superconducting phases. Besides K(x)picene, we discovered new superconductors such as Rb(x)picene and Caxpicene. A most serious problem is that the shielding fraction is <= 615% for K(x)picene and Rb(x)picene, and it is often similar to 1% for other superconductors. Such low shielding fractions have made it difficult to determine the crystal structures of superconducting phases. Nevertheless, many research groups have expended a great deal of effort to make high quality hydrocarbon superconductors in the five years since the discovery of hydrocarbon superconductivity. At the present stage, superconductivity is observed in certain metaldoped aromatic hydrocarbons (picene, phenanthrene and dibenzopentacene), but the shielding fraction remains stubbornly low. The highest priority research area is to prepare aromatic superconductors with a high superconducting volume-fraction. Despite these difficulties, aromatic superconductivity is still a core research target and presents interesting and potentially breakthrough challenges, such as the positive pressure dependence of T-c that is clearly observed in some phases of aromatic hydrocarbon superconductors, suggesting behavior not explained by the standard BCS picture of superconductivity. In this article, we describe the present status of this research field, and discuss its future prospects. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2015.02.015

Intrinsic spin polarized electronic structure of CrO2 epitaxial film revealed by bulk-sensitive spin-resolved photoemission spectroscopy Reviewed

Hirokazu Fujiwara, Masanori Sunagawa, Kensei Terashima, Tomoko Kittaka, Takanori Wakita, Yuji Muraoka, Takayoshi Yokoya

APPLIED PHYSICS LETTERS 106 ( 20 ) 2015.5

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

We have performed bulk-sensitive spin-resolved photoemission spectroscopy in order to clarify the intrinsic spin-resolved electronic states of half-metallic ferromagnet CrO2. We used CrO2 epitaxial films on TiO2(100), which shows a peak at 1 eV with a clear Fermi edge, consistent with the bulk-sensitive PES spectrum for CrO2. In spin-resolved spectra at 40 K, while the Fermi edge was observed in the spin up (majority spin) state, no states at the Fermi level (EF) with an energy gap of 0.5 eV below EF were observed in the spin down (minority spin) state. At 300 K, the gap in the spin down state closes. These results are consistent with resistivity measurements and magnetic hysteresis curves of the fabricated CrO2 film, constituting spectroscopic evidence for the half-metallicity of CrO2 at low temperature and reducing the spin polarization at room temperature. We also discuss the electron correlation effects of Cr 3d. (C) 2015 AIP Publishing LLC.

DOI： 10.1063/1.4921629

Comparative ARPES Study on Iron-Platinum-Arsenide Superconductor Ca-10(Pt4As8)(Fe2-xPtxAs2)(5) (x=0.25 and 0.42) Reviewed

Masanori Sunagawa, Rikiya Yoshida, Toshihiko Ishiga, Koji Tsubota, Taihei Jabuchi, Junki Sonoyama, Satomi Kakiya, Daisuke Mitsuoka, Kazutaka Kudo, Minoru Nohara, Kanta Ono, Hiroshi Kumigashira, Tamio Oguchi, Takanori Wakita, Yuji Muraoka, Takayoshi Yokoya

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 84 ( 5 ) 2015.5

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

We studied the electronic structure of Ca-10(Pt4As8)(Fe2-xPtxAs2)(5) for two platinum concentrations with different T-c using ARPES. From the carrier number estimated from the areas of FS sheets, we found that the Pt substitution induces electron doping into the Fe2As2 layers (effectively two electrons per Pt atom). In addition, we found that the FS nesting condition in x = 0.25 is stronger than that in x = 0.42, indicating close correlation between the FS nesting and T-c.

DOI： 10.7566/JPSJ.84.055001

Signature of high T-c above 25 K in high quality superconducting diamond Reviewed

Hiroyuki Okazaki, Takanori Wakita, Takayuki Muro, Tetsuya Nakamura, Yuji Muraoka, Takayoshi Yokoya, Shin-ichiro Kurihara, Hiroshi Kawarada, Tamio Oguchi, Yoshihiko Takano

APPLIED PHYSICS LETTERS 106 ( 5 ) 2015.2

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

We have observed zero resistivity above 10K and an onset of resistivity reduction at 25.2K in a heavily B-doped diamond film. However, the effective carrier concentration is similar to that of superconducting diamond with a lower T-c. We found that the carrier has a longer mean free path and lifetime than in the previous report, indicating that this highest T-c diamond has better crystallinity compared to that of other superconducting diamond films. In addition, the susceptibility shows a small transition above 20K in the high quality diamond, suggesting a signature of superconductivity above 20 K. These results strongly suggest that heavier carrier doped defect-free crystalline diamond could give rise to high T-c diamond. (C) 2015 AIP Publishing LLC.

DOI： 10.1063/1.4907411

Signature of high $T_c$ around 25K in higher quality heavily boron-doped diamond Reviewed

H. Okazaki, T. Wakita, T. Muro, T. Nakamura, Y. Muraoka, T. Yokoya, S. Kurihara, H. Kawarada, T. Oguchi, Y. Takano

Appl. Phys. Lett. 2015.2

16aDB-6 Angle-resolved photoemission spectroscopy on KxFe_<2-y>Se_2 II

Sunagawa M., Terashima K., Hamada K., Fujiwara H., Fukura T., Takeda A., Tanaka M., Takeya H., Takano Y., Arita M., Shimada K., Namatame H., Taniguchi M., Suzuki K., Usui H., Kuroki K., Wakita T., Muraoka Y., Yokoya T.

Meeting Abstracts of the Physical Society of Japan 70 1681 - 1681 2015

18pCH-1 Intrinsic electronic states of ferromagnetic metal CrO2 revealed by bulk-sensitive spin-resolved photoemission spectroscopy

Fujiwara H., Sunagawa M., Terashima K., Kittaka T., Ogata M., Wakita T., Muraoka Y., Yokoya T.

Meeting Abstracts of the Physical Society of Japan 70 942 - 942 2015

16pDA-7 Relationship between Fermi surface topology and chemical pressure effect on Ln(O,F)BiS_2 studied by ARPES

Terashima K., Wakita T., Sunagawa M., Fujiwara H., Hamada T., Nagao M., Watauchi S., Tanaka I., Ono K., Kumigashira H., Okazaki H., Takano Y., Usui H., Kuroki K., Suzuki K., Muraoka Y., Yokoya T.

Meeting Abstracts of the Physical Society of Japan 70 1720 - 1720 2015

16aAD-4 Fe 3p-3d resonant photoemission study of mixed-valence compound YbFe_2O_4

Fukura T., Wakita T., Takeda A., Terashima K., Fujiwara K., Ikeda N., Muraoka Y., Yokoya T.

Meeting Abstracts of the Physical Society of Japan 70 2405 - 2405 2015

17pCD-9 High-resolution photoelectron study on Ce(Os_<1-y>Re_y)_2Al_<10>

Nagayama T., Terashima K., Kawabata J., Yamada Y., Takabatake T., Wakita T., Muraoka Y., Yokoya T.

Meeting Abstracts of the Physical Society of Japan 70 1933 - 1933 2015

18aCE-9 高濃度ボロンドープダイヤモンド超伝導のB1s-2p共鳴光電子分光

竹田彩, 脇田高徳, 福良哲司, 高野義彦, 栗原慎一郎, 川原田洋, 小口多美夫, 村岡祐治, 横谷尚睦

日本物理学会講演概要集 70 1237 - 1237 2015

Proximity to Fermi-surface topological change in superconducting LaO0.54F0.46BiS2 Reviewed

Terashima Kensei, Sonoyama Junki, Wakita Takanori, Sunagawa Masanori, Ono Kanta, Kumigashira Hiroshi, Muro Takayuki, Nagao Masanori, Watauchi Satoshi, Tanaka Isao, Okazaki Hiroyuki, Takano Yoshihiko, Miura Osuke, Mizuguchi Yoshikazu, Usui Hidetomo, Suzuki Katsuhiro, Kuroki Kazuhiko, Muraoka Yuji, Yokoya Takayoshi

PHYSICAL REVIEW B 90 ( 22 ) 2014.12

Proximity to Fermi-surface topological change in superconducting LaO0.54F0.46BiS2 Reviewed

Kensei Terashima, Junki Sonoyama, Takanori Wakita, Masanori Sunagawa, Kanta Ono, Hiroshi Kumigashira, Takayuki Muro, Masanori Nagao, Satoshi Watauchi, Isao Tanaka, Hiroyuki Okazaki, Yoshihiko Takano, Osuke Miura, Yoshikazu Mizuguchi, Hidetomo Usui, Katsuhiro Suzuki, Kazuhiko Kuroki, Yuji Muraoka, Takayoshi Yokoya

PHYSICAL REVIEW B 90 ( 22 ) 2014.12

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

The electronic structure of nearly optimally doped novel superconductor LaO1-xFxBiS2(x = 0.46) was investigated using angle-resolved photoemission spectroscopy (ARPES). We clearly observed band dispersions from 2 to 6 eV binding energy and near the Fermi level (E-F), which are well reproduced by first-principles calculations when the spin-orbit coupling is taken into account. The ARPES intensity map near E-F shows a squarelike distribution around the Gamma(Z) point in addition to electronlike Fermi-surface (FS) sheets around the X(R) point, indicating that FS of LaO0.54F0.46BiS2 is in close proximity to the theoretically predicted topological change.

DOI： 10.1103/PhysRevB.90.220512

Pressure-dependent magnetization and magnetoresistivity studies on tetragonal FeS (mackinawite): revealing its intrinsic metallic character Reviewed

S. J. Denholme, H. Okazaki, S. Demura, K. Deguchi, M. Fujioka, T. Yamaguchi, H. Takeya, M. ElMassalami, H. Fujiwara, T. Wakita, T. Yokoya, Y. Takano

SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS 15 ( 5 ) 2014.10

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

The transport and magnetic properties of the tetragonal Fe1+delta S were investigated using magnetoresistivity and magnetization within 2 <= T <= 300 K, H <= 70 kOe and P <= 3.0 GPa. In addition, room-temperature x-ray diffraction and photoelectron spectroscopy were also applied. In contrast to previously reported nonmetallic character, Fe1+delta S is intrinsically metallic but due to a presence of a weak localization such metallic character is not exhibited below room temperature. An applied pressure reduces strongly this additional resistive contribution and as such enhances the temperature range of the metallic character which, for similar to 3 GPa, is evident down to 75 K. The absence of superconductivity as well as the mechanism behind the weak localization will be discussed.

DOI： 10.1088/1468-6996/15/5/055007

Electronic Structures of CeM2Al10 (M = Fe, Ru, and Os) Studied by Soft X-ray Resonant and High-Resolution Photoemission Spectroscopies Reviewed

Toshihiko Ishiga, Takanori Wakita, Rikiya Yoshida, Hiroyuki Okazaki, Koji Tsubota, Masanori Sunagawa, Kanta Uenaka, Kozo Okada, Hiroshi Kumigashira, Masaharu Oshima, Keisuke Yutani, Yuji Muro, Toshiro Takabatake, Yuji Muraoka, Takayoshi Yokoya

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 83 ( 9 ) 2014.9

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

We have performed a photoemission spectroscopy (PES) study of CeM2Al10 (M = Fe, Ru, and Os) to directly observe the electronic structure involved in the unusual magnetic ordering. Soft X-ray resonant (SXR) PES provides spectroscopic evidence of the hybridization between conduction and Ce 4f electrons (c-f hybridization) and the order of the hybridization strength (Ru < Os < Fe). High-resolution (HR) PES of CeRu2Al10 and CeOs2Al10, as compared with that of CeFe2Al10, identifies two structures that can be ascribed to structures induced by the c-f hybridization and the antiferromagnetic ordering, respectively. Although the c-f hybridization-induced structure is a depletion of the spectral intensity (pseudogap) around the Fermi level (E-F) with an energy scale of 20-30 meV, the structure related to the antiferromagnetic ordering is observed as a shoulder at approximately 10-11 meV within the pseudogap. The energies of the shoulder structures of CeRu2Al10 and CeOs2Al10 are approximately half of the optical gap (20 meV), indicating that E-F is located at the midpoint of the gap.

DOI： 10.7566/JPSJ.83.094717

Persistent insulator-to-metal transition of a VO2 thin film induced by soft X-ray irradiation Reviewed

Yuji Muraoka, Hiroki Nagao, Shinsuke Katayama, Takanori Wakita, Masaaki Hirai, Takayoshi Yokoya, Hiroshi Kumigashira, Masaharu Oshima

JAPANESE JOURNAL OF APPLIED PHYSICS 53 ( 5 ) 2014.5

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

We present a persistent soft-X-ray-induced insulator-to-metal transition at room temperature for VO2 thin films. Continuous soft X-ray irradiation renders the insulating electronic state metallic in the VO2 thin films, which has been found by photoemission spectroscopy measurement. The observed transition is irreversible and the metallic state persists when soft X-ray irradiation is stopped. The analysis of valence band spectra reveals that the density of states (DOS) of the V 3d band increases with irradiation time, while the DOS of the O 2p band decreases. We propose a simple model where the persistent insulator-to-metal transition is driven by oxygen desorption from VO2 thin films under soft X-ray irradiation. The present results will help in developing a new route for controlling the phase transition of VO2 by light. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.53.05FB09

Characteristic two-dimensional Fermi surface topology of high-T-c iron-based superconductors Reviewed

Masanori Sunagawa, Toshihiko Ishiga, Koji Tsubota, Taihei Jabuchi, Junki Sonoyama, Keita Iba, Kazutaka Kudo, Minoru Nohara, Kanta Ono, Hiroshi Kumigashira, Tomohiro Matsushita, Masashi Arita, Kenya Shimada, Hirofumi Namatame, Masaki Taniguchi, Takanori Wakita, Yuji Muraoka, Takayoshi Yokoya

SCIENTIFIC REPORTS 4 2014.3

Language：English Publishing type：Research paper (scientific journal) Publisher：NATURE PUBLISHING GROUP

Unconventional Cooper pairing originating from spin or orbital fluctuations has been proposed for iron-based superconductors. Such pairing may be enhanced by quasi-nesting of two-dimensional electron and hole-like Fermi surfaces (FS), which is considered an important ingredient for superconductivity at high critical temperatures (high-T-c). However, the dimensionality of the FS varies for hole and electron-doped systems, so the precise importance of this feature for high-T-c materials remains unclear. Here we demonstrate a phase of electron-doped CaFe2As2 (La and P co-doped CaFe2As2) with T-c = 45 K, which is the highest T-c found for the AEFe(2)As(2) bulk superconductors (122-type; AE = Alkaline Earth), possesses only cylindrical hole-and electron-like FSs. This result indicates that FS topology consisting only of two-dimensional sheets is characteristic of both hole-and electron-doped 122-type high-T-c superconductors.

DOI： 10.1038/srep04381

Soft X-ray Photoemission Study of New BiS2-Layered Superconductor LaO1-xFxBiS2 Reviewed

Shinsuke Nagira, Junki Sonoyama, Takanori Wakita, Masanori Sunagawa, Yudai Izumi, Takayuki Muro, Hiroshi Kumigashira, Masaharu Oshima, Keita Deguchi, Hiroyuki Okazaki, Yoshihiko Takano, Osuke Miura, Yoshikazu Mizuguchi, Katsuhiro Suzuki, Hidetomo Usui, Kazuhiko Kuroki, Kozo Okada, Yuji Muraoka, Takayoshi Yokoya

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 83 ( 3 ) 2014.3

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

We use core level and valence band soft X-ray photoemission spectroscopy (SXPES) to investigate the electronic structure of new BiS2 layered superconductor LaO1-xFxBiS2. The core level spectra of doped samples show a new spectral feature on the lower binding energy side of the Bi 4 f main peak, which may be explained by core-hole screening with metallic states near the Fermi level (E-F). The experimental electronic structure and its x dependence (a higher binding energy shift of the valence band as well as appearance of new states near E-F having the dominant Bi 6p character) were found to be consistent with the predictions of band structure calculations in general. Noticeable deviation of the spectral shape of the states near EF from that of calculations might provide insight into interesting physical properties. This study provides the first experimental electronic structure of the new BiS2 layered superconductors.

DOI： 10.7566/JPSJ.83.033703

Evidence for metallic states in potassium-intercalated picene film on graphite Reviewed

Okazaki Hiroyuki, Jabuchi Taihei, Wakita Takanori, Kato Takashi, Muraoka Yuji, Yokoya Takayoshi

PHYSICAL REVIEW B 88 ( 24 ) 2013.12

Evidence for metallic states in potassium-intercalated picene film on graphite Reviewed

Hiroyuki Okazaki, Taihei Jabuchi, Takanori Wakita, Takashi Kato, Yuji Muraoka, Takayoshi Yokoya

PHYSICAL REVIEW B 88 ( 24 ) 2013.12

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We report the results of photoemission spectroscopy on the potassium concentration (x) dependence of K-x picene films on graphite. The results show a systematic shift of a whole valence band and the appearance of a clear Fermi edge for a certain value of x, with additional structures at higher binding energies. Observed spectral evolutions are reminiscent to those of doped C-60, implying that the K-x picene film has multiple phases depending on the K concentration. Appearance of a clear Fermi edge, in contrast to multilayer K-x picene films on Au, SiO2, and Ag(111) substrates, where no clear Fermi edges are observed, suggests that the emergence of the metallic states for K-x picene films depends on the specific molecular orientations in the films on different substrates.

DOI： 10.1103/PhysRevB.88.245414

Collapsed Tetragonal Phase Transition of Ca(Fe1-xRhx)(2)As-2 Studied by Photoemission Spectroscopy Reviewed

Koji Tsubota, Takanori Wakita, Hiroki Nagao, Chiaki Hiramatsu, Toshihiko Ishiga, Masanori Sunagawa, Kanta Ono, Hiroshi Kumigashira, Masataka Danura, Kazutaka Kudo, Minoru Nohara, Yuji Muraoka, Takayoshi Yokoya

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 82 ( 7 ) 2013.7

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

We have investigated the changes in the band structure and core-level energy of Ca(Fe1-xRhx)(2)As-2 across the tetragonal-collapsed tetragonal (T-cT) phase transition using photoemission spectroscopy. Angle-resolved photoemission spectroscopy clearly demonstrates that, at the Gamma point in the Brillouin zone, a hole-like band that crosses the Fermi level in the T phase lowers its energy position and is completely filled in the cT phase. This indicates the disappearance of a hole-like Fermi surface around the zone center in the cT phase, which gives rise to the extinction of nesting. The relationship of the present results with the results of previous transport and magnetic studies is discussed. The temperature dependences of the As 3d and Ca 2p core levels show energy shifts toward higher and lower binding energies, respectively. These data constitute the first direct experimental results on electronic structure and chemical states across the T-cT transition.

DOI： 10.7566/JPSJ.82.073705

Magnetoelectric Effect Driven by Magnetic Domain Modification in LuFe2O4 Reviewed

Takashi Kambe, Yukimasa Fukada, Jun Kano, Tomoko Nagata, Hiroyuki Okazaki, Takayoshi Yokoya, Shuichi Wakimoto, Kazuhisa Kakurai, Naoshi Ikeda

PHYSICAL REVIEW LETTERS 110 ( 11 ) 2013.3

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

The magnetocapacitance effect was investigated using impedance spectroscopy on single crystals of LuFe2O4. The intrinsic impedance response could be separated from the interfacial response and showed a clear hysteresis loop below T-Ferri similar to 240 K under the magnetic field. The neutron diffraction experiment under the magnetic field proves the origin of the dielectric property related to the motion of the nanosized ferromagnetic domain boundary. These results imply that the modification of the microscopic domain structure is responsible for the magnetoelectric effect in LuFe2O4. DOI: 10.1103/PhysRevLett.110.117602

DOI： 10.1103/PhysRevLett.110.117602

Translational Symmetry Breaking and Gapping of Heavy-Quasiparticle Pocket in URu 2 Si 2 Reviewed

Rikiya Yoshida, Koji Tsubota, Toshihiko Ishiga, Masanori Sunagawa, Jyunki Sonoyama, Dai Aoki, Jacques Flouquet, Takanori Wakita, Yuji Muraoka, Takayoshi Yokoya

Scientific Reports 3 2013

Language：English Publishing type：Research paper (scientific journal)

URu 2 Si 2 is a uranium compound that exhibits a so-called 'hidden-order' transition at ∼17.5â€...K. However, the order parameter of this second-order transition as well as many of its microscopic properties remain unclarified despite considerable research. One of the key questions in this regard concerns the type of spontaneous symmetry breaking occurring at the transition
although rotational symmetry breaking has been detected, it is not clear whether another type of symmetry breaking also occurs. Another key question concerns the property of Fermi-surface gapping in the momentum space. Here we address these key questions by a momentum-dependent observation of electronic states at the transition employing ultrahigh-resolution three-dimensional angle-resolved photoemission spectroscopy. Our results provide compelling evidence of the spontaneous breaking of the lattice's translational symmetry and particle-hole asymmetric gapping of a heavy quasiparticle pocket at the transition.

DOI： 10.1038/srep02750

Te concentration dependent photoemission and inverse-photoemission study of FeSe1-xTex Reviewed

Takayoshi Yokoya, Rikiya Yoshida, Yuki Utsumi, Koji Tsubota, Hiroyuki Okazaki, Takanori Wakita, Yoshikazu Mizuguchi, Yoshihiko Takano, Takayuki Muro, Yukako Kato, Hiroshi Kumigashira, Masaharu Oshima, Hisatomo Harima, Yoshihiro Aiura, Hitoshi Sato, Akihiro Ino, Hirofumi Namatame, Masaki Taniguchi, Masaaki Hirai, Yuji Muraoka

SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS 13 ( 5 ) 2012.10

Language：English Publishing type：Research paper (scientific journal) Publisher：NATL INST MATERIALS SCIENCE

We have characterized the electronic structure of FeSe1-xTex for various x values using soft x-ray photoemission spectroscopy (SXPES), high-resolution photoemission spectroscopy (HRPES) and inverse photoemission spectroscopy (IPES). The SXPES valence band spectral shape shows that the 2 eV feature in FeSe, which was ascribed to the lower Hubbard band in previous theoretical studies, becomes less prominent with increasing x. HRPES exhibits systematic x dependence of the structure near the Fermi level (E-F): its splitting near EF and filling of the pseudogap in FeSe. IPES shows two features, near E-F and approximately 6 eV above E-F; the former may be related to the Fe 3d states hybridized with chalcogenide p states, while the latter may consist of plane-wave-like and Se d components. In the incident electron energy dependence of IPES, the density of states near E-F for FeSe and FeTe has the Fano lineshape characteristic of resonant behavior. These compounds exhibit different resonance profiles, which may reflect the differences in their electronic structures. By combining the PES and IPES data the on-site Coulomb energy was estimated at 3.5 eV for FeSe.

DOI： 10.1088/1468-6996/13/5/054403

Comparative photoemission studies on the superconducting gap of the filled skutterudite superconductors LaPt4Ge12 and PrPt4Ge12 Reviewed

Yoshiaki Nakamura, Hiroyuki Okazaki, Rikiya Yoshida, Takanori Wakita, Hiroyuki Takeya, Kazuto Hirata, Masaaki Hirai, Yuji Muraoka, Takayoshi Yokoya

PHYSICAL REVIEW B 86 ( 1 ) 2012.7

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We performed a comparative study of the superconducting gap in the new filled skutterudite superconductors LaPt4Ge12 and PrPt4Ge12 using high-resolution photoemission spectroscopy. We succeeded in observing spectral changes across T-c that reflect the opening of the superconducting gap in both compounds and also in observing a noticeable difference in their respective superconducting spectral shapes near the Fermi level, pointing toward a more complex superconducting gap structure in PrPt4Ge12. In addition, we found that the two-gap model is more suitable for describing the superconducting-state spectrum of PrPt4Ge12 than the single-isotropic-gap and single-anisotropic-gap models, which suggests an explanation that multiband effects may possibly induce the anomalous superconducting properties of PrPt4Ge12.

DOI： 10.1103/PhysRevB.86.014521

Observation of two fine structures related to the hidden order in the spectral functions of URu2Si2 Reviewed

Rikiya Yoshida, Masaki Fukui, Yoshinori Haga, Etsuji Yamamoto, Yoshichika Onuki, Mario Okawa, Walid Malaeb, Shik Shin, Yuji Muraoka, Takayoshi Yokoya

PHYSICAL REVIEW B 85 ( 24 ) 2012.6

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We studied the spectral functions of URu2Si2 in the hidden-order state by means of ultrahigh-resolution angle-resolved photoemission spectroscopy. High signal-to-noise ratio data uncover the existence of two anomalous fine structures, namely a "splitting" in the previously reported narrow dispersive band, and a small, dispersionless "satellite" structure. These structures can be explained by neither ordinary bands nor the development of a hybridization of two states; thus our observation should evoke the consideration of complicated many-body effects. As a possible origin, we suggest the existence of electron-mode coupling in the ordered state.

DOI： 10.1103/PhysRevB.85.241102

Observation of two fine structures related to the hidden order in the spectral functions of URu2Si2 Reviewed

Rikiya Yoshida, Masaki Fukui, Yoshinori Haga, Etsuji Yamamoto, Yoshichika Onuki, Mario Okawa, Walid Malaeb, Shik Shin, Yuji Muraoka, Takayoshi Yokoya

Physical Review B 85 ( 24 ) 241102 2012.6

Effectiveness of a hot-filament chemical vapor deposition method for preparation of a boron-doped superconducting diamond film with higher superconducting transition temperature Reviewed

T. Doi, T. Fukaishi, C. Hiramatsu, T. Wakita, M. Hirai, Y. Muraoka, T. Yokoya, Y. Kato, Y. Izumi, T. Muro, Y. Tamenori

DIAMOND AND RELATED MATERIALS 25 5 - 7 2012.5

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE SA

We have investigated the effectiveness of a hot-filament chemical vapor deposition (HFCVD) method for preparation of a boron-doped superconducting diamond film with higher superconducting transition temperature (T-c). A boron-doped superconducting diamond film has been fabricated on a diamond (111) substrate using the HFCVD method, and studied by means of scanning electron microscopy. X-ray photoelectron spectroscopy (XPS), and resistivity. The film consists of grains with an average size of 200 mm. Analyses of valence band and boron Is core-level XPS spectra indicate the formation of boron-doped diamond film on the substrate. From the resistivity measurements, the film is found to be a superconductor with onset T-c of 7.1 K. Carrier concentration determined by Hall conductivity measurements is 1.1 x 10(21) cm(-3). The value of T-c is higher compared with that in boron-doped superconducting diamond films prepared by a commonly used microwave plasma-assisted chemical vapor deposition (MPCVD) method, at the same carrier concentration [A. Kawano et al., Phys. Rev. B 82 (2010) 085318]. The result of higher T-c in the film by the HFCVD method is consistent with the previous one [Wang et al., Diamond Relat. Mater. 15 (2006) 659], suggesting that the HFCVD method is effective for preparation of boron-doped superconducting diamond films showing higher T-c. (C) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.diamond.2012.02.008

Development of a soft X-ray angle-resolved photoemission system applicable to 100 mu m crystals Reviewed

Takayuki Muro, Yukako Kato, Tomohiro Matsushita, Toyohiko Kinoshita, Yoshio Watanabe, Hiroyuki Okazaki, Takayoshi Yokoya, Akira Sekiyama, Shigemasa Suga

JOURNAL OF SYNCHROTRON RADIATION 18 879 - 884 2011.11

Language：English Publishing type：Research paper (scientific journal) Publisher：WILEY-BLACKWELL

A system for angle-resolved photoemission spectroscopy (ARPES) of small single crystals with sizes down to 100 mm has been developed. Soft X-ray synchrotron radiation with a spot size of similar to 40 mu m x 65 mu m at the sample position is used for the excitation. Using this system an ARPES measurement has been performed on a Si crystal of size 120 mu m x 100 mu m x 80 mu m. The crystal was properly oriented on a sample stage by measuring the Laue spots. The crystal was cleaved in situ with a microcleaver at 100 K. The cleaved surface was adjusted to the beam spot using an optical microscope. Consequently, clear band dispersions along the Gamma-X direction reflecting the bulk electronic states were observed with a photon energy of 879 eV.

DOI： 10.1107/S0909049511034418

Photoemission Study of Rh17S15 Superconductor Reviewed

Masaki Fukui, Rikiya Yoshida, Yoshiaki Nakamura, Yukako Kato, Takayuki Muro, Rikio Settai, Yoshichika Onuki, Hisatomo Harima, Masaaki Hirai, Yuji Muraoka, Takayoshi Yokoya

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 80 2011.7

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

We have investigated the electronic structures of Rh17S15 by using soft x-ray photoemission spectroscopy and first principle band structure calculations. While S 2p core level spectra are explained by symmetric components, Rh 3d core level exhibits asymmetric line shape, suggesting a dominant Rh 4d character at the states near the Fermi level. The binding energy of Rh 4d in Rh17S15 is similar to the one in Rh metal, and this observation implies metallic Rh-Rh bonding. Many features in experimental valence spectra are reasonably described by the calculations, indicating that one particle approximation is a good starting point to understand the electronic properties of Rh17S15. The comparison between the experiment and the calculation suggests that Rh 4d character at the states near the Fermi level is essential for the correlated superconductivity in Rh17S15. However, some deviation between the experiment and the calculation as well as a finite mass enhancement are found, indicating some corrections on the treatment of electron-phonon coupling and/or electron correlations are necessary.

DOI： 10.1143/JPSJS.80SA.SA111

Ultrahigh-resolution laser photoemission study of URu2Si2 across the hidden-order transition Reviewed

Rikiya Yoshida, Yoshiaki Nakamura, Masaki Fukui, Yoshinori Haga, Etsuji Yamamoto, Yoshichika Onuki, Mario Okawa, Shik Shin, Masaaki Hirai, Yuji Muraoka, Takayoshi Yokoya

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 72 ( 5 ) 580 - 581 2011.5

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

We have studied the electronic structures of URu2Si2 employing ultrahigh-resolution laser angle-resolved photoemission spectroscopy. The change of photoemission spectra is investigated across the hidden-order transition, and the emergence of a narrow band is clearly observed near the Fermi level for both (pi,0) and (pi,pi) directions. In addition, it is shown that tuning of light's polarization allows the signal of a hole-like dispersive feature to enhance. These observations prove that laser angle-resolved photoemission spectroscopy is an effective tool for studying the evolution of electronic structures across the hidden-order transition in URu2Si2. (C) 2010 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jpcs.2010.10.048

Photoemission study of electronic structure evolution across the metal-insulator transition of heavily B-doped diamond Reviewed

H. Okazaki, T. Arakane, K. Sugawara, T. Sato, T. Takahashi, T. Wakita, M. Hirai, Y. Muraoka, Y. Takano, S. Ishii, S. Iriyama, H. Kawarada, T. Yokoya

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 72 ( 5 ) 582 - 584 2011.5

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

We studied the electronic structure evolution of heavily B-doped diamond films across the metal-insulator transition (MIT) using ultraviolet photoemission spectroscopy (UPS). From high-temperature UPS, through which electronic states near the Fermi level (E(F)) up to similar to 5k(B)T can be observed (k(B) is the Boltzmann constant and T the temperature), we observed the carrier concentration dependence of spectral shapes near E(F). Using another carrier concentration dependent UPS, we found that the change in energy position of sp-band of the diamond valence band, which corresponds to the shift of E(F), can be explained by the degenerate semiconductor model, indicating that the diamond valence band is responsible for the metallic states for samples with concentrations above MIT. We discuss a possible electronic structure evolution across MIT. (C) 2010 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jpcs.2010.10.052

Spectroscopic evidence of the formation of (V,Ti)O-2 solid solution in VO2 thinner films grown on TiO2(001) substrates Reviewed

Y. Muraoka, K. Saeki, R. Eguchi, T. Wakita, M. Hirai, T. Yokoya, S. Shin

JOURNAL OF APPLIED PHYSICS 109 ( 4 ) 2011.2

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

We have prepared VO2 thin films epitaxially grown on TiO2(001) substrates with thickness systematically varied from 2.5 to 13 nm using a pulsed laser deposition method, and studied the transport property and electronic states of the films by means of resistivity and in situ synchrotron photoemission spectroscopy (SRPES). In resistivity measurements, the 13-nm-thick film exhibits a metal-insulator transition at around 290 K on cooling with change of three orders of magnitudes in resistivity. As the film thickness decreases, the metal-insulator transition broadens and the transition temperature increases. Below 4 nm, the films do not show the transition and become insulators. In situ SRPES measurements of near the Fermi level valence band find that the electronic state of the 2.5-nm-thick film is different than that of the temperature-induced insulator phase of VO2 itself although these two states are insulating. Ti 2p core-level photoemission measurements reveal that Ti ions exist near the interface between the films and TiO2 substrates, with a chemical state similar to that in (V,Ti)O-2 solid solution. These results indicate that insulating (V,Ti)O-2 solid solution is formed in the thinner films. We propose a simple growth model of a VO2 thin film on a TiO2(001) substrate. Near the interface, insulating (V,Ti) O-2 solid solution is formed due to the diffusion of Ti ions from the TiO2 substrate into the VO2 film. The concentration of Ti in (V,Ti) O-2 is relatively high near the interface and decreases toward the surface of the film. Beyond a certain film thickness (about 7 nm in the case of the present 13-nm-thick film), the VO2 thin film without any Ti ions starts to grow. Our work suggests that developing a technique for preparing the sharp interface between the VO2 thin films and TiO2 substrates is a key issue to study the physical property of an ultrathin film of "pure" VO2, especially to examine the presence of the novel electronic state called a semi-Dirac point phase predicted by calculations. (c) 2011 American Institute of Physics. [doi:10.1063/1.3549835]

DOI： 10.1063/1.3549835

Multiple phosphorus chemical sites in heavily phosphorus-doped diamond Reviewed

Hiroyuki Okazaki, Rikiya Yoshida, Takayuki Muro, Tetsuya Nakamura, Takanori Wakita, Yuji Muraoka, Masaaki Hirai, Hiromitsu Kato, Satoshi Yamasaki, Yoshihiko Takano, Satoshi Ishii, Tamio Oguchi, Takayoshi Yokoya

APPLIED PHYSICS LETTERS 98 ( 8 ) 2011.2

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

We have performed high-resolution core level photoemission spectroscopy on a heavily phosphorus (P)-doped diamond film in order to elucidate the chemical sites of doped-phosphorus atoms in diamond. P 2p core level study shows two bulk components, providing spectroscopic evidence for multiple chemical sites of doped-phosphorus atoms. This indicates that only a part of doped-phosphorus atoms contribute to the formation of carriers. From a comparison with band calculations, possible origins for the chemical sites are discussed. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3554699]

DOI： 10.1063/1.3554699

Electronic structure of U(Ru1-xRhx)(2)Si-2 studied by laser angle-resolved photoemission spectroscopy Reviewed

R. Yoshida, Y. Nakamura, M. Fukui, Y. Haga, E. Yamamoto, Y. Onuki, M. Okawa, S. Shin, M. Hirai, Y. Muraoka, T. Yokoya

INTERNATIONAL CONFERENCE ON STRONGLY CORRELATED ELECTRON SYSTEMS (SCES 2010) 273 2011

Language：English Publishing type：Research paper (international conference proceedings) Publisher：IOP PUBLISHING LTD

We have studied electronic structures of U(Ru1-xRhx)(2)Si-2 employing ultrahigh-resolution laser angle-resolved photoemission spectroscopy to understand the effect of Rh substitution. A hole-like dispersive feature, which presumably has Rh d-band character, was observed in U(Ru1-xRhx)(2)Si-2 for both x = 0 and x = 0.03. However, although a heavy quasiparticle band appears in the hidden-order state of URu2Si2 (x = 0), it was not observed for x = 0.03 over the temperature range studied. In addition, it was found that energy-distribution curves at the Fermi vector of the hole-like band for x = 0.03 behave similarly to the Fermi-Dirac function. We also present the mapping of Fermi surfaces formed by the hole-like band, which stay nearly unaffected by Rh substitution.

DOI： 10.1088/1742-6596/273/1/012021

Synthesis and physical properties of the hollandite-type titanium oxide KxTi8O16 Reviewed

Yuji Muraoka, Kengo Noami, Takanori Wakita, Masaaki Hirai, Takayoshi Yokoya, Yukako Kayo, Takayuki Muro, Yusuke Tamenori

PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 8, NO 2 8 ( 2 ) 2011

Language：English Publishing type：Research paper (international conference proceedings) Publisher：WILEY-V C H VERLAG GMBH

Polycrystalline sample of hollandite-type titanium oxide KxTi8O16 has been prepared and studied by magnetization and X-ray photoelectron spectroscopy (XPS). Room temperature ferromagnetic behaviour is observed in the magnetic hysteresis measurements. XPS measurements of the Ti 2p(3/2) core-level finds that the titanium ions take a mixed valence of Ti4+ and Ti3+. In addition, the valence band spectrum reveals that the 3d electrons tend to localize on Ti3+ ions in the hollandite-type TiO2 lattice. Also, analysis of the valence band spectrum shows that the prepared sample is a wide-gap oxide with a band gap of 3.6 eV. These results indicate that the present hollandite-type KxTi8O16 sample can be classified as a TiO2-based wide-gap semiconductor with Curie temperature above room temperature. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssc.201000383

Photoemission study of Ca-intercalated graphite superconductor CaC6 Reviewed

Hiroyuki Okazaki, Rikiya Yoshida, Keisuke Iwai, Kengo Noami, Takayuki Muro, Tetsuya Nakamura, Takanori Wakita, Yuji Muraoka, Masaaki Hirai, Fumiaki Tomioka, Yoshihiko Takano, Asami Takenaka, Masahiro Toyoda, Tamio Oguch, Takayoshi Yokoya

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 470 S637 - S638 2010.12

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

In this work, we have performed resonant photoemission studies of Ca-intercalated graphite superconductor CaC6. Using photon energy of the Ca 2p-3d threshold, the photoemission intensity of the peak at Fermi energy (E-F) is resonantly enhanced. This result provides spectroscopic evidence for the existence of Ca 3d states at E-F, and strongly supports that Ca 3d state plays a crucial role for the superconductivity of this material with relatively high T-c. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.10.039

Angle-resolved photoemission study of Si electronic structure: Boron concentration dependence Reviewed

Takanori Wakita, Hiroyuki Okazaki, Yoshihiko Takano, Masaaki Hirai, Yuji Muraoka, Takayoshi Yokoya

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 470 S641 - S643 2010.12

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

The boron concentration dependence of the Si electronic structure of Si(1 00)2 x 1 surfaces were investigated by angle-resolved photoemission spectroscopy (ARPES). The ARPES spectra exhibit rigid shifts toward lower binding energy as the boron concentration increases. The band dispersion was obtained from fitting procedure, and it is found that the top of the valence band does not exceed the Fermi level even with a boron concentration 35 times larger than the critical concentration of the metal-insulator transition. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.11.028

Analysis on photoemission spectrum of superconducting FeSe Reviewed

Rikiya Yoshida, Takanori Wakita, Hiroyuki Okazaki, Yoshikazu Mizuguchi, Shunsuke Tsuda, Yoshihiko Takano, Hiroyuki Takeya, Kazuto Hirata, Yukako Kato, Takayuki Muro, Mario Okawa, Kyoko Ishizaka, Shik Shin, Hisatomo Harima, Masaaki Hirai, Yuji Muraoka, Takayoshi Yokoya

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 470 S389 - S390 2010.12

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

In this paper, we present the result of soft X-ray photoemission spectroscopy and its comparison with the density functional calculation. Although local density approximation seems to be a good starting point for describing the electronic structure of FeSe, the simulated spectrum poorly reproduced the structure around E-B = 2 eV. This result suggests the necessity of theoretical treatment beyond local density approximation. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.11.172

Electronic Structure of the Novel Filled Skutterudite PrPt4Ge12 Superconductor Reviewed

Yoshiaki Nakamura, Hiroyuki Okazaki, Rikiya Yoshida, Takanori Wakita, Masaaki Hirai, Yuji Muraka, Hiroyuki Takeya, Kazuto Hirata, Hiroshi Kumigashira, Masaharu Oshima, Takayoshi Yokoya

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 79 ( 12 ) 2010.12

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

We have performed soft x-ray photoemission spectroscopy (SXPES) and resonant photoemission spectroscopy (RPES) of the filled skutterudite superconductor PrPt4Ge12 in order to study the electronic structure of valence band and the character of Pr 4f. SXPES of PrPt4Ge12 measured with 1200 eV photon energy, where spectral contribution of Pr 4f is negligible, was found nearly identical with that of LaPt4Ge12, indicating similarity of Pt-Ge derived electronic states of the two compounds. Good correspondence with band calculations allows us to ascribe the dominant Ge 4p character of the density of states at the Fermi level (E-F). Pr 3d -> 4f RPES shows that, although Pr 4f electrons in PrPt4Ge12 are not as strongly hybridized with conduction electrons near E-F as in PrFe4P12, there are finite Pr 4f contribution to the states near E-F in PrPt4Ge12. These PES results give the information of fundamental electronic structure for understanding the physical properties of the novel filled skutterudite superconductor PrPt4Ge12

DOI： 10.1143/JPSJ.79.124701

Signature of hidden order and evidence for periodicity modification in URu2Si2 Reviewed

Rikiya Yoshida, Yoshiaki Nakamura, Masaki Fukui, Yoshinori Haga, Etsuji Yamamoto, Yoshichika Onuki, Mario Okawa, Shik Shin, Masaaki Hirai, Yuji Muraoka, Takayoshi Yokoya

PHYSICAL REVIEW B 82 ( 20 ) 2010.11

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

The detail of electronic structures near the Fermi level in URu2Si2 has been investigated employing state-of-art laser angle-resolved photoemission spectroscopy. The observation of a narrow dispersive band near the Fermi level in the ordered state as well as its absence in a Rh-substituted sample strongly suggest that the emergence of the narrow band is a clear signature of the hidden-order transition. The temperature dependence of the narrow band, which appears at the onset of the hidden-order transition, invokes the occurrence of periodicity modification in the ordered state, which is shown for the first time by any spectroscopic probe. We compare our data to other previous studies and discuss possible implications.

DOI： 10.1103/PhysRevB.82.205108

Electronic structure of pristine and K-doped solid picene: Nonrigid band change and its implication for electron-intramolecular-vibration interaction Reviewed

H. Okazaki, T. Wakita, T. Muro, Y. Kaji, X. Lee, H. Mitamura, N. Kawasaki, Y. Kubozono, Y. Yamanari, T. Kambe, T. Kato, M. Hirai, Y. Muraoka, T. Yokoya

PHYSICAL REVIEW B 82 ( 19 ) 2010.11

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We use photoemission spectroscopy to study electronic structures of pristine and K-doped solid picene. The valence band spectrum of pristine picene consists of three main features with no state at the Fermi level (E(F)) while that of K-doped picene has three structures similar to those of pristine picene with new states near E(F), consistent with the semiconductor-metal transition. The K-induced change cannot be explained with a simple rigid-band model of pristine picene but can be interpreted by molecular-orbital calculations considering electron-intramolecular-vibration interaction. Excellent agreement of the K-doped spectrum with the calculations points to importance of electron-intramolecular-vibration interaction in K-doped picene.

DOI： 10.1103/PhysRevB.82.195114

Angle-resolved photoemission spectroscopy for VO2 thin films grown on TiO2 (001) substrates Reviewed

Y. Muraoka, K. Saeki, Y. Yao, T. Wakita, M. Hirai, T. Yokoya, R. Eguchi, S. Shin

JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 181 ( 2-3 ) 249 - 251 2010.8

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We present the results of angle-resolved photoemission spectroscopy (ARPES) measurements of metallic VO2 thin films. The VO2 thin films have been grown on TiO2 (0 0 1) single crystal substrates using pulsed laser deposition. The films exhibit a first-order metal-insulator transition (MIT) at 305K. In the ARPES spectra of the metallic phase for the films, the O 2p band shows highly dispersive feature in the binding energy range of 3-8 eV along the Gamma-Z direction. The periodicity of the dispersive band is found to be 2.2 A(-1) which is almost identical with the periodicity expected from the c-axis length of the VO2 thin films. The overall feature of the experimental band structure is similar to the band structure calculations. supporting that we have succeeded in observing the dispersive band of the O 2p state in the metallic VO2 thin film. The present work indicates that the ARPES measurements using epitaxial thin films are promising for determining the band structure of VO2. (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2010.02.003

Bulk and surface physical properties of a CrO(2) thin film prepared from a Cr(8)O(21) precursor Reviewed

K. Iwai, Y. Muraoka, T. Wakita, M. Hirai, T. Yokoya, Y. Kato, T. Muro, Y. Tamenori

JOURNAL OF APPLIED PHYSICS 108 ( 4 ) 2010.8

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

We have prepared a CrO(2) thin film by chemical vapor deposition from a Cr(8)O(21) precursor and studied the bulk and surface physical properties. The CrO(2) thin film is grown on a TiO(2) (100) substrate by heating of a Cr(8)O(21) precursor and TiO(2) (100) substrate together in a sealed quartz tube. The prepared film is found from x-ray diffraction analysis to be an (100)-oriented single phase. The magnetization and resistivity measurements indicate that the film is a ferromagnetic metal with a Curie temperature of about 400 K. Cr 3s core-level and valence band photoelectron spectroscopy spectra reveal the presence of a metallic CrO(2) in the surface region of the film. Our work indicates that preparation from a Cr(8)O(21) precursor is promising for obtaining a CrO(2) thin film with the metallic surface. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3471811]

DOI： 10.1063/1.3471811

Lateral Inhomogeneity in the Electronic Structure of a Conjugated Poly(3-hexylthiophene) Thin Film Reviewed

Kaname Kanai, Takahiro Miyazaki, Takanori Wakita, Kouki Akaike, Takayoshi Yokoya, Yukio Ouchi, Kazuhiko Seki

ADVANCED FUNCTIONAL MATERIALS 20 ( 13 ) 2046 - 2052 2010.7

Language：English Publishing type：Research paper (scientific journal) Publisher：WILEY-V C H VERLAG GMBH

How annealing influences the morphology of a highly regioregular poly(3-hexylthiophene) (RR-P3HT) film at the substrate interface as well as the lateral inhomogeneity in the electronic structure of the film are elucidated. Whereas previous studies have reported that high-molecular-weight (MW) RR-P3HT films tend to show low crystallinity even, after annealing, it is found that high-MW RR-P3HT does show high crystallinity after annealing at high temperature for a long time. Photoemission electron microscopy (PEEM), X-ray photoemission spectroscopy, and ultraviolet photoemission spectroscopy results clearly resolve a considerable lateral inhomogeneity in the morphology of RR-P3HT film, which results in a variation of the electronic structure depending on the local crystallinity. The PEEM results show how annealing facilitates crystal growth in a high-MW RR-P3HT film.

DOI： 10.1002/adfm.201000116

Angle-resolved photoemission observation of the superconducting-gap minimum and its relation to the nesting vector in the phonon-mediated superconductor YNi2B2C Reviewed

T. Baba, T. Yokoya, S. Tsuda, T. Watanabe, M. Nohara, H. Takagi, T. Oguchi, S. Shin

PHYSICAL REVIEW B 81 ( 18 ) 2010.5

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy to directly study the large superconducting (SC) gap anisotropy of YNi2B2C. We succeed in measuring momentum (k) dependence of SC gap for individual Fermi surface (FS) sheets, which demonstrates complexity of SC gap in a phonon-mediated superconductor. Within measured k regions on FS sheets, we find a pointlike minimum of SC gap, whose k positions can be connected by the known nesting vector. This shows close correlation between the nesting vector and node formation.

DOI： 10.1103/PhysRevB.81.180509

Room temperature ferromagnetic behavior in the hollandite-type titanium oxide Reviewed

K. Noami, Y. Muraoka, T. Wakita, M. Hirai, Y. Kato, T. Muro, Y. Tamenori, T. Yokoya

JOURNAL OF APPLIED PHYSICS 107 ( 7 ) 2010.4

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

A hollandite-type K(x)Ti(8)O(16) polycrystalline sample has been prepared and studied by magnetization, resistivity and x-ray photoelectron spectroscopy (XPS). Room temperature ferromagnetic behavior is observed in the magnetic hysteresis measurement. The sample shows a semiconductive temperature dependence in the resistivity measurement. Analysis of the Ti 2p(3/2) core-level XPS spectrum indicates that the titanium ions have a mixed valence of Ti(4+) and Ti(3+). In addition, the valence band spectrum reveals that the 3d electrons tend to localize on Ti(3+) ions in the hollandite-type TiO(2) lattice. Also, analysis of the valence band spectrum shows that the prepared sample is a wide-gap oxide with a band gap of 3.6 eV. These results indicate that the present hollandite-type K(x)Ti(8)O(16) sample can be classified as a TiO(2)-based wide-gap semiconductor with Curie temperature above room temperature. Room temperature ferromagnetism (RTFM) decreases in the sample prepared under a strong reducing gas atmosphere, accompanied with the decrease in the resistivity. The results imply that the localized 3d electrons are responsible for the RTFM of the K(x)Ti(8)O(16) sample. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3369500]

DOI： 10.1063/1.3369500

Kondo scaling of the pseudogap in CeOs4Sb12 and CeFe4P12 Reviewed

P. A. Rayjada, A. Chainani, M. Matsunami, M. Taguchi, S. Tsuda, T. Yokoya, S. Shin, H. Sugawara, H. Sato

JOURNAL OF PHYSICS-CONDENSED MATTER 22 ( 9 ) 2010.3

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

CeOs4Sb12 and CeFe4P12 are classified as Kondo semiconductors, which show coupled changes in electrical transport, thermodynamic and magnetic properties with a low-temperature semiconductor-like electrical resistivity. We have carried out core level and valence band photoemission spectroscopy on single crystal CeOs4Sb12 and CeFe4P12 to study their electronic structure and the evolution of states at the Fermi level as a function of temperature (similar to 10-300 K). The Ce 3d core level spectra show the presence of f(0), f(1) and f(2) final states with very different relative intensities in the two compounds. Single-impurity Anderson model calculations provide f electron counts of n(f) = 0.97 and 0.86 per Ce atom, suggestive of a low-and high-T-K (=single ion Kondo temperature) for CeOs4Sb12 and CeFe4P12, respectively. The high-resolution temperature-dependent near-Fermi level spectra show pseudogaps of energy similar to 50 meV and similar to 110 meV in the valence band density of states (DOS) of CeOs4Sb12 and CeFe4P12, respectively. The temperature dependence of the DOS at the Fermi level follows the change in effective magnetic moment estimated from magnetic susceptibility for both materials, confirming the Kondo nature of the pseudogap in CeOs4Sb12 and CeFe4P12. A compilation of measured pseudogaps using photoemission and optical spectroscopy identifies the charge gaps Delta(C) for Ce-based Kondo semiconductors and provides a direct relation with T-K given by Delta(C) similar to 2k(B)T(K). In conjunction with the known behaviour of the spin gaps Delta(S) similar to k(B)T(K), the results establish the coupled energy scaling of the spin and charge gaps in Kondo semiconductors.

DOI： 10.1088/0953-8984/22/9/095502

Low voltage operation in picene thin film field-effect transistor and its physical characteristics Reviewed

Yumiko Kaji, Naoko Kawasaki, Xuesong Lee, Hideki Okamoto, Yasuyuki Sugawara, Shohei Oikawa, Akio Ito, Hiroyuki Okazaki, Takayoshi Yokoya, Akihiko Fujiwara, Yoshihiro Kubozono

APPLIED PHYSICS LETTERS 95 ( 18 ) 2009.11

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

Low voltage operation of picene thin film field-effect transistor (FET) has been realized with 40 nm thick SiO(2) gate dielectrics coated by two polymers, Cytop (TM) and polystyrene. The picene FETs operated in low absolute gate voltage vertical bar V(G)vertical bar below 15 V for Cytop (TM) coated SiO(2) and 30 V for polystyrene coated SiO(2) gate dielectrics, and they showed a significant O(2) gas sensing effect down to similar to 10 ppm. Photoemission spectrum clarified that O(2) molecules penetrate into the thin films at O(2)/picene mole ratio of 1: 1. X-ray diffraction pattern of picene thin films showed highly oriented growth on the polymer-coated SiO(2).

DOI： 10.1063/1.3257373

Study of electronic states for V thin films deposited on 6H-SiC substrates by soft X-ray emission spectroscopy Reviewed

M. Hirai, H. Okazaki, R. Yoshida, M. Tajima, K. Saeki, Y. Muraoka, T. Yokoya

APPLIED SURFACE SCIENCE 256 ( 4 ) 948 - 949 2009.11

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Silicon carbide (SiC) is a candidate material for electronic devices to operate upon crucial environment. Electronic states of silicides and/or carbide/graphite formed in metal/SiC contact system is fundamentally important from the view point of device performance.
We study interface electronic structure of vanadium (V) thin-film deposited on 6H-SiC(0 0 0 1) Si-face by using a soft X-ray emission spectroscopy (SXES). For specimens of V(38 nm)/6H-SiC (substrate) contact systems annealed at 850 degrees C, the Si L(2,3) emission spectra indicate different shapes and peak energies from the substrate. The product of materials such as silicides and/or ternary materials is suggested. Similarly, the C K alpha emission spectra show the shape and peak energy characteristic of vanadium carbide including substrate 6H-SiC signal. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.apsusc.2009.05.120

Band dispersion near the Fermi level for VO2 thin films grown on TiO2 (001) substrates Reviewed

K. Saeki, T. Wakita, Y. Muraoka, M. Hirai, T. Yokoya, R. Eguchi, S. Shin

PHYSICAL REVIEW B 80 ( 12 ) 2009.9

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We have performed angle-resolved photoemission spectroscopy (ARPES) measurements of VO2 using epitaxial thin films and observed the band dispersion near the Fermi level (E-F) for this compound. The VO2 thin films have been grown on TiO2 (001) single-crystal substrates using pulsed laser deposition. The films exhibit a first-order metal-insulator transition (MIT) at 305 K. In the ARPES spectra of the metallic phase for the films, the O 2p band shows highly dispersive features in the binding-energy range of 3-8 eV along the Gamma-Z direction. Also, the V 3d state shows two dispersive bands around the Gamma point near E-F, indicative of two electron pockets centered at the Gamma point. Both electron pockets have an occupied bandwidth of about 0.4 eV. Assuming the parabolic energy bands around the Gamma point, the effective-mass ratios of the two electron pockets are estimated to be about 0.2 and 1. The present work indicates that the ARPES measurements using epitaxial thin films are promising for determining the band structure of VO2 and thus would play a crucial role to elucidate the mechanism of the MIT in VO2.

DOI： 10.1103/PhysRevB.80.125406

The electronic structure of Ca-intercalated superconducting graphite CaC6 Reviewed

H. Okazaki, R. Yoshida, K. Iwai, K. Noami, T. Muro, T. Nakamura, T. Wakita, Y. Muraoka, M. Hirai, F. Tomioka, Y. Takano, A. Takenaka, M. Toyoda, T. Oguchi, T. Yokoya

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 469 ( 15-20 ) 1041 - 1044 2009.8

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We have measured Ca-intercalated graphite superconductor CaC6 (T-c = 11.2 K) by soft X-ray photoemission spectroscopy in order to understand the electronic structure. For the valence band, we observed several structures that correspond to those of calculated density of states with the partial density of states of Ca 3d at the Fermi level (E-F). We also observed core level spectra that are a very large asymmetric Ca 2p and asymmetric C 1s for CaC6, suggesting the existence of conduction electrons derived from Ca 3d and a charge transfer from Ca to graphene layer. These results provide spectroscopic evidence for PDOS of Ca 3d at E-F. From a comparison of electronic structure of CaC6 and other graphite intercalation compounds (GICs), we found the difference between CaC6 and other superconducting GICs, which provides deeper understanding of the superconductivity of CaC6. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.05.172

Bulk-sensitive spectroscopic studies on noncentrosymmetric superconducting system of Mg-Ir-B Reviewed

R. Yoshida, H. Okazaki, K. Iwai, K. Noami, T. Muro, M. Okawa, K. Ishizaka, S. Shin, Z. Li, J. L. Luo, G. -Q. Zheng, T. Oguchi, M. Hirai, Y. Muraoka, T. Yokoya

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 469 ( 15-20 ) 1034 - 1036 2009.8

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We present the data from ultrahigh-resolution laser-excited photoemission spectroscopy on a newly discovered noncentrosymmetric superconcluctor Mg(10)Ir(19)B(16). We observed the evidences for the opening of superconducting gap where numerical fittings show that isotropic s-wave model can satisfactorily explain the behavior of spectra near the Fermi level. We also present the valence band photoemission spectrum and B K fluorescence spectrum to discuss the basic electronic structure of Mg(10)Ir(19)B(16). (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.05.175

Effect of antiferromagnetic ordering on temperature dependent superconducting gap in ErNi2B2C: Laser-photoemission spectroscopy Reviewed

T. Baba, T. Yokoya, S. Tsuda, T. Kiss, T. Shimojima, K. Ishizaka, H. Takeya, K. Hirata, N. Nakai, K. Machida, T. Togashi, C. T. Chen, C. Q. Zhang, S. Watanabe, S. Shin

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 469 ( 15-20 ) 928 - 931 2009.8

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We have performed temperature (T)-dependent laser-photoemission spectroscopy of antiferromagnetic (AF) superconductor ErNi2B2C to study the electronic structure, especially the effect of AF ordering in T-dependent superconducting (SC) gap. To estimate the values of T-dependent SC gap, we fitted the experimental data by Dynes function having an anisotropic s-wave SC gap. From the fitting results we, find a sudden deviation from the BCS prediction just below T-N. This observation can be well explained by the theoretical model, indicating that the origin of anomalous T-dependence is competition between rapid evolution of AF molecular field and SC condensation energy. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.05.101

Spectroscopic evidence of the existence of substantial Ca 3d derived states at the Fermi level in the Ca-intercalated graphite superconductor CaC6 Reviewed

Okazaki H, Yoshida R, Iwai K, Noami K, Muro T, Nakamura T, Wakita T, Muraoka Y, Hirai M, Tomioka F, Takano Y, Takenaka A, Toyoda M, Oguchi T, Yokoya T

PHYSICAL REVIEW B 80 ( 3 ) 2009.7

Spectroscopic evidence of the existence of substantial Ca 3d derived states at the Fermi level in the Ca-intercalated graphite superconductor CaC6 Reviewed

H. Okazaki, R. Yoshida, K. Iwai, K. Noami, T. Muro, T. Nakamura, T. Wakita, Y. Muraoka, M. Hirai, F. Tomioka, Y. Takano, A. Takenaka, M. Toyoda, T. Oguchi, T. Yokoya

PHYSICAL REVIEW B 80 ( 3 ) 2009.7

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We have performed soft x-ray photoemission studies of Ca-intercalated graphite superconductor CaC6 (T-c=11.2 K). The valence-band spectrum shows six main structures, with a peak at the Fermi level (E-F), which correspond to those of calculated DOS. The intensity of the E-F peak is resonantly enhanced at the Ca 2p-3d threshold, providing spectroscopic evidence for the existence of Ca 3d electrons at E-F, which is confirmed experimentally for the first time. The Ca 2p core-level spectrum has a very large asymmetric line shape, which is further possible suggestion of the existence of Ca 3d derived conduction electrons at Ca sites. These results strongly support the picture where Ca 3d derived interlayer band plays a crucial role for the superconductivity of this material with relatively high T-c.

DOI： 10.1103/PhysRevB.80.035420

In situ positioning of a few hundred micrometer-sized cleaved surfaces for soft-x-ray angle-resolved photoemission spectroscopy by use of an optical microscope Reviewed

Takayuki Muro, Yukako Kato, Tomohiro Matsushita, Toyohiko Kinoshita, Yoshio Watanabe, Akira Sekiyama, Hiroshi Sugiyama, Masato Kimura, Satoshi Komori, Shigemasa Suga, Hiroyuki Okazaki, Takayoshi Yokoya

REVIEW OF SCIENTIFIC INSTRUMENTS 80 ( 5 ) 2009.5

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

A method to position samples with small cleaved regions has been developed to be applied to the angle-resolved photoemission spectroscopy (ARPES) which uses soft-x-ray synchrotron radiation focused down to 160x180 mu m(2). A long-working-distance optical microscope is used for the sample observation. A selected region on a sample can be optimally set at the position of measurements, which is realized by the spatial resolution of the photoelectron analyzer. Using this method, electronic band dispersions of bulk silicon have been measured by ARPES for a partially cleaved region with a size of similar to 200x500 mu m(2).

DOI： 10.1063/1.3124145

Ishizaka et al. Reply Reviewed

K. Ishizaka, R. Eguchi, S. Tsuda, A. Chainani, T. Yokoya, T. Kiss, T. Shimojima, T. Togashi, S. Watanabe, C. -T. Chen, Y. Takano, M. Nagao, I. Sakaguchi, T. Takenouchi, H. Kawarada, S. Shin

PHYSICAL REVIEW LETTERS 102 ( 19 ) 2009.5

Soft X-ray Core-Level Photoemission Study of Boron Sites in Heavily Boron-Doped Diamond Films Reviewed

Hiroyuki Okazaki, Rikiya Yoshida, Takayuki Muro, Takanori Wakita, Masaaki Hirai, Yuji Muraoka, Yoshihiko Takano, Shingo Iriyama, Hiroshi Kawarada, Tamio Oguchi, Takayoshi Yokoya

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 78 ( 3 ) 2009.3

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

We have performed high-resolution soft x-ray B 1s core-level photoemission spectroscopy of heavily boron-doped diamond films in order to study the chemical sites of doped-boron atoms and their relation to the superconducting transition temperature (T-c). We find that B 1s core-level spectra exhibit several fine structures and three bulk components can he resolved from photon energy dependent studies and spectral fittings. These results indicate existence of several chemical environments of doped-boron atoms. One of the bulk components having the lowest binding energy can be assigned to a signal from substitutionally doped-boron atoms. Comparisons of bulk components with T-c suggest spectroscopic evidence for the importance of substitutionally doped-boron atoms for effective carrier and superconductivity.

DOI： 10.1143/JPSJ.78.034703

Electronic Structure of Superconducting FeSe Studied by High-Resolution Photoemission Spectroscopy Reviewed

Rikiya Yoshida, Takanori Wakita, Hiroyuki Okazaki, Yoshikazu Mizuguchi, Shunsuke Tsuda, Yoshihiko Takano, Hiroyuki Takeya, Kazuto Hirata, Takayuki Muro, Mario Okawa, Kyoko Ishizaka, Shik Shin, Hisatomo Harima, Masaaki Hirai, Yuji Muraoka, Takayoshi Yokoya

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 78 ( 3 ) 2009.3

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

We have pet-formed soft x-ray and Ultralligh-resolution laser-excited photoemission measurements on tetragonal FeSe, which was recently identified as a superconductor. Energy dependent study of valence band is compared to band Structure Calculations and yields a reasonable assignment of partial densities of states. However, the sharp peak near the Fermi level slightly deviates from the calculated energy position, giving rise to the necessity of self energy correction. We have also performed ultrahigh-resolution laser photoemission experiment on FeSe and observed the suppression of intensity around the Fermi level upon cooling.

DOI： 10.1143/JPSJ.78.034708

Superconducting Gap and Valence Band of Mg10Ir19B16 Studied by Laser and Synchrotron Photoemission Spectroscopy Reviewed

Rikiya Yoshida, Hiroyuki Okazaki, Keisuke Iwai, Kengo Noami, Takayuki Muro, Mario Okawa, Kyoko Ishizaka, Shik Shin, Zheng Li, Jianlin Luo, Guo-qing Zheng, Tamio Oguchi, Masaaki Hirai, Yuji Muraoka, Takayoshi Yokoya

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 78 ( 3 ) 2009.3

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

We report the results of first photoemission spectroscopy on a newly discovered noncentrosymmetric superconductor Mg10Ir19B16. We measured the valence band structure of Mg10Ir19B16 using soft X-ray photoemission technique and found that the first principal calculation explains our observation well. In the bulk-sensitive laser-excited photoemission study, we observed that the intensity at the Fermi level decreases as we lower the sample temperature, indicative of the opening of superconducting energy gap. Using Dynes' function, we performed fitting analysis on the spectra of superconducting state.

DOI： 10.1143/JPSJ.78.034705

Strong-Coupling Superconductivity in Noncentrosymmetric Superconductor Li2Pd3B by Sub-meV Photoemission Spectroscopy Reviewed

Shunsuke Tsuda, Takayoshi Yokoya, Takayuki Kiss, Takahiro Shimojima, Kyoko Ishizaka, Shik Shin, Tadashi Togashi, Shuntaro Watanabe, Chengqian Zhang, Chuangtian Chen, Izumi Hase, Hiroyuki Takeya, Kazuto Hirata, Kazumasa Togano

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 78 ( 3 ) 2009.3

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

Noncentrosymmetric superconductors Li2Pd3B and Li2Pt3B are studied by high-resolution photoemission spectroscopy. The valence band spectra of Li2Pd3B and Li2Pt3B are well explained by first principles band calculations. The sub-meV photoemission result shows that the size of the superconducting gap of Li2Pd3B is about 1.1 meV at 5.4 K with a reduced gap value (2 Delta(0)/k(B)T(c)) of about 4.5, which is greater than the Bardeen-Cooper-Schrieffer (BCS) value of 3.52. The obtained superconducting gap can be explained more clearly with an s-wave superconducting gap than with p- and d-wave superconducting gaps. This result indicates that Li2Pd3B is a conventional strong-coupling superconductor.

DOI： 10.1143/JPSJ.78.034711

Ultraviolet laser photoemission spectroscopy of FeSi: Observation of a opening in density of states Reviewed

K. Ishizaka, T. Kiss, T. Shimojima, T. Yokoya, T. Togashi, S. Watanabe, C. Q. Zhang, C. T. Chen, Y. Onose, Y. Tokura, S. Shin

Physical Review B 72 233202 2008.12

Language：English Publishing type：Research paper (scientific journal)

The temperature (T) dependent gap formation in the density of states (DOS) of FeSi has been investigated by angle-integrated laser photoemission spectroscopy (PES). With decreasing T, the evolution of a small gap (_60meV) at the Fermi level is observed in the DOS, indicating a p-type semiconducting character of this compound. The Fermi edge, which has been controversial in all past PES studies, is extremely small at 5K in accordance with transport and optical experiments. The T dependence of the gap, which gets smeared out quickly at high T as in optical conductivity spectrum, suggests the existence of a strong scattering mechanism beyond thermal excitations.

DOI： 10.1103/PhysRevB.72.233202

Near E-F electronic structure of heavily boron-doped superconducting diamond Reviewed

H. Okazaki, T. Yokoya, J. Nakamura, N. Yamada, T. Nakamura, T. Muro, Y. Tamenori, T. Matsushita, Y. Takata, T. Tokushima, S. Shin, Y. Takano, M. Nagao, T. Takenouchi, H. Kawarada, T. Oguchi

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 69 ( 12 ) 2978 - 2981 2008.12

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

We have performed soft X-ray angle-resolved photoemission spectroscopy (SXARPES) of a heavily boron-doped superconducting diamond film (T-c = 7.2 K) in order to Study the electronic structure near the Fermi level (E-F). Careful determination of measured momentum space that across F point in the Brillouin zone (BZ) and increase of an energy resolution provide further spectroscopic evidence that EF is located at the highly dispersive diamond-like bands, indicating that holes at the top of the diamond-like valence band play an essential role for the conducting properties of the heavily boron-doped superconducting diamond for this boron-doping region (effective carrier concentration of 1.6%). The SXARPES intensities at E-F were also mapped out over BZ to obtain experimental Fermi surface sheets and compared with calculations. Crown Copyright (c) 2008 Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jpcs.2008.06.006

Electronic Structure of Noncentrosymmetric Superconductor Li-2(PdxPt1-x)(3)B Studied by Photoemission Spectroscopy Reviewed

Rikiya Yoshida, Hiroyuki Okazaki, Mitsutoshi Tajima, Takayuki Muro, Izumi Hase, Kozo Okada, Hiroyuki Takeya, Kazuto Hirata, Masaaki Hirai, Yuji Muraoka, Takayoshi Yokoya

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 77 ( 10 ) 2008.10

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

We have performed x-ray photoemission spectroscopy on the system of noncentrosymmetric Superconductor, Li-2(PdxPt1-x)(3)B. For Li2Pt3B, we found two major peaks with two other weak components, and the band calculations were in agreement with the observation. The assignment of valence band features using the calculated partial density of states determined that Pt 5d and B 2p contribute to the density of states at the Fermi level. The effect of antisymmetric spin-orbit coupling on the band Structure might have been probed. and the analysis on the effect of Pt incorporation into the system indicates the smooth evolution of electronic structures. We presented the measurements of core levels (Pd 3d, Pt 4f, and B 1s) and discussed the chemical bonding states and electronic structures from them.

DOI： 10.1143/JPSJ.77.104701

Temperature-dependent localized excitations of doped carriers in superconducting diamond Reviewed

K. Ishizaka, R. Eguchi, S. Tsuda, A. Chainani, T. Yokoya, T. Kiss, T. Shimojima, T. Togashi, S. Watanabe, C. -T. Chen, Y. Takano, M. Nagao, I. Sakaguchi, T. Takenouchi, H. Kawarada, S. Shin

PHYSICAL REVIEW LETTERS 100 ( 16 ) 2008.4

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

Laser-excited photoemission spectroscopy is used to show that the doped carriers in metallic or superconducting diamond couple strongly to the lattice via high-energy (similar to 150 meV) optical phonons, with direct observations of localized Franck-Condon multiphonon sidebands appearing as Fermi-edge replicas. It exhibits a temperature-dependent spectral weight transfer from higher to lower energy sidebands and zero-phonon Fermi-edge states. The quantified coupling strength shows a systematic increase on lowering temperature, implicating its relation to the normal state transport and superconductivity.

DOI： 10.1103/PhysRevLett.100.166402

Bulk electronic structure of the antiferromagnetic superconducting phase in ErNi(2)B(2)C Reviewed

T. Baba, T. Yokoya, S. Tsuda, T. Kiss, T. Shimojima, K. Ishizaka, H. Takeya, K. Hirata, T. Watanabe, M. Nohara, H. Takagi, N. Nakai, K. Machida, T. Togashi, S. Watanabe, X. -Y. Wang, C. T. Chen, S. Shin

PHYSICAL REVIEW LETTERS 100 ( 1 ) 2008.1

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We have performed temperature- (T-)dependent laser-photoemission spectroscopy of the antiferromagnetic (AF) superconductor ErNi(2)B(2)C to study the electronic-structure evolution reflecting the interplay between antiferromagnetism and superconductivity. The spectra at the superconducting (SC) phase show a very broad spectral shape. A T-dependent SC gap shows a sudden deviation from the BCS prediction just below T(N). This observation can be explained well by the theoretical model and thus represents the characteristic bulk electronic structure of the AF SC phase for the first time.

DOI： 10.1103/PhysRevLett.100.017003

Erratum: Bulk screening in core-level photoemission from Mott-Hubbard and charge-transfer systems (Physical Review B - Condensed Matter and Materials Physics (2005) 71, (155102)) Reviewed

M. Taguchi, A. Chainani, N. Kamakura, K. Horiba, Y. Takata, M. Yabashi, K. Tamasaku, Y. Nishino, D. Miwa, T. Ishikawa, S. Shin, E. Ikenaga, T. Yokoya, K. Kobayashi, T. Mochiku, K. Hirata, K. Motoya

Physical Review B - Condensed Matter and Materials Physics 76 ( 16 ) 2007.10

Thickness dependence of magnetic domain formation in La0.6Sr0.4MnO3 epitaxial thin films studied by XMCD-PEEM Reviewed

Toshiyuki Taniuchi, Ryutaro Yasuhara, Hiroshi Kumigashira, Masato Kubota, Hiroyuki Okazaki, Takanori Wakita, Takanori Yokoya, Kanta Ono, Masaharu Oshima, Mikk Lippmaa, Masashi Kawasaki, Hideomi Koinuma

SURFACE SCIENCE 601 ( 20 ) 4690 - 4693 2007.10

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We have used photoelectron emission microscopy (PEEM) and X-ray magnetic circular dichroism (XMCD) to study the effect of thin film thickness on the magnetic domain formation in La0.6Sr0.4MnO3 samples that were epitaxially grown on stepped SrTiO3 (00 1) substrates. The magnetic image exhibited a stripe structure elongated along the step direction, irrespective of film thickness, suggesting that uniaxial magnetic anisotropy induced by step-and-terrace structures plays an important role in the magnetic domain formation. Additional domains evolved gradually with increasing film thickness. In these domains, the direction of magnetization differed from the step direction due to biaxial magneto-crystalline anisotropy. The evolution of additional magnetic domains with increasing film thickness implies that a competition exists between the two anisotropies in LSMO films. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.susc.2007.05.036

Study of electronic structures for Fe thin films deposited on Si- and C-faces of 4H-SiC substrates by soft X-ray emission spectroscopy Reviewed

M. Hirai, T. Wakita, H. Okazaki, D. Koishihara, Y. Muraoka, T. Yokoya

APPLIED SURFACE SCIENCE 254 ( 1 ) 26 - 28 2007.10

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Silicon carbide (SiC) is a candidate material for electronic devices to operate upon crucial environment. Electronic states of silicides and/or carbide/graphite formed in metal/SiC contact system are fundamentally important from the viewpoint of device performance.
We study interface electronic structure of iron thin film deposited on silicon (Si)- and carbon (C)-face of 4H-SiC(0 0 0 1) by using a soft X-ray emission spectroscopy (SXES). For specimens of Fe (50 nm)/4H-SiC (substrate) contact systems annealed at 700 and 900 degrees C, the Si L-2,L-3 emission spectra indicate different shapes and peak energies from the substrate depending on thermal-treated temperature. The product of materials such as silicides is suggested. Further, from comparison of Si L-2,L-3 emission spectra between Si- and C-face for the same annealing temperature at 700 degrees C, it is concluded that the similar silicides and/or ternary materials are formed on the two surfaces. However for those of 900 degrees C, the film on substrate is composed of the different silicide and/or ternary materials. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.apsusc.2007.07.025

Charge-order-maximized momentum-dependent superconductivity Reviewed

T. Kiss, T. Yokoya, A. Chainani, S. Shin, T. Hanaguri, M. Nohara, H. Takagi

NATURE PHYSICS 3 ( 10 ) 720 - 725 2007.10

Language：English Publishing type：Research paper (scientific journal) Publisher：NATURE PUBLISHING GROUP

Charge ordering and superconductivity are observed in the phase diagrams of a variety of materials such as NbSe3, layered transition-metal dichalcogenides and high-temperature copper oxide superconductors, low-dimensional organics, Ba1-xKxBiO3 and so forth. Although both conventional charge-density-wave (CDW) and superconducting transitions show an energy gap in the single-particle density of states at the Fermi level ( E F), their physical properties are poles apart: insulating behaviour for the CDW and zero resistivity in superconductors. Consequently, these two ground states are believed to compete with each other. Here we provide evidence for maximized superconductivity at points in momentum (k) space that are directly connected by the CDW ordering vector. Temperature-dependent angle-resolved photoemission spectroscopy of 2H-NbSe2 across the CDW and superconducting transitions ( T-CDW similar to 33Kand T-c= 7.2 K, respectively) shows CDW-induced spectral-weight depletion at the same Fermi-surface-crossing k points, which evolve into the largest superconducting gaps. These k points also show the highest electron-phonon coupling and lowest Fermi velocities. Our results demonstrate that charge order can boost superconductivity in an electron-phonon coupled system, in direct contrast to the prevailing view that it only competes with superconductivity.

DOI： 10.1038/nphys699

Magnetic domain structure of a technically patterned ferromagnetic La0.6Sr0.4MnO3 thin film Reviewed

M. Kubota, T. Taniuchi, R. Yasuhara, H. Kumigashira, M. Oshima, K. Ono, H. Okazaki, T. Wakita, T. Yokoya, H. Akinaga, M. Lippmaa, M. Kawasaki, H. Koinuma

APPLIED PHYSICS LETTERS 91 ( 18 ) 2007.10

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

By using photoelectron emission microscopy, we systematically investigated the magnetic domain in the La0.6Sr0.4MnO3 patterned thin film with several microns, in which we can control the ratio of a shape anisotropy to a step-induced uniaxial anisotropy in magnitude. We demonstrate that a cooperative effect of a step-induced uniaxial anisotropy with a shape anisotropy causes the formation of a single domain, whereas the competitive effect makes a multimagnetic domain. (C) 2007 American Institute of Physics.

DOI： 10.1063/1.2801699

Bulk screening in core-level photoemission from Mott-Hubbard and charge-transfer systems (vol 71, art no 155102, 2005) Reviewed

M. Taguchi, A. Chainani, N. Kamakura, K. Horiba, Y. Takata, M. Yabashi, K. Tamasaku, Y. Nishino, D. Miwa, T. Ishikawa, S. Shin, E. Ikenaga, T. Yokoya, K. Kobayashi, T. Mochiku, K. Hirata, K. Motoya

PHYSICAL REVIEW B 76 ( 16 ) 2007.10

Sub-meV resolution photoemission study on carbon substituted MgB2 Reviewed

S. Tsuda, T. Yokoya, T. Kiss, T. Shimojima, S. Shin, T. Togashi, S. Watanabe, C. Q. Zhang, C. T. Chen, S. Lee, H. Uchiyama, S. Tajima, N. Nakai, K. Machida

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 460 80 - 83 2007.9

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We have performed a sub-meV photoemission study on Mg(B1-xCx)(2) for x = 0-0.075. The superconducting (SC) transition temperature (T-c) changes systematically from 38 K to 24 K with increasing the carbon concentration. Because of the extremely high-energy resolution of sub-meV, two features corresponding to the two SC gaps originated from pi and sigma bands are clearly observed even in raw spectra up to x = 0.075. From numerical analyses, we found that the gap size of the sigma band is proportional to T-c, while that of the pi band is almost constant. The temperature and carbon concentration dependences of the two gaps are explained within a simple two-band model based on a mean field theory. Using the theoretical model, we deduced interband and intraband coupling strength constants. The transition temperature is dominated by the intraband coupling of the sigma band, while the interband coupling slightly increases with increasing the carbon concentration. The two-band model shows that the interband coupling slightly pushes up T-c. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.03.071

Laser-photoemission study of antiferromagnetic superconductor ErNi2B2C Reviewed

Teruhisa Baba, Takayoshi Yokoya, Shunsuke Tsuda, Takayuki Kiss, Takahiro Shimojima, Kyoko Ishizaka, Shik Shin, Hiroyuki Takeya, Kazuto Hirata, Tadashi Togashi, C. Zhang, C. T. Chen, Shuntaro Watanabe

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 460 634 - 635 2007.9

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We have investigated the electronic structures of antiferromagnetic superconductor ErNi2B2C by using laser-photoemission spectroscopy. The observed spectra below the superconducting transition temperature (T-c = 9.0 K) show a very broad coherent peak and finite density of states at the Fermi level due to strong magnetic pair breaking effects. In addition, we find a dip structure in the temperature dependence of the superconducting order parameter just below the antiferromagnetic ordering temperature (T-N = 6.0 K). This behavior can be well interpreted as a result of competition between the rapid evolution of the antiferromagnetic molecular field and the increase of the superconducting condensation energy with decreasing temperature. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.04.132

Interplay of superconductivity and rattling phenomena in beta-pyrochlore KOs2O6 studied by photoemission spectroscopy Reviewed

T. Shimojima, Y. Shibata, K. Ishizaka, T. Kiss, A. Chainani, T. Yokoya, T. Togashi, X.-Y. Wang, C. T. Chen, S. Watanabe, J. Yamaura, S. Yonezawa, Y. Muraoka, Z. Hiroi, T. Saitoh, S. Shin

PHYSICAL REVIEW LETTERS 99 ( 11 ) 2007.9

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

The electronic structure near the Fermi level (E-F) of the beta-pyrochlore superconductor KOs2O6 is studied using laser-excited ultrahigh-resolution photoemission spectroscopy. The superconducting gap clearly opens across the superconducting transition (T-c=9.6 K), with the strong electron-phonon coupling value of 2 Delta(0)/k(B)T(c)>= 4.56. A fitting analysis identifies clear anomalies at T-p=7.5 K in the temperature dependencies of the superconducting gap size and the quasiparticle relaxation lifetime. These anomalies and the fine spectral structures arising from phonons suggest that the existence of the rattling behavior of K ions significantly affects the superconductivity in KOs2O6.

DOI： 10.1103/PhysRevLett.99.117003

Recent photoemission studies on MgB2 and related materials Reviewed

Shunsuke Tsuda, Takayoshi Yokoya, Shik Shin

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 456 ( 1-2 ) 126 - 133 2007.6

Language：English Publisher：ELSEVIER SCIENCE BV

MgB2 shows the highest superconducting transition temperature among intermetallic compounds. Angle-resolved photoemission spectroscopy revealed their electronic structure, and the result is well explained within a simple band picture except for one band. Other photoemission studies on related materials of AlB2 and CaAlSi also show an extra band similar to that in MgB2. The direct observation of the superconducting gap confirmed a two-gap superconductivity. The larger and smaller gaps open at the sigma and pi bands, respectively. The carbon substitution effect on the two gaps was evaluated by using sub-meV photoemission spectroscopy. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2006.12.003

Core-level electronic structure evolution of heavily boron-doped superconducting diamond studied with hard x-ray photoemission spectroscopy Reviewed

T. Yokoya, E. Ikenaga, M. Kobata, H. Okazaki, K. Kobayashi, A. Takeuchi, A. Awaji, Y. Takano, M. Nagao, I. Sakaguchi, T. Takenouchi, K. Kobayashi, H. Kawarada, T. Oguchi

PHYSICAL REVIEW B 75 ( 20 ) 2007.5

Language：English Publishing type：Research paper (scientific journal) Publisher：AMERICAN PHYSICAL SOC

Hard x-ray photoemission spectroscopy has been used to study intrinsic core-level electronic structure evolution of heavily boron-doped superconducting diamond films made with a microwave plasma-assisted chemical-vapor deposition method. The boron concentration dependent C 1s core-level spectra show systematic changes in the shift of the main peak and in the evolution of an additional feature at 1.1-1.3 eV lower binding energy than the main peak. In comparison to a low boron concentration nonsuperconducting diamond, the higher boron concentration doped diamond films show formation of several additional features in the B 1s core levels. Based on the present results, the local chemical environments around the doped boron atoms, the efficiency of hole doping by boron doping, and the implications for a recent x-ray absorption study are discussed.

DOI： 10.1103/PhysRevB.75.205117

Observation of a superconducting gap in boron-doped diamond by laser-excited photoemission spectroscopy Reviewed

K. Ishizaka, R. Eguchi, S. Tsuda, T. Yokoya, A. Chainani, T. Kiss, T. Shimojima, T. Togashi, S. Watanabe, C. -T. Chen, C. Q. Zhang, Y. Takano, M. Nagao, I. Sakaguchi, T. Takenouchi, H. Kawarada, S. Shin

PHYSICAL REVIEW LETTERS 98 ( 4 ) 2007.1

Language：English Publishing type：Research paper (scientific journal) Publisher：AMERICAN PHYSICAL SOC

We investigate the temperature (T)-dependent low-energy electronic structure of a boron-doped diamond thin film using ultrahigh resolution laser-excited photoemission spectroscopy. We observe a clear shift of the leading edge below T=11 K, indicative of a superconducting gap opening (Delta similar to 0.78 meV at T=4.5 K). The gap feature is significantly broad and a well-defined quasiparticle peak is lacking even at the lowest temperature of measurement (=4.5 K). We discuss our results in terms of disorder effects on the normal state transport and superconductivity in this system.

DOI： 10.1103/PhysRevLett.98.047003

Electronic structures of heavily boron-doped superconducting diamond films Reviewed

Takayoshi Yokoya, Hiroyuki Okazaki, Tetsuya Nakamura, Tomohiro Matsushita, Takayuki Muro, Eiji Ikenaga, Masaaki Kobata, Keisuke Kobayashi, Akihisa Takeuchi, Akihiro Awaji, Yoshihiko Takano, Masanori Nagao, Tomohiro Takenouchi, Hiroshi Kawarada, Tamio Oguchi

DIAMOND ELECTRONICS - FUNDAMENTALS TO APPLICATIONS 956 39 - + 2007

Language：English Publishing type：Research paper (international conference proceedings) Publisher：MATERIALS RESEARCH SOC

Recent photoemission studies on heavily boron-doped superconducting diamond films, reporting the electronic structure evolution as a function of boron concentrations, are reviewed. From soft X-ray angle-resolved photoemission spectroscopy, which directly measures electronic band dispersions, depopulation of electrons (or formation of hole pockets) at the top of the valence band were clearly observed. This indicates that the holes at the top of the valence bands are responsible for the metallic properties and hence superconductivity at lower temperatures. Hard X-ray photoemission spectroscopy observed shift of the main C Is core level and intensity evolution of a lower binding energy additional structure, suggesting chemical potential shift, carrier doping efficiency by boron doping, and possibility of boron-related cluster formations.

Valence band electronic structures of heavily boron-doped superconducting diamond studied by synchrotron photoemission spectroscopy Reviewed

Takayoshi Yokoya, Tetsuya Nakamura, Tomohiro Matsushita, Takayuki Muro, Eiji Ikenaga, Masaaki Kobata, Keisuke Kobayashi, Yoshihiko Takano, Masanori Nagao, Tomohiro Takenouchi, Hiroshi Kawarada, Tamio Oguchi

NEW DIAMOND AND FRONTIER CARBON TECHNOLOGY 17 ( 1 ) 11 - 19 2007

Language：English Publishing type：Research paper (scientific journal) Publisher：MYU K K

The valence band electronic structures of heavily boron-doped superconducting diamond films made by microwave plasma-assisted chemical vapor deposition (MPCVD) were investigated by hard X-ray photoemission spectroscopy (HXPES) and soft X-ray angle-resolved photoemission spectroscopy (SXARPES). The HXPES core-level spectrum of heavily boron-doped diamond shows a new feature at the lower binding energy side of the C Is main peak. The HXPES valence band spectrum of a heavily boron-doped superconducting diamond film shows a broader spectral shape than that of a lightly doped non-superconducting sample. The SXARPES results of homoepitaxial CVD films show clear valence band dispersions with a bandwidth of similar to 23 eV and the top of the valence band at the Gamma point in the Brillouin zone, which are well explained by the calculated valence band dispersions of pure diamond. Boron con centrati on-dependent band dispersions near the Fermi level (E-F) by SXARPES exhibit a systematic shift in E-F, indicating electron depopulation due to hole doping, and an increase in the line shape corresponding to the broader density of states observed by HXPES. These results indicate that holes in the top of the valence band are responsible for the metallic states leading to superconductivity at low temperatures. The HXPES C Is core-level spectra of lightly boron-doped non-superconducting and heavily boron-doped superconducting films are also shown.

24pxA-2 The study of electronic structure of n-type phosphorus-doped diamond by using soft x-ray photoelectron spectroscopy Reviewed

Okazaki H., Yamasaki S., Takano Y., Ishii S., Oguchi T., Yokoya T., Muro T., Nakamura T., Matsushita T., Wakita T., Muraoka Y., Hirai M., Kato S.

Meeting Abstracts of the Physical Society of Japan 62 ( 0 ) 665 - 665 2007

24pXA-1 Soft x-ray photoelectron spectroscopy of heavily B-doped superconducting diamond Reviewed

Okazaki H., Ishii S., Iriyama S., Kawarada H., Yokoya T., Muro T., Nakamura T., Matsushita T., Wakita T., Muraoka Y., Hirai M., Takano Y.

Meeting Abstracts of the Physical Society of Japan 62 ( 0 ) 665 - 665 2007

Angle-resolved photoemission study of the cobalt oxide superconductor NaxCoO2 center dot yH(2)O: Observation of the Fermi surface Reviewed

T. Shimojima, K. Ishizaka, S. Tsuda, T. Kiss, T. Yokoya, A. Chainani, S. Shin, P. Badica, K. Yamada, K. Togano

PHYSICAL REVIEW LETTERS 97 ( 26 ) 2006.12

Language：English Publishing type：Research paper (scientific journal) Publisher：AMERICAN PHYSICAL SOC

The cobalt oxide superconductor NaxCoO2 center dot yH(2)O is studied by angle-resolved photoemission spectroscopy. We report the Fermi surface (FS) topology and electronic structure near the Fermi level (E-F) in the normal state of NaxCoO2 center dot yH(2)O. Our result indicates the presence of the hexagonal FS centered at the Gamma point, while the small pocket FSs along Gamma-K direction are absent, similar to NaxCoO2. The top of the e(g)(') band, which is expected in band calculations to form the small pocket FSs, extends to within similar to 30 meV below E-F, closer to E-F than in NaxCoO2. We discuss its possible role in superconductivity, comparing with other experimental and theoretical results.

DOI： 10.1103/PhysRevLett.97.267003

Laser-excited ultrahigh-resolution photoemission spectroscopy of borocarbide superconductor RNi2B2C(R = Y and Er) Reviewed

T. Baba, T. Yokoya, S. Tsuda, T. Kiss, T. Shimojima, S. Shin, T. Togashi, C. T. Chen, C. Q. Zhang, S. Watanabe, T. Watanabe, M. Nohara, H. Takagi

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 445 46 - 49 2006.10

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We present laser-excited ultrahigh-resolution photoemission spectroscopy of YNi2B2C and ErNi2B2C. The extremely high energy resolution enables us to discuss the superconducting (SC) gap function of YNi2B2C and to measure the density of states which is coexisted between superconductivity and antiferromagnetism in ErNiAC. For YNi2B2C, s + g-wave SC gap symmetry cannot reproduce the experimental data, implying importance of taking the complicated electronic structures into consideration. For ErNi2B2C, the observed coherent peak is very broad and finite density of state exists at Fermi level. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2006.03.074

Observation of step-induced magnetic domain formation in La1-xSrxMnO3 thin films by photoelectron emission microscopy Reviewed

T. Taniuchi, H. Kumigashira, M. Oshima, T. Wakita, T. Yokoya, M. Kubota, K. Ono, H. Akinaga, M. Lippmaa, M. Kawasaki, H. Koinuma

APPLIED PHYSICS LETTERS 89 ( 11 ) 2006.9

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

Magnetic domain formation in thin films of La1-xSrxMnO3 (LSMO) with x=0.2 and 0.4 epitaxially grown on stepped SrTiO3 (001) substrates has been investigated by photoelectron emission microscopy. The magnetic domains show a stripe structure elongated along the step directions, indicating uniaxial magnetic anisotropy induced by step structures. We have also found that the magnetization of the magnetic domains is slightly meandering at domain boundaries. The existence of the additional structures suggests that the magnetic domains in LSMO films are also influenced by biaxial magnetocrystalline anisotropy. The direct observation of the magnetic domain structures suggests that the competition between the two magnetic anisotropies may play an important role in magnetic properties in LSMO films. (c) 2006 American Institute of Physics.

DOI： 10.1063/1.2347895

Superconducting gap of filled skutterudite superconductor LaRu4P12 studied by sub-meV resolution photoemission spectroscopy Reviewed

S. Tsuda, T. Yokoya, T. Kiss, T. Shimojima, S. Shin, T. Togasi, S. Watanabe, C. Q. Zhang, C. T. Chen, H. Sugawara, H. Sato, H. Harima

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 75 ( 6 ) 2006.6

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

We study the superconducting gap of the filled skutterudite superconductor LaRu4P12 (transition temperature T-c = 7.2 K) using a newly developed sub-meV resolution photoemission spectrometer with a vacuum-UV laser as a light source. The result at 4.8 K shows the opening of a superconducting gap, whose spectral shape is well explained with an isotropic s-wave Dynes function using a gap size of 1.3 meV. This corresponds to the reduced gap 2 Delta(0)/k(B)T(c) of 4.2. The temperature dependence of the superconducting gap shows fairly good agreement with the conventional BCS theory. The present results are consistent with those of a moderately strong-coupling and isotropic s-wave superconductor.

DOI： 10.1143/JPSJ.75.064711

Laser-excited ultrahigh-resolution photoemission study of anisotropic s-wave superconductor YNi2B2C Reviewed

T Baba, T Yokoya, S Tsuda, T Kiss, T Shimojima, S Shin, T Togashi, CT Chen, CQ Zhang, S Watanabe, T Watanabe, M Nohara, H Takagi

PHYSICA B-CONDENSED MATTER 378-80 469 - 470 2006.5

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We have performed laser-excited ultrahigh-resolution photoemission spectroscopy of Y(Ni1-xPt2)(2)B2C (x = 0.0, 0.2) and numerical simulation using s, anisotropic s and even s + g symmetry to study the superconducting gap anisotropy in borocarbide supercodnuctors. We observed the reduction of gap anisotropy in the x = 0.2 sample in comparison to the x = 0.0 sample, confirming the anisotropic s-wave gap. We also found that the spectrum of x = 0.0 at 3.5 K cannot be fitted well with the s + g-wave symmetry, implying that the s + g symmetry is too simple to describe over all supercodncuting gap electronic Structure of borocarbides. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2006.01.204

Hard X-ray photoelectron emission microscopy as tool for studying buried layers Reviewed

T Wakita, T Taniuchi, K Ono, M Suzuki, N Kawamura, M Takagaki, H Miyagawa, FZ Guo, T Nakamura, T Muro, H Akinaga, T Yokoya, M Oshima, K Kobayashi

JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS 45 ( 3A ) 1886 - 1888 2006.3

Language：English Publishing type：Research paper (scientific journal) Publisher：JAPAN SOC APPLIED PHYSICS

We have performed photoelectron emission microscopy measurements with hard X-rays (HX-PEEM) and have shown that HX-PEEM has a spatial resolution higher than 106 nm for observing the sample surface and can detect signals from a Au layer buried under a 50-nm-thick Co layer. These results mean that HX-PEEM has a probing depth one order of magnitude deeper than that of PEEM with soft X-rays. This deeper probing depth with HX-PEEM provides a new opportunity to investigate electronic and/or magnetic structures of buried layers under a thick overlayer. The notable advantages and potential applications of HX-PEEM are discussed.

DOI： 10.1143/JJAP.45.1886

Bulk- and surface-sensitive high-resolution photoemission study of two mott-hubbard systems: SrVO3 and CaVO3 Reviewed

R Eguchi, T Kiss, S Tsuda, T Shimojima, T Mizokami, T Yokoya, A Chainani, S Shin, IH Inoue, T Togashi, S Watanabe, CQ Zhang, CT Chen, M Arita, K Shimada, H Namatame, M Taniguchi

PHYSICAL REVIEW LETTERS 96 ( 7 ) 2006.2

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We study the electronic structure of Mott-Hubbard systems SrVO3 and CaVO3 with bulk and surface-sensitive high-resolution photoemission spectroscopy, using a vacuum ultraviolet laser, synchrotron radiation, and a discharge lamp (h nu=7-21 eV). A systematic suppression of the density of states (DOS) within similar to 0.2 eV of the Fermi level (E-F) is found on decreasing photon energy, i.e., on increasing bulk sensitivity. The coherent band in SrVO3 and CaVO3 is shown to consist of surface and bulk-derived features, separated in energy. The stronger distortion on surface of CaVO3 compared to SrVO3 leads to a higher surface metallicity in the coherent DOS at E-F, consistent with recent theory.

DOI： 10.1103/PhysRevLett.96.076402

Anisotropic s-wave superconductors studied by angle-resolved photoemission spectroscopy Reviewed

T Yokoya, T Baba, S Tsuda, T Kiss, A Chainani, S Shin, T Watanabe, M Nohara, T Hanaguri, H Takagi, Y Takano, H Kito, J Itoh, H Harima, T Oguchi

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 67 ( 1-3 ) 277 - 281 2006.1

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

We review our angle-resolved photoemission spectroscopy (ARPES) results across superconducting transitions in 2H-NbSe2 and MgB2 and also show recent results on YNi2B2C. For 2H-NbSe2 and MgB2, we observe that the superconducting gap exhibits clear Fermi surface (FS) sheet dependence for two types of FS sheets with different characters, thus providing evidence that the FS sheet dependence is a general feature for a superconductor having multiple FS sheets of different character. In YNi2B2C, we show experimental FSs, and momentum-dependent superconducting gap showing large anisotropy observed on a single FS. These ARPES results provide direct evidence for and importance of momentum-dependent superconducting gaps of phonon-mediated superconductors. (c) 2006 Published by Elsevier Ltd.

DOI： 10.1016/j.jpcs.2005.10.125

Laser-excited ultrahigh-resolution photoemission spectroscopy of superconducting Na0.35CoO2 center dot 1.3H(2)O Reviewed

T Shimojima, T Yokoya, T Kiss, A Chainani, S Shin, T Togashi, C Zhang, C Chen, S Watanabe, K Takada, T Sasaki, H Sakurai, E Takayama-Muromachi

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 67 ( 1-3 ) 282 - 285 2006.1

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

We have studied the electronic structure near the Fermi level of the layered cobaltate superconductor Na(0.35)CoO2 center dot 1.3H(2)0 using laser-excited ultrahigh-resolution photoemission spectroscopy (PES). In the normal state, we observed the formation of a pseudogap with an energy scale of similar to 20 meV. Across the superconducting transition, we observed the leading-edge shift at 3.5 K compared to that at 10 K, indicating superconducting gap (SC gap) opening having a gap value Delta of 0.5-0.7 meV. The energy scale of the SC gap is similar to 20 times smaller than that of the pseudogap, suggesting the existence of competing order parameters at low temperatures. (c) 2006 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jpcs.2005.10.124

Laser-excited photoemission spectroscopy study of superconducting boron-doped diamond Reviewed

K. Ishizaka, R. Eguchi, S. Tsuda, T. Kiss, T. Shimojima, T. Yokoya, S. Shin, T. Togashi, S. Watanabe, C. -T. Chen, C. Q. Zhang, Y. Takano, M. Nagao, I. Sakaguchi, T. Takenouchi, H. Kawarada

SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS 7 S17 - S21 2006

Language：English Publishing type：Research paper (scientific journal) Publisher：NATL INST MATERIALS SCIENCE

We have investigated the low-energy electronic state of boron-doped diamond thin film by the laser-excited photoemission spectroscopy. A clear Fermi-edge is observed for samples doped above the semiconductor-metal boundary, together with the characteristic structures at 150 x n meV possibly due to the strong electron-lattice coupling effect. In addition, for the superconducting sample, we observed a shift of the leading edge below T-c indicative of a superconducting gap opening. We discuss the electron-lattice coupling and the superconductivity in doped diamond. (c) 2006 NIMS and Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.stam.2006.05.008

Soft X-ray angle-resolved photoemission spectroscopy of heavily boron-doped superconducting diamond films Reviewed

T. Yokoya, T. Nakamura, T. Matushita, T. Muro, H. Okazaki, M. Arita, K. Shimada, H. Namatame, M. Taniguchi, Y. Takano, M. Nagao, T. Takenouchi, H. Kawarada, T. Oguchi

SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS 7 S12 - S16 2006

Language：English Publishing type：Research paper (scientific journal) Publisher：NATL INST MATERIALS SCIENCE

We have performed soft X-ray angle-resolved photoemission spectroscopy (SXARPES) of microwave plasma-assisted chemical vapor deposition diamond films with different B concentrations in order to study the origin of the metallic behavior of superconducting diamond. SXARPES results clearly show valence band dispersions with a bandwidth of similar to 23 eV and with a top of the valence band at gamma point in the Brillouin zone, which are consistent with the calculated valence band dispersions of pure diamond. Boron concentration-dependent band dispersions near the Fermi level (E-F) exhibit a systematic shift of E-F, indicating depopulation of electrons due to hole doping. These SXARPES results indicate that diamond bands retain for heavy boron doping and holes in the diamond band are responsible for the metallic states leading to superconductivity at low temperature. A high-resolution photoemission spectroscopy spectrum near E-F of a heavily boron-doped diamond superconductor is also presented. (c) 2006 NIMS and Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.stam.2006.02.014

Fermi surface topology of borocarbide superconductor YNi2B2C: Angle-resolved photoemission spectroscopy Reviewed

T. Baba, T. Yokoya, S. Tsuda, S. Shin, T. Watanabe, M. Nohara, H. Takagi, T. Oguchi

LOW TEMPERATURE PHYSICS, PTS A AND B 850 683 - + 2006

Language：English Publishing type：Research paper (international conference proceedings) Publisher：AMER INST PHYSICS

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) of the borocarbide superconductor YNi2B2C to Study the Fermi surface (FS) topology of the anisotropic s-wave superconductor. We successfully observed experimental valence band dispersions and Fermi surfaces (FSs), and indicated two nested parts connected by the known nesting vector Q similar to(0.55,0,0).

Experimental investigations on the electronic structure and the low-temperature stability of Cd-based quasicrystals and their 1/1 cubic approximants Reviewed

R Tamura, K Minoda, S Takeuchi, T Takeuchi, K Edagawa, T Kiss, T Yokoya, S Shin

PHILOSOPHICAL MAGAZINE 86 ( 3-5 ) 489 - 497 2006

Language：English Publishing type：Research paper (scientific journal) Publisher：TAYLOR & FRANCIS LTD

Valence band spectra of icosahedral quasicrystals and their 1/1 cubic approximants in the Cd-Yb and Cd-Ca systems are investigated systematically. A sharp Fermi edge, indicative of sizable density of states at the Fermi level (EF), is clearly observed for all the compounds and EF is located not at the bottom of the pseudogap but at its shoulder. In addition, a peak structure appears just below E-F for the Cd-Ca system, which is also found for the Cd-Yb system when subtracting a Yb 4f derived doublet. Ca 2p-3d resonant photoemission spectroscopies on the Cd-Ca compounds reveal the existence of Ca 3d states at EF, indicating that the 3d states do contribute to the peak structure at EF, which is attributed to hybridization with the Cd sp states. The low-temperature structural properties of the Cd-based compounds are also reported. An occurrence of a structural phase transition at 110K and 100K for Cd6Yb and Cd6Ca, respectively, is well documented. The transitions are understood as a consequence of an orientational ordering of Cd-4 tetrahedron located at the center of the icosahedral clusters. It is suggested that two orientations of the Cd4 tetrahedron alternatively occur along the [110] direction of the high temperature bcc phase.

DOI： 10.1080/14786430500267778

Temperature-dependent luttinger surfaces Reviewed

T Ito, A Chainani, T Haruna, K Kanai, T Yokoya, S Shin, R Kato

PHYSICAL REVIEW LETTERS 95 ( 24 ) 2005.12

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

The Luttinger surface of an organic metal (TTF-TCNQ), possessing charge order and spin-charge separated band dispersions, is investigated using temperature-dependent angle-resolved photoemission spectroscopy. The Luttinger surface topology, obtained from momentum distribution curves, changes from quasi-2D (dimensional) to quasi-1D with temperature. The high temperature quasi-2D surface exhibits 4k(F) charge-density-wave (CDW) superstructure in the TCNQ derived holon band, in the absence of 2k(F) order. Decreasing temperature results in quasi-1D nested 2k(F) CDW order in the TCNQ spinon band and in the TTF surface. The results establish the link in momentum space between charge order and spin-charge separation in a Luttinger liquid.

DOI： 10.1103/PhysRevLett.95.246402

Origin of the metallic properties of heavily boron-doped superconducting diamond Reviewed

T Yokoya, T Nakamura, T Matsushita, T Muro, Y Takano, M Nagao, T Takenouchi, H Kawarada, T Oguchi

NATURE 438 ( 7068 ) 647 - 650 2005.12

Language：English Publishing type：Research paper (scientific journal) Publisher：NATURE PUBLISHING GROUP

The physical properties of lightly doped semiconductors are well described by electronic band- structure calculations and impurity energy levels(1). Such properties form the basis of present- day semiconductor technology. If the doping concentration n exceeds a critical value n(c), the system passes through an insulator- to- metal transition and exhibits metallic behaviour; this is widely accepted to occur as a consequence of the impurity levels merging to form energy bands(2). However, the electronic structure of semiconductors doped beyond nc have not been explored in detail. Therefore, the recent observation of superconductivity emerging near the insulator- to- metal transition(3) in heavily boron- doped diamond(4,5) has stimulated a discussion on the fundamental origin of the metallic states responsible for the superconductivity. Two approaches have been adopted for describing this metallic state: the introduction of charge carriers into either the impurity bands(6) or the intrinsic diamond bands(7-9). Here we show experimentally that the doping- dependent occupied electronic structures are consistent with the diamond bands, indicating that holes in the diamond bands play an essential part in determining the metallic nature of the heavily boron- doped diamond superconductor. This supports the diamond band approach and related predictions, including the possibility of achieving dopant- induced superconductivity in silicon and germanium(7). It should also provide a foundation for the possible development of diamond- based devices(10).

DOI： 10.1038/nature04278

Ultraviolet laser photoemission spectroscopy of FeSi: Observation of a gap opening in density of states Reviewed

K Ishizaka, T Kiss, T Shimojima, T Yokoya, T Togashi, S Watanabe, CQ Zhang, CT Chen, Y Onose, Y Tokura, S Shin

PHYSICAL REVIEW B 72 ( 23 ) 2005.12

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

The temperature (T) dependent gap formation in the density of states (DOS) of FeSi has been investigated by angle-integrated laser photoemission spectroscopy (PES). With decreasing T, the evolution of a small gap (similar to 60 meV) at the Fermi level is observed in the DOS, indicating a p-type semiconducting character of this compound. The Fermi edge, which has been controversial in all past PES studies, is extremely small at 5 K in accordance with transport and optical experiments. The T dependence of the gap, which gets smeared out quickly at high T as in optical conductivity spectrum, suggests the existence of a strong scattering mechanism beyond thermal excitations.

DOI： 10.1103/PhysRevB.72.233202

Evidence for suppressed screening on the surface of high temperature La2-xSrxCuO4 and Nd2-xCexCuO4 superconductors Reviewed

M Taguchi, A Chainani, K Horiba, Y Takata, M Yabashi, K Tamasaku, Y Nishino, D Miwa, T Ishikawa, T Takeuchi, K Yamamoto, M Matsunami, S Shin, T Yokoya, E Ikenaga, K Kobayashi, T Mochiku, K Hirata, J Hori, K Ishii, F Nakamura, T Suzuki

PHYSICAL REVIEW LETTERS 95 ( 17 ) 2005.10

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

Hard x-ray photoemission spectroscopy (PES) of Cu core electronic states, with a probing depth of similar to 60 angstrom, is used to show that the Zhang-Rice singlet feature is present in La2CuO4 but is absent in Nd2CuO4. Hole and electron doping in La2-xSrxCuO4 (LSCO) and Nd2-xCexCuO4 (NCCO) result in new well-screened features which are missing in soft x-ray PES. Impurity Anderson model calculations establish screening from doped states as its origin, which is strongly suppressed within 15 angstrom of the surface. Complemented with x-ray absorption spectroscopy, the small chemical-potential shift in core levels (similar to 0.2 eV) are shown to be consistent with modifications of valence and conduction band states spanning the band gap (similar to 1 eV) upon hole and electron doping in LSCO and NCCO.

DOI： 10.1103/PhysRevLett.95.177002

Carbon-substitution dependent multiple superconducting gap of Mg B2: A sub-meV resolution photoemission study Reviewed

S. Tsuda, T. Yokoya, T. Kiss, T. Shimojima, S. Shin, T. Togashi, S. Watanabe, C. Zhang, C. T. Chen, S. Lee, H. Uchiyama, S. Tajima, N. Nakai, K. Machida

Physical Review B - Condensed Matter and Materials Physics 72 ( 6 ) 2005.8

Language：English Publishing type：Research paper (scientific journal)

"Sub-meV" resolution photoemission spectroscopy was used to study carbon-substitution dependence of the multiple superconducting gap of Mg (B1-x Cx) 2. Two features corresponding to σ and π gaps are clearly observed in the raw spectra up to carbon concentration x=0.075. The observed x dependence of the two gaps shows a qualitatively different behavior: the σ gap is proportional to Tc while the π gap shows negligible change. Doping as well as temperature dependence can be explained within the two-band mean-field theory. Implications from the present study are discussed. © 2005 The American Physical Society.

DOI： 10.1103/PhysRevB.72.064527

Angle-resolved photoemission study of K0.3MoO3: direct observation of temperature-dependent Fermi surface across the Peierls transition Reviewed

H Ando, T Yokoya, K Ishizaka, S Tsuda, T Kiss, S Shin, T Eguchi, M Nohara, H Takagi

JOURNAL OF PHYSICS-CONDENSED MATTER 17 ( 32 ) 4935 - 4940 2005.8

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

The temperature dependence of the low-energy electronic state for a typical quasi-one-dimensional material K0.3MoO3 has been investigated using high-resolution angle-resolved photoemission spectroscopy. We observed well-defined open quasi-one-dimensional Fermi surfaces (FSs) throughout the entire Brillouin zone. The warping of the FSs arising from the finite interchain transfer is also clearly discernible; this seems to be reduced at low temperature. It is indicative of the variation of the nesting condition, from imperfect to nearly perfect, on decreasing the temperature across the Peierls transition.

DOI： 10.1088/0953-8984/17/32/007

Carbon-substitution dependent multiple superconducting gap of MgB2: A sub-meV resolution photoemission study Reviewed

S Tsuda, T Yokoya, T Kiss, T Shimojima, S Shin, T Togashi, S Watanabe, C Zhang, CT Chen, S Lee, H Uchiyama, S Tajima, N Nakai, K Machida

PHYSICAL REVIEW B 72 ( 6 ) 2005.8

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

"Sub-meV" resolution photoemission spectroscopy was used to study carbon-substitution dependence of the multiple superconducting gap of Mg(B1-xCx)(2). Two features corresponding to sigma and pi gaps are clearly observed in the raw spectra up to carbon concentration x=0.075. The observed x dependence of the two gaps shows a qualitatively different behavior: the sigma gap is proportional to T-c while the pi gap shows negligible change. Doping as well as temperature dependence can be explained within the two-band mean-field theory. Implications from the present study are discussed.

DOI： 10.1103/PhysRevB.72.064527

Hard-X-ray photoelectron spectroscopy of NaxCoO2 center dot yH(2)O Reviewed

A Chainani, T Yokoya, Y Takata, K Tamasaku, M Taguchi, T Shimojima, N Kamakura, K Horiba, S Tsuda, S Shin, D Miwa, Y Nishino, T Ishikawa, M Yabashi, K Kobayashi, H Namatame, M Taniguchi, K Takada, T Sasaki, H Sakurai, E Takayama-Muromachi

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT 547 ( 1 ) 163 - 168 2005.7

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We study the bulk electronic structure of Na(x)CoO(2 center dot)yH(2)O using Hard X-ray (HX, hv = 5.95KeV) synchrotron photoelectron spectroscopy (PES). The Co 2p core level spectra show well-separated Co3+ and Co4+ ions. Cluster calculations suggest low spin Co3+ and Co4+ character, and a moderate on-site Coulomb correlation energy U-dd similar to 3-5.5 eV. Photon-dependent valence band PES identifies Co 3d and 0 2p derived states, in near agreement with band structure calculations. We discuss the importance of HX-PES for studying correlated transition metal oxides. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nima.2005.05.021

Contribution of electronic structure to thermoelectric power in (Bi,Pb)(2)(Sr,La)(2)CuO6+delta Reviewed

T Kondo, T Takeuchi, U Mizutani, T Yokoya, S Tsuda, S Shin

PHYSICAL REVIEW B 72 ( 2 ) 2005.7

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

The energy-momentum (epsilon-k) dispersion and the shape of the Fermi surface in the (Bi,Pb)(2)(Sr,La)(2)CuO6+delta (Bi2201) superconductors with various hole concentrations were determined by the high resolution angle-resolved photoemission spectroscopy. On the basis of the epsilon-k dispersion thus obtained, temperature and hole-concentration dependence of the thermoelectric power S(T) was calculated within the framework of the Boltzmann transport theory. S(T) of the optimally and overdoped samples was quantitatively reproduced by the present calculation, strongly indicating that characteristics in S(T) of the high-T-c cuprates under the optimally to overdoped conditions is unambiguously caused by the band structure characterized by the van Hove singularity near the Fermi level.

DOI： 10.1103/PhysRevB.72.024533

Hole-concentration dependence of electrical resistivity in the (Bi, Pb)(2)(Sr, La)(2)CuO6+delta quantitative evaluation with angle-resolved photoemission spectroscopy Reviewed

T Kondo, T Takeuchi, T Yokoya, S Tsuda, S Shin, U Mizutani

JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 144 1249 - 1252 2005.6

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Relaxation time of Bloch states at the Fermi level (4), Fermi velocity (110, and mean free path (IF) of the conduction electrons in the (Bi, Pb)(2)(Sr, La)(2)CuO6+delta (Bi2201) superconductors with various hole-concentration (p) were determined by the angle-resolved photoemission spectroscopy (ARPES) measurement with high energy- and momentum-resolutions; Delta E = 10 meV and Delta k = 0.005 angstrom(-1). We found that tau(F) at k(F) along (0, 0)-(pi, pi) cut is kept constant over a wide Hole-concentration range while it in the vicinity of (pi, 0) is drastically shortened with decreasing p and eventually reaches nearly zero in the under-doped sample with the transition temperature of 27 K. By calculating electrical resistivity and comparing it with the measured one, we confirmed that tau(F), v(F), and l(F) determined by the ARPES measurement can be used for the quantitative estimation of the electrical transport properties, such as the electrical resistivity, in high-T-c cuprates. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2005.01.198

Ultrahigh-resolution photoemission spectroscopy of superconductors using a VUV laser Reviewed

T Kiss, T Shimojima, F Kanetaka, K Kanai, T Yokoya, S Shin, Y Onuki, T Togashi, CQ Zhang, CT Chen, S Watanabe

JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 144 953 - 956 2005.6

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We have developed an ultrahigh-resolution photoemission system using a quasi continuous wave (quasi-CW) VUV laser. The photon energy of the VUV laser is 6.994 eV. It has a line width of 260 mu eV and a repetition rate of 80 MHz. The photon energy of this laser is the highest for a quasi-CW laser. Using a quasi-CW laser removes the space charge effect and hence does not degrade the energy resolution. For the electron analyser, we use the newly designed ultrahigh-resolution photoelectron analyser GAMMADATA-SCIENTA R4000. The analyser resolution is about 250 mu eV with a wide collecting angle of about 38 degrees. The total energy resolution of this system is measured to be 360 mu eV for the Fermi edge of gold. Using this system, we have measured several low transition temperature superconductors and succeeded to observe the bulk superconducting state of these materials. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2005.01.154

Role of charge-density-wave fluctuations on the spectral function in a metallic charge-density-wave system Reviewed

T Yokoya, T Kiss, A Chainani, S Shin, K Yamaya

PHYSICAL REVIEW B 71 ( 14 ) 2005.4

Language：English Publishing type：Research paper (scientific journal) Publisher：AMERICAN PHYSICAL SOC

Angle-resolved photoemission spectroscopy of ZrTe3 has been performed from room temperature (T) down to 6 K (across T-CDW=63 K) to study the charge-density-wave (CDW) fluctuation effects in a metallic CDW compound having Fermi surface (FS) sheets with differing dimensionality. While spectra on the three-dimensional (3D) FS show typical Fermi-Dirac functionlike T dependence, those along the quasi-one-dimensional (1D) FS show formation of a pseudogap, starting at a much higher T than T-CDW. Simultaneously, a van-Hove singularity consisting of the quasi-1D FS intersecting the 3D FS shows an increase of coherence. This demonstrates the role of CDW fluctuations on the spectral function, and relation to the dimensionality of the states, in a metallic CDW system.

DOI： 10.1103/PhysRevB.71.140504

Bulk screening in core-level photoemission from Mott-Hubbard and charge-transfer systems Reviewed

M Taguchi, A Chainani, N Kamakura, K Horiba, Y Takata, M Yabashi, K Tamasaku, Y Nishino, D Miwa, T Ishikawa, S Shin, E Ikenaga, T Yokoya, K Kobayashi, T Mochiku, K Hirata, K Motoya

PHYSICAL REVIEW B 71 ( 15 ) 2005.4

Language：English Publishing type：Research paper (scientific journal) Publisher：AMERICAN PHYSICAL SOC

We report bulk-sensitive hard x-ray (h nu=5.95 keV) core-level photoemission spectroscopy (PES) of single crystal V1.98Cr0.02O3 and the high-T-c cuprate Bi2Sr2CaCu2O8+delta (Bi2212). V1.98Cr0.02O3 exhibits low binding energy "satellites" to the V 2p "main lines" in the metallic phase, which are suppressed in the antiferromagnetic insulator phase. In contrast, the Cu 2p spectra of Bi2212 do not show temperature-dependent features, but a comparison with soft x-ray PES indicates a large increase in the 2p(5)3d(9) satellites or 3d(9) weight in the bulk. Cluster model calculations, including a full multiplet structure and a screening channel derived from the coherent band at the Fermi energy, give very satisfactory agreement with the experiments.

DOI： 10.1103/PhysRevB.71.155102

Absence of strong correlation in Li 2Pd 3B Reviewed

T. Yokoya, T. Muro, I. Hase, H. Takeya, K. Hirata, K. Togano

Physical Review B - Condensed Matter and Materials Physics 71 ( 9 ) 2005.3

Language：English Publishing type：Research paper (scientific journal)

We use x-ray photoemission spectroscopy to study the electronic structure of new superconductor Li 2Pd 3B. Binding energies of the Pd 3p and 3d indicate valency that is intermediate between Pd metal (0±) and PdO(2+), or effectively Pd 4d 9 configuration. Comparison of satellite features in core level spectra of Li 2Pd 3B with that of PdO suggests that a larger hybridization strength of Li 2Pd 3B than that of PdO. The valence band spectrum is in excellent agreement with first principle band structure calculations, allowing us to ascribe the states at the Fermi level responsible for the superconductivity to be mainly Pd 4d hybridized with B 2p and Li 2p. These observations indicate that correlation effects play a negligible role for the physical properties of the new superconductor. ©2005 The American Physical Society.

DOI： 10.1103/PhysRevB.71.092507

Absence of strong correlation in Li2Pd3B Reviewed

T Yokoya, T Muro, Hase, I, H Takeya, K Hirata, K Togano

PHYSICAL REVIEW B 71 ( 9 ) 2005.3

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We use x-ray photoemission spectroscopy to study the electronic structure of new superconductor Li2Pd3B. Binding energies of the Pd 3p and 3d indicate valency that is intermediate between Pd metal (0) and PdO(2+), or effectively Pd 4d(9) configuration. Comparison of satellite features in core level spectra of Li2Pd3B with that of PdO suggests that a larger hybridization strength of Li2Pd3B than that of PdO. The valence band spectrum is in excellent agreement with first principle band structure calculations, allowing us to ascribe the states at the Fermi ,level responsible for the superconductivity to be mainly Pd 4d hybridized with B 2p and Li 2p. These observations indicate that correlation effects play a negligible role for the physical properties of the new superconductor.

DOI： 10.1103/PhysRevB.71.092507

Photoemission spectroscopic evidence of gap anisotropy in an f-electron superconductor Reviewed

T Kiss, F Kanetaka, T Yokoya, T Shimojima, K Kanai, S Shin, Y Onuki, T Togashi, C Zhang, CT Chen, S Watanabe

PHYSICAL REVIEW LETTERS 94 ( 5 ) 2005.2

Language：English Publishing type：Research paper (scientific journal) Publisher：AMERICAN PHYSICAL SOC

We used low-temperature ultrahigh-resolution (360 mueV) photoemission spectroscopy with a laser as a photon source (Laser-PES) to study the superconducting (SC) gap of an f-electron superconductor CeRu2. The unique combination of the large escape depth expected from the known universal behavior and extremely high-energy resolution has enabled us to directly measure the bulk SC gap of an f-electron superconductor for the first time. The present study provides direct evidence for an anisotropic SC gap in CeRu2, and also demonstrates the potential of Laser-PES in investigating unconventional superconductivity realized in correlated d- and f-electron superconductors.

DOI： 10.1103/PhysRevLett.94.057001

Laser-excited ultrahigh-resolution photoemission spectroscopy of NaxCoO2 center dot yH(2)O: Evidence for pseudogap formation Reviewed

T Shimojima, T Yokoya, T Kiss, A Chainani, S Shin, T Togashi, S Watanabe, C Zhang, CT Chen, K Takada, T Sasaki, H Sakurai, E Takayama-Muromachi

PHYSICAL REVIEW B 71 ( 2 ) 2005.1

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We have studied the temperature (T)-dependent electronic structure near the Fermi level of the layered cobaltate superconductor, Na0.35CoO2.1.3H(2)O, and related materials, using laser-excited ultrahigh-resolution photoemission spectroscopy. We observe the formation of a pseudogap (PG) with an energy scale of similar to20 meV in Na0.35CoO2.1.3H(2)O and Na0.35CoO2.0.7H(2)O, which is clearly absent in Na0.7CoO2. The energy scale of the PG is larger than the expected value for the superconducting gap, suggesting an additional competing order parameter at low T. We discuss implications of the PG in relation to available transport and magnetic susceptibility results.

DOI： 10.1103/PhysRevB.71.020505

Laser-excited ultrahigh-resolution photoemission spectroscopy of Na xCoO 2·yH 2O: Evidence for pseudogap formation Reviewed

T. Shimojima, T. Yokoya, T. Kiss, A. Chainani, S. Shin, T. Togashi, S. Watanabe, C. Zhang, C. T. Chen, K. Takada, T. Sasaki, H. Sakurai, E. Takayama-Muromachi

Physical Review B - Condensed Matter and Materials Physics 71 ( 2 ) 2005.1

Language：English Publishing type：Research paper (scientific journal)

We have studied the temperature (T)-dependent electronic structure near the Fermi level of the layered cobaltate superconductor, Na 0.35CoO 2·1.3H 2O, and related materials, using laser-excited ultrahigh-resolution photoemission spectroscopy. We observe the formation of a pseudogap (PG) with an energy scale of ∼20 meV in Na 0.35CoO 2·1.3H 2O and Na 0.35CoO 2·0.7H 2O, which is clearly absent in Na 0.7CoO 2. The energy scale of the PG is larger than the expected value for the superconducting gap, suggesting an additional competing order parameter at low T. We discuss implications of the PG in relation to available transport and magnetic susceptibility results. ©2005 The American Physical Society.

DOI： 10.1103/PhysRevB.71.020505

Electronic structure changes across the valence transition in EuNi <inf>2</inf>(Si<inf>0.2</inf>Ge<inf>0.8</inf>)<inf>2</inf>

P. A. Rayjada, P. A. Rayjada, A. Chainani, A. Chainani, K. Kanai, K. Kanai, T. Haruna, T. Yokoya, S. Shin, S. Shin, H. Wada, M. Shiga

Physical Review B - Condensed Matter and Materials Physics 70 1 - 7 2004.12

We study the electronic structure of EuNi2(Si 0.2Ge0.8)2, which exhibits a temperature dependent mixed valence transition, using 4d-4f resonant photoemission spectroscopy (RESPES), x-ray absorption spectroscopy (XAS) and temperature-dependent ultraviolet photoemission spectroscopy (UPS). The RESPES studies identify the divalent and trivalent Eu 4f character density of states (DOS) which participate in the valence transition. Using the photoionization cross section variation as a function of photon energy, we discuss the Eu, Ni, and Ge-Si partial DOS in the valence band. The bulk divalent Eu 4f character states are centered at a binding energy of about 0.75 eV, significantly away from the Fermi level. While the surface divalent feature is negligibly affected, the spectra obtained using He IIα UPS exhibit temperature dependent bulk Eu 4f character states. The bulk divalent spectral weight is transferred to the high energy trivalent states, across the valence transition temperature, T υ ∼ 80 K. The He Iα UPS also exhibit spectral intensity changes across Tυ. The non-f character conduction band states at and near the Fermi level exhibit spectral weight changes up to 350 meV with a small energy (∼25 meV) temperature dependent pseudogaplike feature. The results suggest an increase in effective hybridization strength between the conduction and 4f electrons in the low temperature nearly trivalent phase. While the 4f character changes across Tυ are qualitatively consistent with change in valence configurations, the temperature dependent spectral changes in the non-f character DOS indicate direct participation in the valence transition in EuNi2(Si 0.2Ge0.8)2.

DOI： 10.1103/PhysRevB.70.235105

Electronic structure changes across the valence transition in EuNi2(Si0.2Ge0.8)(2) Reviewed

PA Rayjada, A Chainani, K Kanai, T Haruna, T Yokoya, S Shin, H Wada, M Shiga

PHYSICAL REVIEW B 70 ( 23 ) 2004.12

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We study the electronic structure of EuNi2(Si0.2Ge0.8)(2), which exhibits a temperature dependent mixed valence transition, using 4d-4f resonant photoemission spectroscopy (RESPES), x-ray absorption spectroscopy (XAS) and temperature-dependent ultraviolet photoemission spectroscopy (UPS). The RESPES studies identify the divalent and trivalent Eu 4f character density of states (DOS) which participate in the valence transition. Using the photoionization cross section variation as a function of photon energy, we discuss the Eu, Ni, and Ge-Si partial DOS in the valence band. The bulk divalent Eu 4f character states are centered at a binding energy of about 0.75 eV, significantly away from the Fermi level. While the surface divalent feature is negligibly affected, the spectra obtained using He IIalpha UPS exhibit temperature dependent bulk Eu 4f character states. The bulk divalent spectral weight is transferred to the high energy trivalent states, across the valence transition temperature, T(v)similar to80 K. The He Ialpha UPS also exhibit spectral intensity changes across T-v. The non-f character conduction band states at and near the Fermi level exhibit spectral weight changes up to 350 meV with a small energy (similar to25 meV) temperature dependent pseudogaplike feature. The results suggest an increase in effective hybridization strength between the conduction and 4f electrons in the low temperature nearly trivalent phase. While the 4f character changes across T-v are qualitatively consistent with change in valence configurations, the temperature dependent spectral changes in the non-f character DOS indicate direct participation in the valence transition in EuNi2(Si0.2Ge0.8)(2).

DOI： 10.1103/PhysRevB.70.235105

Valence-band photoemission study of beta-ZrNCl and the quasi-two-dimensional superconductor NaxZrNCl Reviewed

T Yokoya, T Takeuchi, S Tsuda, T Kiss, T Higuchi, S Shin, K Iizawa, S Shamoto, T Kajitani, T Takahashi

PHYSICAL REVIEW B 70 ( 19 ) 2004.11

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We have performed photoemission (PE) spectroscopy of beta-ZrNCl and NaxZrNCl to study intercalation-induced change in the electronic structure. The valence band PE spectrum of beta-ZrNCl shows a small intensity region within similar to2.5 eV of E-F with a very small hump at E-F that can be ascribed to hydrogen impurity states. These provide spectroscopic evidence that beta-ZrNCl is an n-type semiconductor. Upon Na intercalation, new states possibly involving hybridized Zr4d and N2p systematically evolve at E-F, with a negligible shift in the valence band structures. Similar results in beta-HfNCl and NaxHfNCl indicate that these results represent common electronic features of the layered nitride chloride superconductors.

DOI： 10.1103/PhysRevB.70.193103

Valence-band photoemission study of β-ZrNCl and the quasi-two-dimensional superconductor NaxZrNCl Reviewed

T. Yokoya, T. Takeuchi, S. Tsuda, T. Kiss, T. Higuchi, S. Shin, K. Iizawa, S. Shamoto, T. Kajitani, T. Takahashi

Physical Review B - Condensed Matter and Materials Physics 70 ( 19 ) 1 - 4 2004.11

Language：English Publishing type：Research paper (scientific journal)

We have performed photoemission (PE) spectroscopy of β-ZrNCl and NaxZrNCl to study intercalation-induced change in the electronic structure. The valence band PE spectrum of β-ZrNCl shows a small intensity region within ∼2.5 eV of EF with a very small hump at E F that can be ascribed to hydrogen impurity states. These provide spectroscopic evidence that β-ZrNCl is an n-type semiconductor. Upon Na intercalation, new states possibly involving hybridized Zr4d and N2p systematically evolve at EF, with a negligible shift in the valence band structures. Similar results in β-HfNCl and NaxHfNCl indicate that these results represent common electronic features of the layered nitride chloride superconductors.

DOI： 10.1103/PhysRevB.70.193103

Electronic structure and superconducting gap of silicon clathrate Ba8Si46 studied with ultrahigh-resolution photoemission spectroscopy (vol B 64, art no 172504, 2001) Reviewed

T Yokoya, A Fukushima, T Kiss, K Kobayashi, S Shin, K Moriguchi, A Shintani, H Fukuoka, S Yamanaka

PHYSICAL REVIEW B 70 ( 15 ) 2004.10

The Fermi surface and sheet-dependent superconducting gap of MgB2 Reviewed

S Tsuda, T Yokoya, S Shin, Y Takano, H Kito, A Matsushita, F Yin, J Itoh, H Harima

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 412 36 - 40 2004.10

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

The Fermi surface and superconducting gap of MgB2 have been studied by angle-resolved photoemission spectroscopy. Two Fermi surfaces composed of the a band and the pi band are clearly observed without contaminated by the "surface-derived" band. The high-resolution photoemission spectra show that superconducting gaps with values of 5.5 and 2.2 meV open on the sigma band and the pi band, respectively, both of which close at the bulk transition temperature (T-c). This result provides an experimental evidence for the two-band superconductivity on MgB2 and, moreover, validates the role of momentum-dependent electron-phonon coupling as the origin of multiple-gap superconductivity as well as the high transition temperature (T-c) of MgB2. (C) 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2004.01.037

Electronic structure and large thermoelectric power in Ca3Co4O9 Reviewed

T Takeuchi, T Kondo, K Soda, U Mizutani, R Funahashi, M Shikano, S Tsuda, T Yokoya, S Shin, T Muro

JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 137 595 - 599 2004.7

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Resonant photoemission spectroscopy, soft X-ray emission spectroscopy, soft X-ray absorption spectroscopy, and high-resolution (DeltaE approximate to 10 meV) ultraviolet photoemission spectroscopy were performed on a layered cobalt oxide, Ca3Co4O9, which has attracted interests as one of the potential thermoelectric materials because of its possession of high thermoelectric power S, relatively low electrical resistivity rho, and small thermal conductivity kappa. A narrow band of 1.5-2 eV in width was observed in the photoemission spectra with its center at 1.0 eV below the Fermi level (E-F). This peak turns out to be less significant when Co 2p-3d resonance takes place, indicating that it consists mainly of O 2p and of small amount of Co 3d component. Since E-F is located near the high-energy edge of this narrow band, the density of states at E-F is finite but negligibly small at room temperature. An energy-gap across E-F opens below 50 K with decreasing temperature. This development of the energy-gap causes the insulating behavior; divergence both in electrical resistivity and Hall coefficient. We calculated thermoelectric power S(T) using the photoemission spectra near E-F. The calculated S(T) shows almost half of the measured value. Large thermoelectric power up to 200 muV/K observed for the Ca3CO4O9 is closely related to the metallic electron transport in a less dispersive band with EF near its band-edge. (C) 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2004.02.013

Hole-concentration dependence of band structure in (Bi,Pb)(2)(Sr,La)(2)CUO6+delta determined by the angle-resolved photoemission spectroscopy Reviewed

T Kondo, T Takeuchi, T Yokoya, S Tsuda, S Shin, U Mizutani

JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 137 663 - 668 2004.7

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Energy-momentum (E-k) dispersion and the shape of the Fermi surface (FS) in the (Bi,Pb)(2)(Sr,La)(2)CuO6+delta (Bi2201) superconductors of various hole-concentrations were investigated by the angle-resolved photoemission spectroscopy (ARPES) with high energy and momentum resolutions (DeltaE = 10 meV and Deltak = 0.005 Angstrom(-1)). The E-k dispersion is kept rigid and the chemical potential moves towards lower or higher energies with increasing or decreasing hole-concentration, respectively, over a wide hole-concentration range from the nearly optimal-doped condition with a transition temperature of 32 K to a heavily over-doped condition where the superconducting transition disappears. By comparing the shape of the FS determined for the present Bi2201 with that reported for the Bi2Sr2CaCu2O8+delta (Bi2212), we found that the area surrounded by the FS in the Bi2201 is fairly larger than that in the Bi2212. Estimated hole-concentration in the Bi2201 was found to vary from 0.25 to 0.43 holes/Cu, that is more than twice as large as that in the optimally doped La(2-)xSrCuO(4) (LSCO) and Bi2212 superconductors of 0.17 holes/Cu. (C)2004 Elsevier B.V. All fights reserved.

DOI： 10.1016/j.elspec.2004.02.104

Advantage of thin-film filter for reliable photoemission spectroscopy using high-flux discharging lamp Reviewed

T Yokoya, S Tsuda, T Kiss, S Shin, T Mochiku, K Hirata

JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 43 ( 6A ) 3618 - 3619 2004.6

Language：English Publishing type：Research paper (scientific journal) Publisher：INST PURE APPLIED PHYSICS

An aluminum film with a thickness of 1500Angstrom has been used as a filter for the He Ialpha resonance line (21.2182eV) from a microwave-driven high-flux discharging lamp to reduce the degradation of sample surfaces during photoemission spectroscopy (PES) measurements. A marked increase in the lifetime of sample surfaces, which overcomes a similar to90% intensity reduction, has been observed. The thin-film filter, if combined with a high-flux discharging lamp, provides clean vacuum ultraviolet lights for reliable PES measurements with an ultrahigh resolution.

DOI： 10.1143/JJAP.43.3618

Bulk electronic structure of Na<inf>0.35</inf>CoO<inf>2</inf>·1. 3H<inf>2</inf>O Reviewed

A. Chainani, T. Yokoya, Y. Takata, K. Tamasaku, M. Taguchi, T. Shimojima, N. Kamakura, K. Horiba, S. Tsuda, S. Shin, S. Shin, D. Miwa, Y. Nishino, T. Ishikawa, M. Yabashi, K. Kobayashi, H. Namatame, M. Taniguchi, K. Takada, K. Takada, T. Sasaki, T. Sasaki, H. Sakurai, E. Takayama-Muromachi

Physical Review B - Condensed Matter and Materials Physics 69 ( 18 ) 2004.5

Publishing type：Research paper (scientific journal)

High-energy (hv = 5.95 keV) synchrotron photoemission spectroscopy (PES) is used to study bulk electronic structure of Na0.35CoO2· 1.3H2O, the layered superconductor. In contrast to three-dimensional doped Co oxides, Co 2p core level spectra show well-separated Co3+and Co4+ions. Cluster calculations suggest low-spin Co3+and Co4+character, and a moderate on-site Coulomb correlation energy Udd∼3-5.5 eV. Photon dependent valence band PES identifies Co 3d and O 2p derived states, in near agreement with band-structure calculations.

DOI： 10.1103/PhysRevB.69.180508

Other Link： http://orcid.org/0000-0002-1124-0451

Bulk electronic structure of Na0.35CoO2 center dot 1.3H(2)O Reviewed

A Chainani, T Yokoya, Y Takata, K Tamasaku, M Taguchi, T Shimojima, N Kamakura, K Horiba, S Tsuda, S Shin, D Miwa, Y Nishino, T Ishikawa, M Yabashi, K Kobayashi, H Namatame, M Taniguchi, K Takada, T Sasaki, H Sakurai, E Takayama-Muromachi

PHYSICAL REVIEW B 69 ( 18 ) 2004.5

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

High-energy (hnu=5.95 keV) synchrotron photoemission spectroscopy (PES) is used to study bulk electronic structure of Na0.35CoO2.1.3H(2)O, the layered superconductor. In contrast to three-dimensional doped Co oxides, Co 2p core level spectra show well-separated Co3+ and Co4+ ions. Cluster calculations suggest low-spin Co3+ and Co4+ character, and a moderate on-site Coulomb correlation energy U(dd)similar to3-5.5 eV. Photon dependent valence band PES identifies Co 3d and O 2p derived states, in near agreement with band-structure calculations.

DOI： 10.1103/PhysRevB.69.180508

Bulk electronic structure of Na0.35CoO2 center dot 1.3H(2)O Reviewed

Chainani, A, Yokoya, T, Takata, Y, Tamasaku, K, Taguchi, M, Shimojima, T, Kamakura, N, Horiba, K, Tsuda, S, Shin, S, Miwa, D, Nishino, Y, Ishikawa, T, Yabashi, M, Kobayashi, K, Namatame, H, Taniguchi, M, Takada, K, Sasaki, T, Sakurai, H, Takayama-Muromachi, E

Physical Review B 69 ( 18 ) 180508 2004.5

Experimental evidence for the p-d hybridization in the Cd-Ca quasicrystal: Origin of the pseudogap Reviewed

R Tamura, T Takeuchi, C Aoki, S Takeuchi, T Kiss, T Yokoya, S Shin

PHYSICAL REVIEW LETTERS 92 ( 14 ) 2004.4

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

Ca 2p-3d resonant photoemission spectroscopy of a Cd6Ca crystalline approximant unambiguously demonstrates that the low-lying unoccupied 3d levels of calcium are lowered below the Fermi energy by the formation of the approximant, as suggested from electronic structure calculations [Y Ishii and T Fujiwara, Phys. Rev. Lett. 87, 206408 (2001)]. Moreover, the Ca 3d partial density of states (DOS) obtained near the Fermi energy is in reasonable agreement with theoretical Ca 3d DOS. These results verify the unconventional Picture that the origin of the pseudogap in the Cd-based quasicrystals is due to hybridization of the Ca 3d band with the Cd 5p band.

DOI： 10.1103/PhysRevLett.92.146402

Contribution of electronic structure to the large thermoelectric power in layered cobalt oxides Reviewed

Tsunehiro Takeuchi, Takeshi Kondo, Tsuyoshi Takami, Hirofumi Takahashi, Hiroshi Ikuta, Uichiro Mizutani, Kazuo Soda, Ryoji Funahashi, Masahiro Shikano, Masashi Mikami, Syunsuke Tsuda, Takayoshi Yokoya, Shik Shin, Takayuki Muro

Physical Review B - Condensed Matter and Materials Physics 69 ( 12 ) 2004.3

Language：English Publishing type：Research paper (scientific journal)

With a strong help of high-resolution photoemission spectroscopy we demonstrate in this paper that the large thermoelectric power observed in the layered cobalt oxides, such as Ca3Co4O9, Na0.6CoO2, and Bi2Sr2Co2O9, can be well accounted for with the Boltzmann-type metallic electrical conduction. An intense peak with 1.5-2 eV in width was observed in the photoemission spectra with its center at 1.0 eV below the Fermi level EF in these compounds. The density of states at EF is finite but negligibly small at room temperature, because EF is located near the high-energy edge of this narrow band. We calculated thermoelectric power S using the Boltzmann transport equation with the electronic structure near EF determined by the photoemission measurement. The calculated S shows fairly good consistency with the measured value both in its magnitude and the temperature dependence. © 2004 The American Physical Society.

DOI： 10.1103/PhysRevB.69.125410

Identical superconducting gap on different Fermi surfaces of Ca(Al0.5Si0.5)(2) with the AlB2 structure Reviewed

S Tsuda, T Yokoya, S Shin, M Imai, Hase, I

PHYSICAL REVIEW B 69 ( 10 ) 2004.3

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

Angle-resolved photoemission spectroscopy of Ca(Al0.5Si0.5)(2) (CaAlSi), which is a superconductor (transition temperature is 7.7 K) with the AlB2 structure, revealed that superconducting gaps on two Fermi surfaces (FSs) with three-dimensional character around Gamma(A) and M(L) in the Brillouin zone provide essentially the same superconducting gap value (similar to1.2 meV+/-0.2 meV). This is in contrast to the case of MgB2, in which different FSs exhibit different gap values. The reduced gap value 2Delta(0)/k(B)T(c) of similar to4.2+/-0.2 classifies CaAlSi as a moderately strong-coupling superconductor.

DOI： 10.1103/PhysRevB.69.100506

Contribution of electronic structure to the large thermoelectric power in layered cobalt oxides Reviewed

T Takeuchi, T Kondo, T Takami, H Takahashi, H Ikuta, U Mizutani, K Soda, R Funahashi, M Shikano, M Mikami, S Tsuda, T Yokoya, S Shin, T Muro

PHYSICAL REVIEW B 69 ( 12 ) 2004.3

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

With a strong help of high-resolution photoemission spectroscopy we demonstrate in this paper that the large thermoelectric power observed in the layered cobalt oxides, such as Ca3Co4O9-, Na0.6CoO2, and Bi2Sr2Co2O9, can be well accounted for with the Boltzmann-type metallic electrical conduction. An intense peak with 1.5-2 eV in width was observed in the photoemission spectra with its center at 1.0 eV below the Fermi level E-F in these compounds. The density of states at E-F is finite but negligibly small at room temperature, because EF IS located near the high-energy edge of this narrow band. We calculated thermoelectric power S using the Boltzmann transport equation with the electronic structure near EF determined by the photoemission measurement. The calculated S shows fairly good consistency with the measured value both in its magnitude and the temperature dependence.

DOI： 10.1103/PhysRevB.69.125410

Erratum: Electronic structure and superconducting gap of silicon clathrate Ba<inf>8</inf>Si<inf>46</inf> studied with ultrahigh-resolution photoemission spectroscopy (Physical Review B - Condensed Matter and Materials Physics (2001) 64 (172504)) Reviewed

Yokoya, T., Fukushima, A., Kiss, T., Kobayashi, K., Shin, S., Moriguchi, K., Shintani, A., Fukuoka, H., Yamanaka, S.

Physical Review B - Condensed Matter and Materials Physics 70 ( 15 ) 2004

Comparative study of the electronic structure of MgB2 and ZrB2 Reviewed

C. Jariwala, A. Chainani, S. Tsuda, T. Yokoya, S. Shin, Y. Takano, K. Togano, S. Otani, H. Kito

Physical Review B - Condensed Matter and Materials Physics 68 ( 17 ) 2003.11

Language：English Publishing type：Research paper (scientific journal)

X-ray photoemission spectroscopy is used to compare the electronic structures of MgB2 and ZrB2. The B 1s core levels in high-quality MgB2 and ZrB2 exhibit a single asymmetric peak typical of a metallic boride. The Mg 2p core level shows a single peak with negligible intensity in charge-transfer satellites and no correlation effects. The Mg 2p and B 1s core-level spectra exhibit a broad bulk plasmon feature centered at about 22 eV from the main peak, in good accord with calculations. The measured valence bands are consistent with band-structure calculations indicating a higher density of states (DOS) at EF for MgB2 compared to ZrB2. The high Tc in MgB2 is due to p-derived DOS, while ZrB2 is dominated by d-derived states at EF. © 2003 The American Physical Society.

DOI： 10.1103/PhysRevB.68.174506

Comparative study of the electronic structure of MgB2 and ZrB2 Reviewed

C Jariwala, A Chainani, S Tsuda, T Yokoya, S Shin, Y Takano, K Togano, S Otani, H Kito

PHYSICAL REVIEW B 68 ( 17 ) 2003.11

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

X-ray photoemission spectroscopy is used to compare the electronic structures of MgB2 and ZrB2. The B 1s core levels in high-quality MgB2 and ZrB2 exhibit a single asymmetric peak typical of a metallic boride. The Mg 2p core level shows a single peak with negligible intensity in charge-transfer satellites and no correlation effects. The Mg 2p and B 1s core-level spectra exhibit a broad bulk plasmon feature centered at about 22 eV from the main peak, in good accord with calculations. The measured valence bands are consistent with band-structure calculations indicating a higher density of states (DOS) at E-F for MgB2 compared to ZrB2. The high T-c in MgB2 is due to p-derived DOS, while ZrB2 is dominated by d-derived states at E-F.

DOI： 10.1103/PhysRevB.68.174506

Soft X-ray emission and high-resolution photoemission study of quasi-two-dimensional superconductor NaxHfNCl Reviewed

T Takeuchi, S Tsuda, T Yokoya, T Tsukamoto, S Shin, A Hirai, S Shamoto, T Kajitani

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 392 127 - 129 2003.10

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We have performed soft X-ray emission (SXE) and high-resolution photoemission (PE) spectroscopy to study the electronic structures of beta-HfNCl and Na-intercalated HfNCl. Both the PE and SXE spectra of Na0.37HfNCl show an additional structure within the band gap of beta-HfNCl, without an apparent shift in the energy position of valence-band features. These results clearly reveal intercalant-induced changes in the electronic structure of the layered nitride chlorides. From these results, we propose non-rigid band like behavior for this system in contrast to generally expected rigid-band filling upon intercalation. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/S0921-4534(03)00752-4

X-ray absorption and resonant photoemission spectroscopy of ZrB2 Reviewed

S Tsuda, R Eguchi, A Kosuge, T Yokoya, A Fukushima, S Shin, A Chainaini, S Otani, Y Takano, K Togano, H Kito

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 392 259 - 262 2003.10

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We have performed X-ray absorption and valence band photoemission spectroscopy on ZrB2, which has the same AlB2 structure of MgB2, to study the electronic structure. From X-ray absorption spectra near the B 1s-2p threshold, we found a small absorption edge at 188.23 eV with a negligible angular dependence upto 190 eV photon energy. This indicates that B 2p partial density of states near the Fermi level (E-F) is three dimensional, in sharp contrast to that of MgB2. Resonant photoemission study across the B 1s-2p threshold was found to be difficult due to a large contribution of Auger electrons. On the other hand, resonant photoemission across the Zr 3p-4d threshold does show small change in intensity within 4 eV of EF. This indicates that the occupied states near EF have substantial Zr 4d character. These observations are in relatively good agreement with band structure calculations. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/S0921-4534(03)00947-X

Definitive experimental evidence for two-band superconductivity in MgB2 Reviewed

S Tsuda, T Yokoya, Y Takano, H Kito, A Matsushita, F Yin, J Itoh, H Harima, S Shin

PHYSICAL REVIEW LETTERS 91 ( 12 ) 2003.9

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

The superconducting-gap of MgB2 has been studied by high-resolution angle-resolved photoemission spectroscopy. The results show that superconducting gaps with values of 5.5 and 2.2 meV open on the sigma band and the pi band, respectively, but both the gaps close at the bulk transition temperature, providing a definitive experimental evidence for the two-band superconductivity with strong interband pairing interaction in MgB2. The experiments validate the role of k-dependent electron-phonon coupling as the origin of multiple-gap superconductivity as well as the high transition temperature of MgB2.

DOI： 10.1103/PhysRevLett.91.127001

Soft x-ray energy-dependent angle-resolved photoemission study of CeIrIn5 Reviewed

T Ito, PA Rayjada, N Kamakura, Y Takata, T Yokoya, A Chainani, S Shin, M Nohara, H Takagi

JOURNAL OF PHYSICS-CONDENSED MATTER 15 ( 28 ) S2149 - S2152 2003.7

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

Soft x-ray energy-dependent angle-resolved photoemission measurements are performed on the heavy-fermion superconductor CeIrIn5 to study the Fermi surface (FS) and the electronic structure near the Fermi level in the GammaMZA plane. The non-dispersive band designated as the Cc 4f(7/2)(1) final state is found at 280 meV along with the main body of the valence band consisting of highly dispersive In 5p bands, which are in good qualitative agreement with the band structure calculation based on the itinerant 4f electron model. Furthermore, we found two electron-like and one hole-like cylindrical FSs at the M (A) and Gamma (Z) points. The Fermi momenta of the observed FSs are consistent with the de Haas-van Alphen experiment, while they contradict the stronger anisotropy of the FS along the k(z) direction predicted by the band calculation. The results suggest stronger two-dimensionality of the electronic structure of CeIrIn5 and point to the need for its theoretical verification.

DOI： 10.1088/0953-8984/15/28/340

Photoemission results of intermetallic superconductors: Nb3Al and MgB2 Reviewed

T Yokoya, S Tsuda, A Chainani, T Kiss, K Hirata, Y Takano, K Togano, H Kito, H Ihara, S Shin

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 63 ( 12 ) 2141 - 2144 2002.12

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

We study the superconducting electronic structures of Nb3Al and MgB2 using high-resolution spectroscopy. The obtained spectrum of Nb3Al measured below T-c shows clear opening of the superconducting gap with a sharp pile up in the density of states and a shift of the leading edge. In addition, the spectrum shows a peak-dip-hump line shape expected from the strong-coupling theory. On the other hand, for MgB2, the superconducting-state spectrum measured at 5.4 K shows a coherent peak with a shoulder structure, in sharp contrast to that expected from a single isotropic gap. The superconducting spectral shape of MgB2 can be explained in terms of a multicomponent gap. (C) 2002 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(02)00236-6

Ultrahigh resolution photoemission study of CeRu4Sb12 Reviewed

K. Kanai, N. Takeda, S. Nozawa, T. Yokoya, M. Ishikawa, S. Shin

J. Phys. Chem. Solids 63 1175 - 1178 2002.12

Photoemission study of pyrochlore superconductor Cd2Re2O7 Reviewed

R. Eguchi, T. Yokoya, T. Baba, M. Hanawa, Z. Hiroi, N. Kamakura, Y. Takata, H. Harima, S. Shin

Physical Review B - Condensed Matter and Materials Physics 66 ( 1 ) 125161 - 125164 2002.7

Language：English Publishing type：Research paper (scientific journal)

We report photoemission study of the pyrochlore superconductor Cd2Re2O7. The valence-band spectra show the Re 5d dominant band close to the Fermi level (EF), consistent with the recent band calculations. Temperature-dependent study of the Re 5d band reveals a gradual change in spectral shape, as is seen from a gradual modification of difference spectra below 200 K. These results are related to another transition at 200 K reported from resistivity and susceptibility measurements and indicate that the transition involves a change in electronic structures of the Re 5d band near EF.

DOI： 10.1103/PhysRevB.66.012516

Superconducting transitions studied by ultrahigh-resolution photoemission Reviewed

T Yokoya, T Kiss, A Chainani, S Shin

JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 124 ( 2-3 ) 99 - 105 2002.7

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Superconducting transitions of elemental metals and Ni borocarbides are studied using ultrahigh-resolution and low-temperature photoemission spectroscopy. Photoemission spectra of elemental metals (Nb and Pb) show the appearance of a sharp peak just below the Fermi level and a shift of the leading edge, indicating clear opening of the superconducting gap. Furthermore, Pb spectrum shows a dip and hump structure beyond the peak, characteristic of the strong-coupling superconductivity. On the other hand, comparative study of Y(Ni1-xPtx)(2)B2C (x=0.0 and 0.2) shows a difference in the slope of the leading edge, which can be explained in terms of a highly anisotropic s-wave gap in YNi2B2C, consistent to magnetic field dependent specific heat measurements. These observations demonstrate that photoemission spectroscopy is a useful and reliable technique to study electronic structures and phase transitions at very small energy scales. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0368-2048(02)00047-6

Photoemission study of pyrochlore superconductor Cd2Re2O7 Reviewed

R Eguchi, T Yokoya, T Baba, M Hanawa, Z Hiroi, N Kamakura, Y Takata, H Harima, S Shin

PHYSICAL REVIEW B 66 ( 1 ) 2002.7

Language：English Publishing type：Research paper (scientific journal) Publisher：AMERICAN PHYSICAL SOC

We report photoemission study of the pyrochlore superconductor Cd2Re2O7. The valence-band spectra show the Re 5d dominant band close to the Fermi level (E-F), consistent with the recent band calculations. Temperature-dependent study of the Re 5d band reveals a gradual change in spectral shape, as is seen from a gradual modification of difference spectra below 200 K. These results are related to another transition at 200 K reported from resistivity and susceptibility measurements and indicate that the transition involves a change in electronic structures of the Re 5d band near E-F.

DOI： 10.1103/PhysRevB.66.012516

High-resolution photoemission spectroscopy for the layered antiferromagnetic (La1-zNdz)(0.46)Sr0.54MnO3 Reviewed

T Takeuchi, H Nozaki, K Soda, T Kondo, U Mizutani, T Yokoya, T Sato, T Takahashi, S Shin, T Muro, Y Saitoh, Y Moritomo

JOURNAL OF SYNCHROTRON RADIATION 9 237 - 241 2002.7

Language：English Publishing type：Research paper (scientific journal) Publisher：BLACKWELL MUNKSGAARD

High-resolution HeI photoemission spectroscopy (UPS), Mn 2p-3d resonant photoemission spectroscopy (RPES) and Mn 2p X-ray absorption spectroscopy (XAS) have been performed to investigate the electronic structure and its effect on the electrical resistivity in (La1-zNdz)(0.46)Sr0.54MnO3 (z = 0, 0,2, 0.6 and 1.0). It was found that in the UPS and RPES spectra the Fermi edge persisted over the temperature range 15 less than or equal to T less than or equal to 340 K regardless of the magnetic structure or the composition of the samples. The density of states at the Fermi level [N(E-F)] in the samples where 0 less than or equal to z less than or equal to 0.6 was increased drastically at the Curie temperature (T-C) with decreasing temperature, but essentially kept unchanged across the Neel temperature (T-N). A fairly large reduction at T-C and a small increase at T-N in the electrical resistivity with decreasing temperature are found to be well accounted for in terms of the temperature dependence of N(E-F). The presence of a finite N(E-F) in the insulating Nd0.46Sr0.54MnO3 was also found. Thus the origin of the insulating behavior in this sample can be regarded as the Anderson localization associated with the small density of states and the chemical disorder between Nd and Sr.

DOI： 10.1107/S0909049502008889

High-resolution photoelectron spectroscopy of Heusler-type Fe2VAl alloy Reviewed

K Soda, T Mizutani, O Yoshimoto, S Yagi, U Mizutani, H Sumi, Y Nishino, Y Yamada, T Yokoya, S Shin, A Sekiyama, S Suga

JOURNAL OF SYNCHROTRON RADIATION 9 233 - 236 2002.7

Language：English Publishing type：Research paper (scientific journal) Publisher：BLACKWELL MUNKSGAARD

The electronic structure of Heusler-type Fe2VAl has been studied by high-resolution photoelectron spectroscopy with the excitation photon energy hnu ranging from 21.2 eV (the He I laboratory light source) to 904 eV (the soft X-ray synchrotron light source) for clean surfaces prepared by scraping or fracturing polycrystalline and single crystalline specimens. Photoelectron spectra recorded for the fractured surfaces show a 10 eV-wide valence band with fine structures and a clear decrease in the intensity towards the Fermi level E-F, while a high intensity at EF and no fine structures are observed for the scraped surface. Comparison with the theoretical density of states (DOS) indicates that the vacuum ultraviolet photoelectron spectra emphasize the transition-metal 3d bands but the soft X-ray photoelectron spectra agree remarkably well with the DOS including the fine structures and the pseudogap at EF. The present results suggest that the electronic structure of Fe2VAl is highly sensitive to possible strain and defects induced by scraping. Bulk electronic structures of Fe2VAl are discussed in relation to the reported fascinating transport properties.

DOI： 10.1107/S0909049502009214

Comparative study of the binary icosahedral quasicrystal Cd5.7Yb and its crystalline approximant Cd6Yb by low-temperature ultrahigh-resolution photoemission spectroscopy Reviewed

Ryuji Tamura, Yohsuke Murao, Shin Takeuchi, Takayuki Kiss, Takayoshi Yokoya, Shik Shin

Physical Review B - Condensed Matter and Materials Physics 65 ( 22 ) 2242071 - 2242075 2002.6

Language：English Publishing type：Research paper (scientific journal)

Low-temperature ultrahigh-resolution photoemission (PE) spectroscopy of the recently discovered binary icosahedral (i) Cd5.7Yb and its 1/1-cubic approximant Cd6Yb has commonly revealed a sharp Fermi edge with a marked spectral intensity decrease towards the Fermi level (EF), indicating the presence of a dip near EF which is consistent with the recent result of electronic structure calculations [Y. Ishii and T. Fujiwara, Phys. Rev. Lett. 87, 206408 (2001)], and the dip at EF is found to be much deeper in i-Cd5.7Yb than in 1/1-Cd6Yb. Moreover, surface core-level shifts (SCS's) are clearly observed in the quasicrystal and the approximant, and the SCS's are found to be substantially different between these two compounds of different long-range orders.

DOI： 10.1103/PhysRevB.65.224207

Comparative study of the binary icosahedral quasicrystal Cd5.7Yb and its crystalline approximant Cd6Yb by low-temperature ultrahigh-resolution photoemission spectroscopy Reviewed

R Tamura, Y Murao, S Takeuchi, T Kiss, T Yokoya, S Shin

PHYSICAL REVIEW B 65 ( 22 ) 2002.6

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

Low-temperature ultrahigh-resolution photoemission (PE) spectroscopy of the recently discovered binary icosahedral (i) Cd5.7Yb and its 1/1-cubic approximant Cd6Yb has commonly revealed a sharp Fermi edge with a marked spectral intensity decrease towards the Fermi level (E-F), indicating the presence of a dip near E-F which is consistent with the recent result of electronic structure calculations [Y. Ishii and T. Fujiwara, Phys. Rev. Lett. 87, 206408 (2001)], and the dip at E-F is found to be much deeper in i-Cd5.7Yb than in 1/1-Cd6Yb. Moreover, surface core-level shifts (SCS's) are clearly observed in the quasicrystal and the approximant, and the SCS's are found to be substantially different between these two compounds of different long-range orders.

DOI： 10.1103/PhysRevB.65.224207

Angle-resolved photoemission study of the mixed valence oxide V6O13: Quasi-one-dimensional electronic structure and its change across the metal-insulator transition

R. Eguchi, T. Yokoya, T. Kiss, Y. Ueda, S. Shin

Physical Review B - Condensed Matter and Materials Physics 65 ( 20 ) 2051241 - 2051244 2002.5

Language：English Publishing type：Research paper (scientific journal)

We have performed angle-resolved photoemission spectroscopy of mixed valence oxide V6O13 that shows a metal-insulator transition (MIT) at ≃ 150 K. In the metallic phase, we observe two bands near the Fermi level (EF). One is a prominent band located around 0.8 eV and the other is a weak structure around 0.2 eV that shows dispersion toward EF only along the b axis. Furthermore, though the momentum distribution curve at EF shows a peak at kb/π=0.29±0.01 indicative of a EF crossing, the intensity of the band near EF is strongly suppressed in the region of EF. These observations indicate quasi-one-dimensional electronic states of V6O13, consistent with a highly anisotropic behavior observed from resistivity and optical conductivity measurements. Across the MIT, the band near EF shifts to higher binding energy and becomes less dispersive, resulting in opening of an energy gap of 0.2 eV. We discuss some implications of the experimental results.

Angle-resolved photoemission study of the mixed valence oxide V6O13: Quasi-one-dimensional electronic structure and its change across the metal-insulator transition Reviewed

R Eguchi, T Yokoya, T Kiss, Y Ueda, S Shin

PHYSICAL REVIEW B 65 ( 20 ) 2002.5

Language：English Publishing type：Research paper (scientific journal) Publisher：AMERICAN PHYSICAL SOC

We have performed angle-resolved photoemission spectroscopy of mixed valence oxide V6O13 that shows a metal-insulator transition (MIT) at similar or equal to150 K. In the metallic phase, we observe two bands near the Fermi level (E-F). One is a prominent band located around 0.8 eV and the other is a weak structure around 0.2 eV that shows dispersion toward E-F only along the b axis. Furthermore, though the momentum distribution curve at E-F shows a peak at kb/pi=0.29+/-0.01 indicative of a E-F crossing, the intensity of the band near E-F is strongly suppressed in the region of E-F. These observations indicate quasi-one-dimensional electronic states of V6O13, consistent with a highly anisotropic behavior observed from resistivity and optical conductivity measurements. Across the MIT, the band near E-F shifts to higher binding energy and becomes less dispersive, resulting in opening of an energy gap of 0.2 eV. We discuss some implications of the experimental results.

DOI： 10.1103/PhysRevB.65.205124

Electronic structure of black SmS. I. 4d-4f resonance and angle-integrated valence-band photoemission spectroscopy Reviewed

A. Chainani, H. Kumigashira, T. Ito, T. Sato, T. Takahashi, T. Yokoya, T. Higuchi, T. Takeuchi, S. Shin, N. K. Sato

Physical Review B - Condensed Matter and Materials Physics 65 ( 15 ) 1552011 - 1552017 2002.4

Language：English Publishing type：Research paper (scientific journal)

We study the electronic structure of single-crystal "black" semiconducting SmS using x-ray absorption and resonance photoemission spectroscopy across the 4d-4f threshold and high-resolution temperature-dependent valence-band photoemission spectroscopy. The 4d-4f on-resonance spectra show mixed valency of Sm2+ and Sm3+ states in semiconducting SmS at low temperature (T = 30 K). The high-resolution spectra show a pseudogap within 20 meV of the Fermi level at low temperatures. This pseudogap is gradually filled up accompanied by a redistribution of spectral weight over a larger energy scale of ∼200 meV on increasing temperature, resulting in an incipient metallic phase at room temperature. The two energy scales can be associated with the coherence temperature Tc and the Kondo temperature TK, respectively. The changes in the single-particle density of states at and about the Fermi level are compatible with recent theoretical results on "exhaustion physics" in the periodic Anderson model. The present study indicates a Kondo lattice picture for SmS and suggests a relation between the mixed valency and the temperature dependence of the pseudogap for "black" semiconducting SmS.

DOI： 10.1103/PhysRevB.65.155201

Electronic structure of black SmS. I. 4d-4f resonance and angle-integrated valence-band photoemission spectroscopy Reviewed

A Chainani, H Kumigashira, T Ito, T Sato, T Takahashi, T Yokoya, T Higuchi, T Takeuchi, S Shin, NK Sato

PHYSICAL REVIEW B 65 ( 15 ) 2002.4

Language：English Publishing type：Research paper (scientific journal) Publisher：AMERICAN PHYSICAL SOC

We study the electronic structure of single-crystal "black" semiconducting SmS using x-ray absorption and resonance photoemission spectroscopy across the 4d-4f threshold and high-resolution temperature-dependent valence-band photoemission spectroscopy. The 4d-4f on-resonance spectra show mixed valency of Sm2+ and Sm3+ states in semiconducting SmS at low temperature (T=30 K). The high-resolution spectra show a pseudogap within 20 meV of the Fermi level at low temperatures. This pseudogap is gradually filled up accompanied by a redistribution of spectral weight over a larger energy scale of similar to200 meV on increasing temperature, resulting in an incipient metallic phase at room temperature. The two energy scales can be associated with the coherence temperature T-c and the Kondo temperature T-K, respectively. The changes in the single-particle density of states at and about the Fermi level are compatible with recent theoretical results on "exhaustion physics" in the periodic Anderson model. The present study indicates a Kondo lattice picture for SmS and suggests a relation between the mixed valency and the temperature dependence of the pseudogap for "black" semiconducting SmS.

DOI： 10.1103/PhysRevB.65.155201

Superconducting gap of MgB2 observed using ultrahigh-resolution photoemission spectroscopy Reviewed

S Tsuda, T Yokoya, T Kiss, A Chainani, Y Takano, K Togano, H Kito, H Ihara, S Shin

PHYSICA B-CONDENSED MATTER 312 150 - + 2002.3

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We study the superconducting gap of MgB2 using high-resolution photoemission. In the spectrum measured at 5.4 K, a coherent peak with a shoulder structure is observed. We find that a simulation using two Dynes functions with the gap sizes of 1.7 and 5.6 meV reproduces the superconducting-state spectrum better than that using a single Dynes function and anisotropic Dynes functions. We also find that both of the smaller and larger gap close at the bulk transition temperature. These results indicate a multiple superconducting gap of MgB2. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(01)01282-0

Fermi surface and superconducting gap of 2H-NbSe2 using low-temperature ultrahigh-resolution angle-resolved photoemission spectroscopy Reviewed

T Kiss, T Yokoya, A Chainani, S Shin, M Nohara, H Takagi

PHYSICA B-CONDENSED MATTER 312 666 - 667 2002.3

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We have studied the Fermi surface (FS) and superconducting gap of 2H-NbSe2, using angle-resolved photoemission spectroscopy with an energy resolution of 2.2 meV and an angle resolution of +/-0.13degrees. We succeeded in distinguishing all FS sheets predicted from band calculations and found that the observed hexagonal Fermi surface nesting vector centered at the Gamma point is larger than the CDW nesting vector reported from neutron diffraction measurements. Furthermore, a superconducting gap at one FS sheet is successfully observed. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(01)01293-5

Angle-resolved photoemission spectroscopy, optical conductivity, and soft X-ray Raman spectroscopy of quasi-one-dimensional V6O13 Reviewed

R Eguchi, T Yokoya, T Kiss, Y Ueda, H Tajima, J Yamazaki, S Shin

PHYSICA B-CONDENSED MATTER 312 600 - 602 2002.3

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We study the electronic properties of quasi-one-dimensional V6O13 which exhibits a metal-insulator transition using angle-resolved photoemission spectroscopy (ARPES), optical conductivity, and soft X-ray Raman spectroscopy (SXRS). From ARPES, we observe an energy gap of 0.2eV across the metal-insulator transition with change in the lower Hubbard band. Band dispersion is found only along the b-axis, consistent with highly anisotropic behavior observed in optical conductivity and SXRS measured on the same samples. These results indicate the electronic properties of V6O13 are dominated by b-axis electronic structure. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(01)01186-3

Electronic structure of the filled skutterudite compound CeRu<inf>4</inf>Sb<inf>12</inf>

K. Kanai, N. Takeda, S. Nozawa, T. Yokoya, M. Ishikawa, S. Shin, S. Shin

Physical Review B - Condensed Matter and Materials Physics 65 ( 4 ) 411051 - 411054 2002.1

Publishing type：Research paper (scientific journal)

Ultrahigh-resolution photoemission spectra of CeRu4Sb12 reveal crossover behavior into a low carrier state. The density of states (DOS) around the Fermi level (EF) decreases on lowering temperature and the semimetalliclike DOS is observed at 5.9 K. The temperature dependence of DOS at EF is explained in terms of the development of the coherence. The smaller hybridization strength between the Ru-d state and the Ce-4f state prevents the hybridization gap from opening in CeRu4Sb12, which is realized in CeRu4P12.

Electronic structure of the filled skutterudite compound CeRu4Sb12 Reviewed

K Kanai, N Takeda, S Nozawa, T Yokoya, M Ishikawa, S Shin

PHYSICAL REVIEW B 65 ( 4 ) 2002.1

Language：English Publishing type：Research paper (scientific journal) Publisher：AMERICAN PHYSICAL SOC

Ultrahigh-resolution photoemission spectra of CeRu4Sb12 reveal crossover behavior into a low carrier state. The density of states (DOS) around the Fermi level (E-F) decreases on lowering temperature and the semimetalliclike DOS is observed at 5.9 K. The temperature dependence of DOS at E-F is explained in terms of the development of the coherence. The smaller hybridization strength between the Ru-d state and the Ce-4f state prevents the hybridization gap from opening in CeRu4Sb12, which is realized in CeRu4P12.

DOI： 10.1103/PhysRevB.65.041105

Photoemission spectroscopy of the strong-coupling superconducting transitions in Lead and Niobium

T. Yokoya, T. Kiss, A. Chainani, S. Shin

Journal of Electron Spectroscopy and Related Phenomena 124 99 2002

Electronic structure of the filled skutterudite compound (formula presented) Reviewed

K. Kanai, N. Takeda, S. Nozawa, T. Yokoya, M. Ishikawa, S. Shin

Physical Review B - Condensed Matter and Materials Physics 65 ( 4 ) 1 - 4 2002

Language：English Publishing type：Research paper (scientific journal)

Ultrahigh-resolution photoemission spectra of (formula presented) reveal crossover behavior into a low carrier state. The density of states (DOS) around the Fermi level (formula presented) decreases on lowering temperature and the semimetalliclike DOS is observed at 5.9 K. The temperature dependence of DOS at (formula presented) is explained in terms of the development of the coherence. The smaller hybridization strength between the Ru- (formula presented) state and the (formula presented) state prevents the hybridization gap from opening in (formula presented) which is realized in (formula presented). © 2002 The American Physical Society.

DOI： 10.1103/PhysRevB.65.041105

Fermi surface sheet-dependent superconductivity in 2H-NbSe2 Reviewed

T Yokoya, T Kiss, A Chainani, S Shin, M Nohara, H Takagi

SCIENCE 294 ( 5551 ) 2518 - 2520 2001.12

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER ASSOC ADVANCEMENT SCIENCE

High-resolution angle-resolved photoemission spectroscopy was used to study the superconducting energy gap and changes in the spectra[ function across the superconducting transition in the quasi-two-dimensional superconductor 2H-NbSe2. The momentum dependence of the superconducting gap was determined on different Fermi surface sheets. The results indicate Fermi surface sheet-dependent superconductivity in this Low-transition temperature multiband system and provide a description consistent with thermodynamic measurements and the anomalous de Haas-van Alphen oscillations observed in the superconducting phase. The present data suggest the importance of Fermi surface sheet-dependent superconductivity in explaining exotic superconductivity in other multiband systems with complex Fermi surface topology, such as the borides and f-electron superconductors.

DOI： 10.1126/science.1065068

Electron-phonon coupling induced pseudogap and the superconducting transition in Ba0.67K0.33BiO3

A. Chainani, T. Yokoya, T. Kiss, S. Shin, T. Nishio, H. Uwe

Physical Review B - Condensed Matter and Materials Physics 64 ( 18 ) 1805091 - 1805094 2001.11

Language：English Publishing type：Research paper (scientific journal)

We study the single-particle density of states (DOS) across the superconducting transition (Tc=31 K) in single-crystal Ba0.67K0.33BiO3 using ultrahigh resolution angle-integrated photoemission spectroscopy. The superconducting gap opens with a pileup in the DOS, Δ(5.3 K) = 5.2 meV and 2Δ(0)/kBTc = 3.9. In addition, we observe a pseudogap below and above Tc, occurring as a suppression in intensity over an energy scale up to the breathing mode phonon (∼70 meV). The results indicate electron-phonon coupling induces a pseudogap in Ba0.67K0.33BiO3.

Electronic structure and superconducting gap of silicon clathrate Ba8Si46 studied with ultrahigh-resolution photoemission spectroscopy

T. Yokoya, A. Fukushima, T. Kiss, K. Kobayashi, S. Shin, K. Moriguchi, A. Shintani, H. Fukuoka, S. Yamanaka

Physical Review B - Condensed Matter and Materials Physics 64 ( 17 ) 1725041 - 1725044 2001.11

Language：English Publishing type：Research paper (scientific journal) Publisher：American Physical Society

We study the electronic structure and superconducting transition of silicon clathrate Ba8Si46 (Tc=8 K) using photoemission spectroscopy. We observe a narrow band at the Fermi level (EF), whose width (∼0.3 eV) is substantially smaller than that of band structure calculations (∼1.5 eV). Ultrahigh-resolution measurements show a superconducting gap at 5.4 K [2Δ(0)/kBTc=3.51]. Fine structures associated with phonons are observed within 70 meV of EF. These results characterize Ba8Si46 as a weak-coupling superconductor most probably driven by phonon.

Electron-phonon coupling induced pseudogap and the superconducting transition in Ba0.67K0.33BiO3 Reviewed

A Chainani, T Yokoya, T Kiss, S Shin, T Nishio, H Uwe

PHYSICAL REVIEW B 64 ( 18 ) 2001.11

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We study the single-particle density of states (DOS) across the superconducting transition (T-c = 31 K) in single-crystal Ba0.67K0.33BiO3 using ultrahigh resolution angle-integrated photoemission spectroscopy. The superconducting gap opens with a pileup in the DOS, Delta (5.3 K) = 5.2 meV and 2 Delta (0)/k(B)T(c) = 3.9. In addition, we observe a pseudogap below and above T-c occurring as a suppression in intensity over an energy scale up to the breathing mode phonon (similar to 70 meV). The results indicate electron-phonon coupling induces a pseudogap in Ba0.67K0.33BiO3.

DOI： 10.1103/PhysRevB.64.180509

Electronic structure and superconducting gap of silicon clathrate Ba8Si46 studied with ultrahigh-resolution photoemission spectroscopy Reviewed

T Yokoya, A Fukushima, T Kiss, K Kobayashi, S Shin, K Moriguchi, A Shintani, H Fukuoka, S Yamanaka

PHYSICAL REVIEW B 64 ( 17 ) 2001.11

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We study the electronic structure and superconducting transition of silicon clathrate Ba8Si46 (T-c = 8 K) using photoemission spectroscopy. We observe a narrow band at the Fermi level (E-F), whose width (similar to0.3 eV) is substantially smaller than that of band structure calculations (similar to1.5 eV). Ultrahigh-resolution measurements show a superconducting gap at 5.4 K [2 Delta (0)/k(B)T(c)=3.51]. Fine structures associated with phonons are observed within 70 meV of E-F. These results characterize Ba8Si46 as a weak-coupling superconductor most probably driven by phonon.

DOI： 10.1103/PhysRevB.64.172504

Evidence for a multiple superconducting gap in MgB2 from high-resolution photoemission spectroscopy Reviewed

S Tsuda, T Yokoya, T Kiss, Y Takano, K Togano, H Kito, H Ihara, S Shin

PHYSICAL REVIEW LETTERS 87 ( 17 ) 2001.10

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We study the new binary intermetallic superconductor MgB2 using high-resolution photoemission spectroscopy. The superconducting-state spectrum measured at 5.4 K shows a coherent peak with a shoulder structure, in sharp contrast to that expected from a simple isotropic-gap opening. The spectrum can be well reproduced using the weighted sum of two Dynes functions with the gap sizes of 1.7 and 5.6 meV. Temperature-dependent study shows that both gaps close at the bulk transition temperature. These results provide spectroscopic evidence for a multiple gap of MgB2.

DOI： 10.1103/PhysRevLett.87.177006

Changes of electronic structure across the insulator-to-metal transition of quasi-two-dimensional Na-intercalated beta-HfNCl studied by photoemission and x-ray absorption Reviewed

T Yokoya, Y Ishiwata, S Shin, S Shamoto, K Iizawa, T Kajitani, Hase, I, T Takahashi

PHYSICAL REVIEW B 64 ( 15 ) 2001.10

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We use photoemission (PE) and x-ray absorption (XA) spectroscopy to study intercalation-induced changes in the electronic structure of beta -HfNCl and Na0.22HfNCl. PE shows appearance of new states in Na0.22HfNCl within the gap of beta -HfNCl, with negligible changes in the energy positions of valence-band features of the parent, suggesting a non-rigid-band behavior. N 1 s,XA results show that the tail of the conduction band, with a quasi-two-dimensional character, extends to lower energy in Na0.22HfNCl than in beta -HfNCl. The study provides information on the electronic structure and its change across the insulator-to-metal transition of the. new class of superconductors.

DOI： 10.1103/PhysRevB.64.153107

Determination of the Fermi surface in high-T-c superconductors by angle-resolved photoemission spectroscopy Reviewed

J Mesot, M Randeria, MR Norman, A Kaminski, HM Fretwell, JC Campuzano, H Ding, T Takeuchi, T Sato, T Yokoya, T Takahashi, Chong, I, T Terashima, M Takano, T Mochiku, K Kadowaki

PHYSICAL REVIEW B 63 ( 22 ) 2001.6

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We study the normal-state electronic excitations probed by angle-resolved photoemission spectroscopy (ARPES) in Bi1.6Pb0.4Sr2CuO6 (Bi2201) and Bi2Sr2CaCu2O8 divided by delta (Bi2212). Our main goal is to establish explicit criteria for determining the Fermi surface from ARPES data on strongly interacting systems where sharply defined quasiparticles do not exist and the dispersion is very weak in parts of the Brillouin zone. Additional complications arise from strong matrix element variations within the zone. We present detailed results as a function of incident photon energy, and show simple experimental tests to distinguish between an intensity drop due to matrix element effects and spectral weight loss due to a Fermi crossing. We reiterate the use of polarization selection rules in disentangling the effect of umklapps due to the BiO superlattice in Bi2212. We conclude that, despite all the complications, the Fermi surface can be determined unambiguously; it is a single large hole barrel centered about (pi,pi) in both materials.

DOI： 10.1103/PhysRevB.63.224516

Topology of the fermi surface and band structure near the fermi level in the Ph-doped Bi2Sr2CuO6+delta superconductor Reviewed

T Takeuchi, T Yokoya, S Shin, K Jinno, M Matsuura, T Kondo, H Ikuta, U Mizutani

JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 114 629 - 634 2001.3

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We report the doping level dependence of the band dispersion and the shape of the Fermi surface for Pb-doped Bi2Sr2CuO6+delta superconductor by use of angle resolved photoemission spectroscopy with extremely high angular and energy resolutions. It is found that the band dispersion of Bi2Sr2CuO6+delta is fairly rigid regardless of its doping level in the over-doped range, and that it shifts toward higher binding energy with decreasing doping. We also found that one of the heavily over-doped samples possesses an electron-like Fermi surface centered at (0,0) point, while less doped samples show a hole-like Fermi surface. This is the first observation of the transformation between electron-like and hole-like Fermi surfaces with varying the doping level in Bi2Sr2CuO6+delta. (C) 2001 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0368-2048(00)00354-6

Ultra-high resolution photoemission spectroscopy of simple metals: direct observation of superconducting gap and phonon-induced fine structures Reviewed

T Kiss, T Yokoya, A Chainani, S Shin

JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 114 635 - 639 2001.3

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We have performed ultra-high resolution photoemission spectroscopy of simple metals, Nb (T-c=9.26 K) and Pb (T-c=7.19 K) in order to study the changes in the density of states across the transition temperatures. The spectra of both compounds measured at 5.3 K show an opening of the superconducting gap accompanied by a leading-edge shift and an appearance of a sharp peak just below the Fermi level. Furthermore, the ph spectrum clearly shows peak-dip-hump structures due to strong electron-phonon coupling, as seen in the high-T-c cuprates. This makes it a characteristic of strong-coupling superconductivity. (C) 2001 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0368-2048(00)00366-2

Ultrahigh-resolution photoemission and resonant-inverse photoemission spectroscopy on CePd3 Reviewed

K Kanai, T Kiss, T Yokoya, G Schmerber, JP Kappler, JC Parlebas, S Shin

JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 114 741 - 745 2001.3

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We carried out ultrahigh-resolution photoemission (URPE) and resonant-inverse photoemission spectroscopy on CePd3 and CePd2.9Cu0.1 compounds. The URPE spectra of CePd3 and CePd2.9Cu0.1 clearly show small humps in the vicinity of Fermi level well below the Kondo temperature. The hump is gradually formed below the characteristic coherent temperature T*. We conclude that it gives direct evidence of the appearance of the small pseudogap below T* in CePd3 and CePd2.9Cu0.1. (C) 2001 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0368-2048(00)00333-9

Two different types of pseudogaps in high-T-c superconductors Reviewed

T Takahashi, T Sato, T Yokoya, T Kamiyama, Y Naitoh, T Mochiku, K Yamada, Y Endoh, K Kadowaki

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 62 ( 1-2 ) 41 - 45 2001.1

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

Ultrahigh-resolution (DeltaE = 7 meV angle-resolved (ARPES) and angle-integrated (AIPES) photoemission spectroscopy have been performed on optimally doped Bi2Sr2CaCu2O8 (Bi2212) and La1.85Sr0.15CuO4 (LSCO). The ARPES spectrum of Bi2212 at the (pi, 0)-(pi, pi) crossing shows a pseudogap at EF above T-c, which is smoothly connected from the superconducting gap across T-c and closes slightly above T-c(T-* = 110 K). On the other hand, the AIPES spectra both in Bi2212 and LSCO show a remarkable depletion of the density of states in a relatively wider energy (100-150 meV) region near EF The temperature-induced filling of the depletion persists far above T-c and T-*. This suggests that there are two different pseudogaps in the high-T-c superconductors: one is likely associated with pairing and the other is attributed to the development of antiferromagnetic correlation. (C) 2000 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(00)00098-6

High-resolution angle-resolved photoemission study of Pb-substituted Bi2201 Reviewed

T Sato, Y Naitoh, T Kamiyama, T Takahashi, T Yokoya, J Mesot, A Kaminski, H Fretwell, JC Campuzano, H Ding, Chong, I, T Terashima, M Takano, K Kadowaki

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 62 ( 1-2 ) 157 - 161 2001.1

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

We have performed a systematic doping-dependent angle-resolved photoemission spectroscopy (ARPES) of Pb-substituted Bi2201 (Bi1.80Pb0.38Sr2.01CuO6-delta) With high energy (7-11 meV) and momentum (0.01 Angstrom (-1)) resolutions. We found that all the samples show a large hole-like Fermi surface centered at (pi,pi), whose volume changes systematically as a function of doping. The superconducting gap exhibits a d(x2-y2)-like anisotropy with a typical gap value of 10-15 meV near (pi ,0). By comparing with Bi2Sr2CaCu2O8 (Bi2212), we found that the size of superconducting gap is well scaled with the maximum T-c (T-c(max)) Doping-dependence of spectral line shape at (pi ,0) is very similar between Bi2201 and Bi2212, while its energy scale is different by a factor of 4, corresponding to the ratio of T-c(max) between the two compounds. These indicate that the superconducting and pseudogap properties are characterized by T-c(max) irrespective of the number of CuO2 layers or the BiO superstructure. (C) 2000 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(00)00119-0

Determination of the Fermi surface in high-Tc superconductors by angle-resolved photoemission spectroscopy Reviewed

J. Mesot, M. Randeria, M. R. Norman, A. Kaminski, H. M. Fretwell, J. C. Campuzano, H. Ding, T. Takeuchi, T. Sato, T. Yokoya, T. Takahashi, I. Chong, T. Terashima, M. Takano, T. Mochiku, K. Kadowaki

Physical Review B - Condensed Matter and Materials Physics 63 ( 22 ) 2245161 - 22451614 2001

Language：English Publishing type：Research paper (scientific journal)

We study the normal-state electronic excitations probed by angle-resolved photoemission spectroscopy (ARPES) in Bi1.6Pb0.4Sr2CuO6 (Bi2201) and Bi2Sr2CaCu2O8+δ (Bi2212). Our main goal is to establish explicit criteria for determining the Fermi surface from ARPES data on strongly interacting systems where sharply defined quasiparticles do not exist and the dispersion is very weak in parts of the Brillouin zone. Additional complications arise from strong matrix element variations within the zone. We present detailed results as a function of incident photon energy, and show simple experimental tests to distinguish between an intensity drop due to matrix element effects and spectral weight loss due to a Fermi crossing. We reiterate the use of polarization selection rules in disentangling the effect of umklapps due to the BiO superlattice in Bi2212. We conclude that, despite all the complications, the Fermi surface can be determined unambiguously
it is a single large hole barrel centered about (π, π) in both materials.

DOI： 10.1103/PhysRevB.63.224516

Ultrahigh-resolution photoemission spectroscopy of Ni borocarbides: direct observation of the superconducting gap and a change in gap anisotropy by impurity

T. Yokoya, T. Kiss, T. Watanabe, S. Shin, M. Nohara, H. Takagi, T. Oguchi

Physical Review Letters 85 ( 23 ) 4952 - 4955 2000.12

Language：English Publishing type：Research paper (scientific journal)

The superconducting gaps (SGs) of YNi2B2C and Y(Ni0.8Pt0.2)2CB2C were studied using ultrahigh-resolution photoemission (PE) spectroscopy. A small but important difference in spectral shape was observed and attributed to the change in anisotropy of the SG between x=0.0 and 0.2 compounds.

DOI： 10.1103/PhysRevLett.85.4952

Hole-state and defect structure of proton conductor SrTiO3 observed by high-resolution X-ray absorption spectroscopy

T. Higuchi, T. Tsukamoto, N. Sata, M. Ishigame, K. Kobayashi, S. Yamaguchi, Y. Ishiwata, T. Yokoya, M. Fujisawa, S. Shin

Solid State Ionics 136-137 261 - 264 2000.11

Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier Science Publishers B.V.

The hole-state and defect structure of proton conductor SrTi1-xScxO3 have been studied by high-resolution X-ray absorption spectroscopy. The proton-induced level is observed in proton-doped SrTi1-xScxO3. The energy separation between the top of the valence band and the bottom of the acceptor or proton level is considered to be the activation energy determined by electronic or proton conductivity.

DOI： 10.1016/S0167-2738(00)00319-2

Pseudogap formation in Kondo insulators CeRhSb and CeRhAs studied by ultrahigh-resolution photoemission spectroscopy Reviewed

H Kumigashira, T Sato, T Yokoya, T Takahashi, S Yoshii, M Kasaya

PHYSICA B 281 284 - 285 2000.6

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

We report an ultrahigh-resolution (Delta E similar to 8 meV) temperature-dependent photoemission spectroscopy on the Kondo insulators CeRhAs and CeRhSb. We have observed a pseudogap at the Fermi level at low temperature for both compounds, which is gradually filled-in with increasing temperature. We found that, the, size of pseudogap is well scaled with the Kondo temperature (T-K) while the temperature evolution is dominated by another characteristic temperature much lower than T-K. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(99)00827-3

Angle-resolved photoemission study of 1D chain and two-leg ladder Reviewed

T Sato, T Yokoya, H Fujisawa, T Takahashi, M Uehara, T Nagata, J Akimitsu, S Miyasaka, M Kibune, H Takagi

STRIPES AND RELATED PHENOMENA 263 - 270 2000

Language：English Publishing type：Research paper (international conference proceedings) Publisher：KLUWER ACADEMIC/PLENUM PUBL

We performed angle-resolved photoemission spectroscopy (ARPES) on Sr2CuO3 and Sr14Cu24O41 to study the difference of electronic structure between the corner-sharing one-dimensional (1D)chain and the two-leg ladder cuprate. ARPES spectra of Sr2CuO3 show two independent dispersive bands near the Fermi level (E-F), which are ascribed to the spinon and the holon band due to the spin-charge separation in the ID chain. In contrast, ARPES spectra of Sr14Cu24O41 exhibit only one dispersive band near E-F With the periodicity of ladder sublattice, which is assigned to a quasi-particle band where holon and spinon are confined together owing to the opening of the spin gap on the ladder. The present results show a clear difference in the nature of the electronic structure near E-F between the Tomonaga-Luttinger liquid (Sr2CuO3) and the Luther-Emery liquid (Sr14Cu24O41).

Photoemission spectroscopy of the strong-coupling superconducting transitions in lead and niobium

A. Chainani, T. Yokoya, T. Kiss, S. Shin

Physical Review Letters 85 ( 9 ) 1966 - 1969 2000

Language：English Publishing type：Research paper (scientific journal)

We study the changes in the electronic structure associated with low-Tc strong-coupling superconducting transitions in Pb and Nb using ultrahigh-resolution (2.3 meV) temperature-dependent (5.3-12.0 K) photoemission spectroscopy. We observe peaks in the density of states on entering the superconducting-phase accompanying gap formation and spectacular redistribution of spectral weight at low energy scales as a function of temperature. The well-known peak-dip feature of the high-Tc cuprates is seen in Pb, making it a characteristic of strong-coupling superconductivity. © 2000 The American Physical Society.

DOI： 10.1103/PhysRevLett.85.1966

Electronic structure in the band gap of lightly doped by high-resolution x-ray absorption spectroscopy

T. Higuchi, T. Tsukamoto, K. Kobayashi, Y. Ishiwata, M. Fujisawa, T. Yokoya, S. Yamaguchi, S. Shin

Physical Review B - Condensed Matter and Materials Physics 61 ( 19 ) 12860 - 12863 2000

Language：English Publishing type：Research paper (scientific journal)

The electronic structure of lightly Nb-doped (Formula presented) has been investigated using high resolution x-ray absorption spectroscopy (XAS). Below the O (Formula presented) threshold, XAS spectra show two features due to empty states whose energy positions match with those of the (Formula presented) photoemission spectra in the band gap energy region of the parent undoped (Formula presented) Similar features are also observed in lightly La-doped (Formula presented) These features exhibit systematic temperature dependence, which is well explained by the Fermi-Dirac distribution function. This indicates that the two features in the band gap are real bulk states such as simple donor states. © 2000 The American Physical Society.

DOI： 10.1103/PhysRevB.61.12860

Superconducting gap, pseudogap, and fermi surface of Bi2201: high energy- and momentum-resolution photoemission study

T. Sato, Y. Naitoh, T. Kamiyama, T. Takahashi, T. Yokoya, J. Mesot, A. Kaminski, H. Fretwell, J. C. Campuzano, H. Ding, I. Chong, T. Terashima, M. Takano, K. Kadowaki

Physica C: Superconductivity and its Applications 341-348 2091 - 2094 2000

Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier Science Publishers B.V.

We have performed a systematic high energy (7-11 meV) and momentum (0.01 angstrom-1) resolution angle-resolved photoemission spectroscopy (ARPES) on superstructure-free Bi1.80Pb0.38Sr2.01CuO6-δ with various dopings. All the samples from overdoped Tc = 0K to underdoped Tc = 18K possess a large hole-like Fermi surface centered at X point, whose volume shows a systematic evolution with doping. The superconducting gap exhibits a dx(2)-y(2)-like anisotropy with a typical gap value of 10-15 meV. By comparing with Bi2Sr2CaCu2O8, we found that the size of superconducting gap and the ARPES spectral feature near EF are well scaled with the maximum Tc (Tc max). This suggests that the superconducting properties are characterized by Tc max irrespective of the number of CuO2 layers or the structural modulation in BiO layer.

DOI： 10.1016/S0921-4534(00)01099-6

Ti-O Hybridization Effect on Ferroelectric Phase Transition of BaTiO₃(共著)

Higuchi Tohru, Tsukamoto Takeyo, Oka Kunihiko, Yokoya Takayoshi, Tezuka Yasuhisa, Shin Shik

Jpn. J. Appl. Phys. 38 ( 9 ) 5667 - 5669 1999.9

Language：English Publisher：The Japan Society of Applied Physics

The Ti-O hybridization effect on the ferroelectric phase transition of BaTiO3 has been studied by resonant-photoemission spectroscopy. The Ti 3d partial-density-of-state in the valence band of the paraelectric phase is larger than that of the ferroelectric phase. This fact indicates that the Ti–O hybridization effect is stronger in the paraelectric phase.

DOI： 10.1143/JJAP.38.5667

Other Link： https://jlc.jst.go.jp/DN/JALC/00063927863?from=CiNii

Spectral evidence for pseudogap formation in kondo insulators CeRhAs and CeRhSb

H. Kumigashira, T. Sato, T. Yokoya, T. Takahashi, S. Yoshii, M. Kasaya

Physical Review Letters 82 ( 9 ) 1943 - 1946 1999.1

Language：English Publishing type：Research paper (scientific journal)

High-resolution photoemission spectroscopy has been performed on CeRhAs and CeRhSb to study the temperature-induced evolution of the “Kondo-insulator” gap. We have observed that a pseudogap opens at EF at low temperatures while it is gradually filled with increasing temperature. The size of the pseudogap is well scaled with the Kondo temperature (TK) of each compound while the temperature evolution is dominated by another characteristic temperature Tcoh (&lt
TK). The results are qualitatively consistent with the theoretical prediction from the Anderson lattice model. © 1999 The American Physical Society.

DOI： 10.1103/PhysRevLett.82.1943

Electronic spectra and their relation to the (π, π) collective mode in high-Tc superconductors

J. C. Campuzano, H. Ding, M. R. Norman, H. M. Fretwell, M. Randeria, A. Kaminski, J. Mesot, T. Takeuchi, T. Sato, T. Yokoya, T. Takahashi, T. Mochiku, K. Kadowaki, P. Guptasarma, D. G. Hinks, Z. Konstantinovic, Z. Z. Li, H. Raffy

Physical Review Letters 83 ( 18 ) 3709 - 3712 1999.1

Language：English Publishing type：Research paper (scientific journal)

The resonant inelastic x-ray scattering spectrum in insulating cuprates is examined by using the exact diagonalization technique on small clusters in the two-dimensional Hubbard model with second and third neighbor hopping terms. When the incident photon energy is tuned near the Cu K absorption edges, we find that the features of the unoccupied upper Hubbard band can be extracted from the spectrum through an anisotropic momentum dependence. They provide an opportunity for the understanding of the different behavior of hole- and electron-doped superconductors. © 1999 The American Physical Society.

DOI： 10.1103/PhysRevLett.83.3709

Pseudogap of optimally doped La1.85Sr0.15CuO4 observed by ultrahigh-resolution photoemission spectroscopy

T. Sato, T. Yokoya, Y. Naitoh, T. Takahashi, K. Yamada, Y. Endoh

Physical Review Letters 83 ( 11 ) 2254 - 2257 1999.1

Language：English Publishing type：Research paper (scientific journal)

We have performed an ultrahigh-resolution \\(ΔE˜7 meV\\) photoemission spectroscopy on La1.85Sr0.15CuO4 \\(Tc = 38 K\\) at 16-250 K. We found a pseudogap at EF with the size of 30-35 meV, which is gradually filled in by a transfer of states from the higher energy region with increasing temperature, but persists over 200 K. The gap energy is consistent with the characteristic temperatures of the magnetic susceptibility and the Hall coefficient. The temperature evolution is well scaled with that of the Hall coefficient. The pseudogap is not smoothly connected to the superconducting gap \\(Δ˜8 meV\\) below Tc, suggesting a different origin from that in Bi2Sr2CaCu2O8. © 1999 The American Physical Society.

DOI： 10.1103/PhysRevLett.83.2254

Superconducting gap anisotropy and quasiparticle interactions: A doping dependent photoemission study

J. Mesot, M. R. Norman, H. Ding, M. Randeria, J. C. Campuzano, A. Paramekanti, H. M. Fretwell, A. Kaminski, T. Takeuchi, T. Yokoya, T. Sato, T. Takahashi, T. Mochiku, K. Kadowaki

Physical Review Letters 83 ( 4 ) 840 - 843 1999.1

Language：English Publishing type：Research paper (scientific journal)

Comparing photoemission measurements on Bi2212 with penetration depth data, we show that a description of the nodal excitations of the d-wave superconducting state in terms of noninteracting quasiparticles is inadequate, and we estimate the magnitude and doping dependence of the Landau interaction parameter which renormalizes the linear T contribution to the superfluid density. Furthermore, although consistent with d-wave symmetry, the gap with underdoping cannot be fit by the simple coskx−cosky form, which suggests an increasing importance of long range interactions as the insulator is approached. © 1999 The American Physical Society.

DOI： 10.1103/PhysRevLett.83.840

Angle-resolved photoemission study of Sr2CuO3

H. Fujisawa, T. Yokoya, T. Takahashi, S. Miyasaka, M. Kibune, H. Takagi

Physical Review B - Condensed Matter and Materials Physics 59 ( 11 ) 7358 - 7361 1999

Language：English Publishing type：Research paper (scientific journal)

We report angle-resolved photoemission spectroscopy (ARPES) on a single Cu-O chain compound Sr2CuO3 with Ne I photons. In contrast with previous ARPES with higher-energy photons, the present result clearly shows two well-separated branches of dispersions near EF ascribable to spinon and holon excitations, respectively. This indicates that spin and charge degrees of freedom are separated in one-dimensional Cu-O chains, supporting the theoretical prediction as well as the previous ARPES interpretation. Comparison with previous ARPES results indicates that surface, photoionization cross section, and finite-temperature effects substantially modify ARPES spectra. © 1999 ThAmerican Physical Society.

DOI： 10.1103/PhysRevB.59.7358

BSCCO superconductors : Hole-like Fermi surface and doping dependence of the gap function(共著)

J. Mesot, M. R. Norman, H. Ding, M. Randeria, J. C. Campuzano, A. Paramekanti, H. M. Fretwell, A. Kaminski, T. Takeuchi, T. Yokoya, T. Sato, T. Takahashi, T. Mochiku, K. Kadowaki

JOURNAL OF LOW TEMPERATURE PHYSICS 117 ( 3-4 ) 365 - 369 1999

Publishing type：Research paper (scientific journal)

We use the gradient of the energy-integrated angle resolved photoemission (ARPES) intensity in order to define precisely the Fermi surface (FS) in BSCCO superconductors. We show that, independent of the photon energy, the FS is a hole barrel centered at (π, π). Then, the superconducting gap along the FS is precisely determined from ARPES measurements on overdoped and underdoped samples of Bi2212. As the doping decreases, the maximum gap increases, but the slope of the gap near the nodes decreases. Though consistent with d-wave symmetry, the gap with underdoping cannot be fit by the simple cos(k ) - cos(k ) form. A comparison of our ARPES results with available penetration depth data indicates that the renormalization of the linear T suppression of the superfluid density at low temperatures due to quasiparticle excitations around the d-wave nodes is large and doping dependent. x y

DOI： 10.1023/A:1022569616840

Electronic structure of (formula presented) comparison between substitutional and vacancy doping

T. Yokoya, T. Sato, H. Fujisawa, T. Takahashi, A. Chainani, M. Onoda

Physical Review B - Condensed Matter and Materials Physics 59 ( 3 ) 1815 - 1818 1999

Language：English Publishing type：Research paper (scientific journal)

The change of the electronic structure across the metal-insulator transition in a Mott-Hubbard system (Formula presented) has been investigated by x-ray absorption and photoemission spectroscopy. The results are compared with (Formula presented) where hole doping is achieved by excess oxygen instead of Sr substitution. It is found that additional doped-hole states are created in the insulator gap for both cases, while the density of states at the Fermi level in (Formula presented) is nearly half of that in (Formula presented) at the same nominal doping. This suggests a strong reduction in the mobility of carriers due to cation vacancies produced by excess oxygen. © 1999 The American Physical Society.

DOI： 10.1103/PhysRevB.59.1815

Metallic-covalent bonding conversion in Alminum-based and Boron-based icosahedral cluster solids Reviewed

K. Kimura, H. Matsuda, M. Fujimori, M. Terauchi, M. Tanaka, H. Kumigashira, T. Yokoya, T. Takahashi

Proc. 6th Int. Conf. Quasicrystals 595 - 598 1998.12

Destruction of the Fermi surface in underdoped high-T(c) superconductors

M. R. Norman, H. Ding, M. Randeria, J. C. Campuzano, T. Yokoya, T. Takeuchi, T. Takahashi, T. Mochiku, K. Kadowaki, P. Guptasarma, D. G. Hinks

Nature 392 ( 6672 ) 157 - 160 1998.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Macmillan Magazines Ltd

The Fermi surface - the set of points in momentum space describing gapless electronic excitations - is a central concept in the theory of metals. In this context, the normal 'metallic' state of the optimally doped high-temperature superconductors is not very unusual: above the superconducting transition temperature, T(c), there is evidence for a large Fermi surface1-3 despite the absence of well-defined elementary excitations. In contrast, the normal state of underdoped high-temperature superconductors differs in that there is evidence for a 'pseudogap' above T(c) (refs 4-6). Here we examine, using angle-resolved photoemission spectroscopy, the temperature dependence of the Fermi surface in underdoped Bi2Sr2CaCu2O(8+δ). We find that, on cooling the sample, the pseudo-gap opens up at different temperatures for different points in momentum space. This leads to an initial breakup of the Fermi surface, at a temperature T*, into disconnected arcs, which then shrink with decreasing temperature before collapsing to the point nodes of the superconducting ground state below T(c). This unusual behaviour, where the Fermi surface does not form a continuous contour in momentum space as in conventional metals, is unprecedented in that it occurs in the absence of long-range order. Moreover, although the superconducting gap below T(c) evolves smoothly into the pseudogap above T(c), the pseudogap differs in its unusual temperature-dependent anisotropy, implying an intimate but non-trivial relationship between the pseudo-gap and the superconducting gap.

DOI： 10.1038/32366

Spin-charge separation in single-chain compound Sr2CuO3 studied by angle-resolved photoemission

H. Fujisawa, T. Yokoya, T. Takahashi, S. Miyasaka, M. Kibune, H. Takagi

Solid State Communications 106 ( 8 ) 543 - 547 1998

Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier Ltd

Angle-resolved photoemission measurements were performed for one-dimensional Sr2CuO3 which consists of single Cu-O chains. Two dispersive bands were observed near EF in the first half of the Brillouin zone (k = 0 to π/2) of the chain-direction. Both bands have a common maximum point closest to EF at k = π/2 while they show different dispersions of about 0.25 and 1 eV, respectively, with a continuous intensity distribution in between. The less dispersive band is almost diminished in another half of the Brillouin zone (k = π/2 to π). These results are consistent with the solution of the one-dimensional Hubbard model as well as the t-J model calculation, suggesting the spin-charge separation in the one-dimensional Cu-O chain. The experimental results are compared with those of the double-chain compound SrCuO2. © 1998 Published by Elsevier Science Ltd.

DOI： 10.1016/S0038-1098(98)00099-4

Angle-resolved photoemission study of superconducting ladder compound Sr14-xCaxCu24O41

T. Sato, T. Yokoya, T. Takahashi, M. Uehara, T. Nagata, J. Goto, J. Akimitsu

Journal of Physics and Chemistry of Solids 59 ( 10-12 ) 1912 - 1916 1998

Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier Ltd

Angle-resolved photoemission spectroscopy has been performed on Sr14-xCaxCu24O41 (x = 0-11.5) which consists of two-leg Cu2O3 ladders and edge-sharing CuO2 chains. A dispersive band with the periodicity of ladder sublattice is observed near the Fermi level (EF) for x=0. The band shows a minimum binding energy at ka= (2n + 1)π/2, suggesting that a Mott-Hubbard gap opens on the ladder. This indicates that copper atoms on the ladder take essentially the divalent state (Cu2+) and a majority of self-doped holes reside on the chain. We found that the Ca substitution causes a transfer of holes from the chain to the ladder. © 1998 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(98)00148-6

Magnetic phase transition of CeSb studied by high-resolution angle-resolved photoemission

T. Takahashi, H. Kumigashira, H. D. Kim, A. Ashihara, A. Chainani, T. Yokoya, A. Uesawa, T. Suzuki

Journal of Magnetism and Magnetic Materials 177-181 ( 2 ) 1027 - 1028 1998

Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier

We observed a change of Fermi-surface topology of CeSb across the magnetic phase transition using temperature-dependent high-resolution angle-resolved photoemission spectroscopy. The experimental results show that the complicated magnetic structure of CeSb strongly relates to its unique electronic structure as predicted by the anisotropic p-f mixing model. © 1998 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0304-8853(97)00440-X

Arpes study of the superconducting gap and pseudogap in Bi2Sr2CaCu2O8+x

H. Ding, J. C. Campuzano, M. R. Norman, M. Randeria, T. Yokoya, T. Takahashi, T. Takeuchi, T. Mochiku, K. Kadowaki, P. Guptasarma, D. G. Hinks

Journal of Physics and Chemistry of Solids 59 ( 10-12 ) 1888 - 1891 1998

Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier Ltd

In this paper, we review some of our ARPES results on the superconducting and pseudo gaps in Bi2Sr2CaCu2O8+x. We find that optimally and overdoped samples exhibit a d-wave gap, which closes at the same temperature, Tc, for all k points. In underdoped samples, a leading edge gap is found up to a temperature T* &gt
Tc. We find that T* scales with the maximum low temperature gap, increasing as the doping is reduced. The momentum dependence of the pseudogap is similar to that of the superconducting gap
however, the pseudogap closes at different temperatures for different k points. © 1998 Published by Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(98)00136-X

Unusual dispersion and line shape of the superconducting state spectra of Bi2Sr2CaCu2O8 + δ

M. R. Norman, H. Ding, J. C. Campuzano, T. Takeuchi, M. Randeria, T. Yokoya, T. Takahashi, T. Mochiku, K. Kadowaki

Physical Review Letters 79 ( 18 ) 3506 - 3509 1997.11

Language：English Publishing type：Research paper (scientific journal)

Photoemission spectra of Bi2Sr2CaCu2O8 + δ below Tc show two features near the (π, 0) point of the zone: a sharp peak at low energy and a higher binding energy hump. We find that the sharp peak persists at low energy even as one moves towards (0, 0), while the broad hump shows significant dispersion which correlates well with the normal state dispersion. We argue that these features are naturally explained by the interaction of electrons with a sharp mode which appears only below Tc, and speculate that the latter may be related to the resonance seen in recent neutron data. © 1997 American Physical Society.

DOI： 10.1103/PhysRevLett.79.3506

Evolution of the fermi surface with carrier concentration in Bi2Sr2CaCu2O8+δ

H. Ding, M. R. Norman, T. Yokoya, T. Takeuchi, M. Randeria, J. C. Campuzano, T. Takahashi, T. Mochiku, K. Kadowaki

Physical Review Letters 78 ( 13 ) 2628 - 2631 1997.3

Language：English Publishing type：Research paper (scientific journal)

We show, by use of angle-resolved photoemission spectroscopy, that underdoped Bi2Sr2CaCu2O8+δ appears to have a large Fermi surface centered at (π, π), even for samples with a Tc as low as 15 K. No clear evidence of a Fermi surface pocket around (π/2, π/2) has been found. These conclusions are based on a determination of the minimum gap locus in the pseudogap regime Tc &lt
T &lt
T*, which is found to coincide with the locus of gapless excitations in momentum space (Fermi surface) determined above T* . These results suggest that the pseudogap is more likely of precursor pairing rather than magnetic origin. © 1997 American Physical Society.

DOI： 10.1103/PhysRevLett.78.2628

Extended Van Hove singularity in a noncuprate layered superconductor Sr2RuO4 - Reply Reviewed

T Yokoya, A Chainani, T Takahashi, H KatayamaYoshida, M Kasai, Y Tokura

PHYSICAL REVIEW LETTERS 78 ( 11 ) 2272 - 2272 1997.3

Comment on “extended van hove singularity in a noncuprate layered superconductor Sr2RuO4”

A. P. Mackenzie, S. R. Julian, G. G. Lonzarich, Y. Maeno, T. Fujita

Physical Review Letters 78 ( 11 ) 2271 1997

Angle-resolved photoemission study of Sr2RuO4; An extended van-Hove singularity in non-cuprate superconductor

T. Takahashi, T. Yokoya, A. Chainani, H. Ding, J. C. Campuzano, M. Kasai, Y. Tokura

Physica C: Superconductivity and its Applications 282-287 ( 1 ) 218 - 221 1997

Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier

We have performed high-resolution angle-resolved photoemission spectroscopy of a non-cuprate layered superconductor Sr2RuO4 to study the electronic structure near the Fermi level (EF). The result shows an extended van Hove singularity (vHs) along the Ru-O bonding direction just like the cuprate, regardless of the character of the electronic states at EF. This suggests that the extended vHs is a general electronic structure of a two dimensional correlated d-electron metal. We also have found that the observed Fermi surface topology (one electron-like Fermi surface sheet centered at Γ point and two hole-like ones centered at X point) is qualitatively different from the calculated Fermi surface topology (two electron-like Fermi sheets centered at Γ point and one hole-like one centered at X point) in the consequence of the existence of the vHs.

DOI： 10.1016/S0921-4534(97)00208-6

Electronic Band Structure of SrCuO2

Naoyuki Nagasako, Tamio Oguchi, Hideki Fujisawa, Osamu Akaki, Takayoshi Yokoya, Takashi Takahashi, Masayuki Tanaka, Masashi Hasegawa, Humihiko Takei

Journal of the Physical Society of Japan 66 ( 6 ) 1756 - 1761 1997

Language：English Publishing type：Research paper (scientific journal) Publisher：Physical Society of Japan

The energy band structure of a quasi one-dimensional cuprate SrCuO2 is studied by angle-resolved photoemission spectroscopy and the linear augmented-plane-wave band structure calculation within local density approximation. It was found that the band structure has strongly one-dimensional nature with a finite dispersion along the Cu-O chains. The calculated band structure of SrCuO2 is in good agreement with the angle-resolved photoemission result except for the region near the Fermi level.

DOI： 10.1143/JPSJ.66.1756

Unconventional spectral changes across the metal-insulator transition

O. Akaki, A. Chainani, T. Yokoya, H. Fujisawa, T. Takahashi, M. Onoda

Physical Review B - Condensed Matter and Materials Physics 56 ( 19 ) 12050 - 12053 1997

Language：English Publishing type：Research paper (scientific journal)

We study the change in the electronic structure of (Formula presented) across the δ-controlled metal-insulator (M-I) transition. X-ray-absorption spectroscopy shows the formation of hole states with substantial (Formula presented) character as a function of δ. Ultraviolet photoemission spectroscopy confirms the gap (Formula presented) closure across the M-I transition due to the doped hole states. The results show that a Mott-Hubbard system can have a charge-transfer character due to the strong hybridization, indicating a very mixed character ground state. © 1997 The American Physical Society.

DOI： 10.1103/PhysRevB.56.12050

ARPES studies of Pb substituted Bi2201 compounds

T. Takeuchi, H. Ding, J. C. Campuzano, T. Yokoya, T. Takahashi, I. Chong, T. Terashima, M. Takano

Physica C: Superconductivity and its Applications 282-287 ( 2 ) 999 - 1000 1997

Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier

We report angle-resolved photoemission of overdoped Bi1.80Pb0.38Sr2.01CuO6 with different Tc's of 4, 8, and 20 K. All samples show similar large Fermi surfaces, centered at (π, π). However, the spectral function linewidth decreases as doping increases. The 4K sample is found to have the typical Fermi liquid linewidth. Comparison is also made to the two layer Bi2Sr2CaCu2O8 compound.

DOI： 10.1016/S0921-4534(97)00572-8

Angle-resolved photoemission study of the ladder compound

T. Takahashi, T. Yokoya, A. Ashihara, O. Akaki, H. Fujisawa, A. Chainani, M. Uehara, T. Nagata, J. Akimitsu

Physical Review B - Condensed Matter and Materials Physics 56 ( 13 ) 7870 - 7873 1997

Language：English Publishing type：Research paper (scientific journal)

Angle-resolved photoemission measurements have been performed on (Formula presented) which consists of two-leg (Formula presented) ladders and edge-sharing (Formula presented) chains. The data are consistent with the existence of a dispersive band near the Fermi level which has the periodicity of the ladder sublattice. The band shows a minimum binding energy at (Formula presented) suggesting that a Mott-Hubbard gap opens on the ladder. The experimental result shows a qualitative agreement with the (Formula presented) ladder calculation. It was also found that replacement of Sr with Ca causes a transfer of holes from the chain to the ladder. © 1997 The American Physical Society.

DOI： 10.1103/PhysRevB.56.7870

High-resolution temperature-dependent valence-band photoemission spectroscopy of gold:mNature of electron-phonon coupling

T. Yokoya, A. Chainani, T. Takahashi

Physical Review B - Condensed Matter and Materials Physics 55 ( 9 ) 5574 - 5577 1997

Language：English Publishing type：Research paper (scientific journal)

Using high-resolution temperature-dependent (20-400 K) valence-band photoemission spectroscopy, we study the nature of electron-phonon coupling in polycrystalline gold. The valence band shows systematic broadening for increasing temperatures which is well-reproduced by a Gaussian broadening of the 20 K spectrum. It depends linearly on temperature and is found to be excessive (∼9 (Formula presented) T). The results suggest the existence of anharmonic lattice vibrations and a quadratic electron-phonon coupling, similar to the case of aluminum 2p core levels. © 1997 The American Physical Society.

DOI： 10.1103/PhysRevB.55.5574

Yokoya et al. Reply

T. Yokoya, A. Chainani, T. Takahashi, T. Katayama-Yoshida, M. Kasai, Y. Tokura

Physical Review Letters 78 ( 11 ) 2272 1997

High-resolution angle-resolved photoemission study of CeP: Narrow-band formation of 4f electrons

H. Kumigashira, S. Yang, T. Yokoya, A. Chainani, T. Takahashi, A. Uesawa, T. Suzuki

Physical Review B - Condensed Matter and Materials Physics 55 ( 6 ) R3355 - R3357 1997

Language：English Publishing type：Research paper (scientific journal)

High-resolution angle-resolved photoemission spectroscopy at low temperature has been performed on the low-carrier Kondo material CeP. It was found that the p-f bonding state with a dominant 4f character exhibits a finite-energy dispersion of about 40 meV along the ΓX direction in the Brillouin zone. This implies that 4f electrons in CeP form a narrow band owing to the strong anisotropic p-f and d-f mixing. © 1997 The American Physical Society.

DOI： 10.1103/PhysRevB.55.R3355

Paramagnetic-to-antiferroparamagnetic phase transition of CeSb studied by high-resolution angle-resolved photoemission

H. Kumigashira, H. Kim, A. Ashihara, A. Chainani, T. Yokoya, T. Takahashi, A. Uesawa, T. Suzuki

Physical Review B - Condensed Matter and Materials Physics 56 ( 21 ) 13654 - 13657 1997

Language：English Publishing type：Research paper (scientific journal)

The paramagnetic-to-antiferroparamagnetic phase transition of CeSb has been studied by high-resolution angle-resolved photoemission spectroscopy. The hole and electron pockets are clearly observed at the Brillouin-zone center and boundary, respectively. The volume of both hole and electron pockets increases when the temperature is lowered across the magnetic phase transition (Formula presented). The observed change of Fermi-surface topology is discussed in terms of the temperature-dependent anisotropic (Formula presented) mixing. © 1997 The American Physical Society.

DOI： 10.1103/PhysRevB.56.13654

Angle-resolved photoemission spectroscopy study of the superconducting gap anisotropy in Bi2Sr2CaCu2O8+x Reviewed

H Ding, MR Norman, JC Campuzano, M Randeria, AF Bellman, T Yokoya, T Takahashi, T Mochiku, K Kadowaki

PHYSICAL REVIEW B 54 ( 14 ) R9678 - R9681 1996.10

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

We report measurements of the momentum dependence of die superconducting gap in Bi2Sr2CaCu2O8+x with angle-resolved photoemission spectroscopy using a dense sampling of the Brillouin zone in the vicinity of the Fermi surface. In the Y quadrant of the zone, where there are no complications from ghost bands caused by the superlattice, we find a gap function consistent within error bats to the form cos(k(x))-cos(k(y)) expected for a d-wave order parameter. Similar results are found in the X quadrant with the photon polarization chosen to enhance main band emission over that due to ghost bands.

ARPES study of the superconducting gap anisotropy in Bi₂Sr₂CaCu₂O_8+δ (共著)

Ding H., Norman M. R., Campuzano J. C., Randeria M., Bellman A. F., Yokoya T., Takahashi T., Mochiku T., Kadowaki K.

Phys. Rev. B 54 ( 14 ) R9678-R9681 - R9681 1996.10

High-resolution angle-resolved photoemission spectroscopy of the momentum dependence of the superconducting gap in Bi2Sr2CaCu2O8

Yokoya T., Takahashi T., Mochiku T., Kadowaki K.

Physical review B 53 ( 21 ) 14055 - 14057 1996.6

Direct observation of particle-hole mixing in the superconducting state by angle-resolved photoemission(共著)

Campuzano J. C., Ding H., Norman M. R., Randeira M., Bellman A. F., Yokoya T., Takahashi T., Katayama-Yoshida H., Mochiku T., Kadowaki K.

Phys. Rev. B 53 ( 22 ) R14737-R14740 - R14740 1996.6

High-resolution photoemission study of CeRu2: The dual character of 4f electrons

Yang S. -H., Kumigashira H., Yokoya T., Chainani A., Takahashi T., Takeya H., Kadowaki K.

Physical review B 53 ( 18 ) R11946 - R11948 1996.5

Extended van hove singularity in a noncuprate layered superconductor sr2RuO4

T. Yokoya, A. Chainani, T. Takahashi, H. Katayama-Yoshida, M. Kasai, Y. Tokura

Physical Review Letters 76 ( 16 ) 3009 - 3012 1996.4

Language：English Publishing type：Research paper (scientific journal)

Angle-resolved photoemission spectroscopy was performed on a ruthenate superconductor Sr2RuO4(Tc∼1 K) which has the same crystal structure as La2CuO4, but with RuO2 layers replacing CuO2 layers. Sr2RuO4 shows an extended Van Hove singularity (VHS) at 20±2 meV below EF just like the cuprates, regardless of the character of the electronic states at EF. This suggests that the extended VHS is a general feature of a two-dimensional correlated d-electron metal. The observed Fermi surface is in contrast to local-density-approximation calculations. © 1996 The American Physical Society.

DOI： 10.1103/PhysRevLett.76.3009

Dual character of 5f electron in UPd₂Al₃ observed by high-resolution photoemission spectroscopy (共著)

Takahashi Takashi, Sato Noriaki, Yokoya Takayoshi, Chainani Ashish, Morimoto Takayuki, Komatsubara Takemi

J. Phys. Soc. Jpn. 65 ( 1 ) 156 - 159 1996.1

Language：English Publisher：INSTITUTE OF PURE AND APPLIED PHYSICS

Comparative high-resolution temperature-dependent photoemission measurements were performed for UPd2Al3 and UPd3. The photoemission spectrum near E F of UPd2Al3 consists of a sharp peak at E F and a broad band at 0.5–1.0 eV binding energy, both features having U 5f character. The sharp feature at E F shows good agreement with a recent band structure calculation, while the broad band matches well with the U 5f-derived localized feature in UPd3, indicating the dual character of 5f electrons in UPd2Al3. The intensity of the broad band relative to the E F-peak increases with increasing temperature, suggesting that the localized character of 5f electrons in UPd2Al3 increases at higher temperatures. These observations are consistent with the transport and magnetic measurements which imply a temperature-dependent dual (itinerant and localized) character of U 5f electrons in UPd2Al3.

DOI： 10.1143/JPSJ.65.156

Other Link： https://jlc.jst.go.jp/DN/JALC/00038017919?from=CiNii

"Anisotropy and Doping Dependence of Superconducting Gap in Bi₂Sr₂CaCuO₈" Reviewed

T.Yokoya, H.Ding, T.Takeuchi, J.C.Campuzano, M.Randeria, M.R.Norman, T.Moshiku, K.Kadowaki, 門脇, 和男

Advances in Superconductivity (IX) 133-137 1996.1

Extended van-Hove singularity in non-cuprate layered perovskite superconductor Sr2RuO4 Reviewed

T Takahashi, T Yokoya, A Chainani, H KatayamaYoshida, M Kasai, Y Tokura

SPECTROSCOPIC STUDIES OF SUPERCONDUCTORS, PTS A AND B 2696 507 - 512 1996

ARPES studies of the superconducting gap in high temperature superconductors Reviewed

H Ding, MR Norman, J Giapintzakis, JC Campuzano, H Claus, H Wuhl, M Randeria, A Bellman, T Yokoya, T Takahashi, T Mochiku, K Kadowaki, DM Ginsberg

SPECTROSCOPIC STUDIES OF SUPERCONDUCTORS, PTS A AND B 2696 496 - 506 1996

High-resolution photoemission study of C: The dual character of electrons

S. H. Yang, H. Kumigashira, T. Yokoya, A. Chainani, T. Takahashi, H. Takeya

Physical Review B - Condensed Matter and Materials Physics 53 ( 18 ) R11946 - R11948 1996

Language：English Publishing type：Research paper (scientific journal)

High-resolution low-temperature photoemission spectroscopy was performed for Ce(Formula presented) to study the nature of low-lying excitations around the Fermi level ((Formula presented)). It was found that the photoemission spectrum near (Formula presented) shows two well-resolved peaks at and about 270 meV below (Formula presented), which are ascribed to the tail of the Ce 4 (Formula presented) final state located just above (Formula presented) and its spin-orbit satellite ((Formula presented) meV), respectively. This suggests that the many-body-correlation effect is necessary for understanding the electronic structure near (Formula presented) of Ce(Formula presented), although the Ce (Formula presented) electron exhibits a substantially itinerant character through the strong hybridization with the conduction electrons. © 1996 The American Physical Society.

DOI： 10.1103/PhysRevB.53.R11946

High-energy-resolution photoemission study of and

H. Kumigashira, A. Chainani, T. Yokoya, O. Akaki, T. Takahashi, M. Ito, M. Kasaya, O. Sakai

Physical Review B - Condensed Matter and Materials Physics 53 ( 5 ) 2565 - 2568 1996

Language：English Publishing type：Research paper (scientific journal)

We studied the electronic structure of isostructural (Formula presented) and (Formula presented) using high-resolution low-temperature photoemission spectroscopy. (Formula presented) is a typical valence fluctuation material, while (Formula presented) is a low (Formula presented)(∼10 K) heavy-fermion system. The valence-band spectra show that Ni 3d derived states appear at energies closer to the Fermi level ((Formula presented)) compared to the Pt 5d derived states. The near-(Formula presented) spectra exhibit the characteristic spin-orbit splitting of 4f states (4(Formula presented) and 4(Formula presented)), with higher 4(Formula presented) intensity in (Formula presented) than in (Formula presented). Numerical simulations based on the single-impurity Anderson model show that the hybridization between the conduction band and f electrons is stronger in (Formula presented) than in (Formula presented). This is qualitatively understood in terms of proximity of Ni 3d states to the f level. The obtained spectroscopic results were discussed in comparison with transport and magnetic measurements. © 1996 The American Physical Society.

DOI： 10.1103/PhysRevB.53.2565

Cooperative effects of electron correlation and charge ordering on the metal-insulator transition in quasi-one-dimensional deuteratedCu

T. Takahashi, T. Yokoya, A. Chainani, H. Kumigashira, O. Akaki, R. Kato

Physical Review B - Condensed Matter and Materials Physics 53 ( 4 ) 1790 - 1794 1996

Language：English Publishing type：Research paper (scientific journal)

We study the changes in the electronic structure of a quasi-one-dimensional organic conductor, deuterated (Formula presented)Cu, across the CDW/M-I (charge-density-wave/metal-insulator) transition at (Formula presented) K, using temperature-dependent (30-300 K) high-resolution photoemission spectroscopy. Above the mean-field (MF) transition temperature ((Formula presented)), the system shows typical metallic behavior with small but finite density of states at (Formula presented). The spectral weight in the vicinity of (Formula presented) decreases systematically upon decreasing temperature below (Formula presented) even in the metallic phase, resulting in a pseudogap formation above (Formula presented). This behavior continues across (Formula presented) and causes the M-I transition. The spectral weight is transferred to energies much larger than the gap, indicating a cooperative effect of strong electron correlation and structural changes which imply decreases in interchain coupling across the M-I transition. © 1996 The American Physical Society.

DOI： 10.1103/PhysRevB.53.1790

Evidence for correlation effects i from resonant and x-ray photoemission spectroscopy

T. Yokoya, A. Chainani, T. Takahashi, H. Katayama-Yoshida, M. Kasai, Y. Tokura

Physical Review B - Condensed Matter and Materials Physics 53 ( 13 ) 8151 - 8154 1996

Language：English Publishing type：Research paper (scientific journal)

We study the electronic structure of (Formula presented)(Formula presented), a noncuprate layered superconductor ((Formula presented)=0.93 K), using electron spectroscopy. X-ray photoemission spectroscopy shows that the single particle occupied density of states (DOS) is in fair agreement with the calculated DOS. However, resonant photoemission spectroscopy across the Ru 4p-4d threshold establishes the existence of a correlation satellite to the Ru 4d band. The results indicate substantial charge-transfer character at the Fermi level, with on-site correlations (Formula presented) comparable in magnitude to the Ru-O hopping integral, like the cuprates. © 1996 The American Physical Society.

DOI： 10.1103/PhysRevB.53.8151

High-resolution angle-resolved photoemission spectroscopy of CeBi

H. Kumigashira, S. Yang, T. Yokoya, A. Chainani, T. Takahashi, A. Uesawa, T. Suzuki, O. Sakai, Y. Kaneta

Physical Review B - Condensed Matter and Materials Physics 54 ( 13 ) 9341 - 9345 1996

Language：English Publishing type：Research paper (scientific journal)

High-resolution angle-resolved photoemission spectroscopy (HR-ARPES) has been performed on a CeBi single crystal to study the complicated electronic structure near the Fermi level ((Formula presented)). The experimental result was compared with the band-structure calculation based on the p-f mixing model as well as the de Haas-van Alphen (dHvA) effect measurements. It was found that the overall feature of the valence band shows a remarkably good agreement between the experiment and the calculation, suggesting the essential validity of the p-f mixing model. HR-ARPES measurement near (Formula presented) has established the existence of a small electron pocket centered at the M point in the Brillouin zone, supporting the band calculation and the dHvA measurement. HR-ARPES spectra around the Γ point show some dispersive bands near (Formula presented) indicative of a hole pocket centered at the Γ point, though it was not so clearly resolved as the electron pocket due to close proximity of individual bands. These results are consistent with the semimetallic nature of CeBi. The observed quantitative discrepancy between the experiment and the calculation is discussed. © 1996 The American Physical Society.

DOI： 10.1103/PhysRevB.54.9341

High-resolution angle-resolved photoemission spectroscopy of the momentum dependence of the superconducting gap i

T. Yokoya, T. Takahashi, T. Mochiku, K. Kadowaki

Physical Review B - Condensed Matter and Materials Physics 53 ( 21 ) 14055 - 14057 1996

Language：English Publishing type：Research paper (scientific journal)

The momentum dependence of the superconducting gap in (Formula presented)(Formula presented)Ca(Formula presented)(Formula presented) has been studied using high-resolution angle-resolved photoemission spectroscopy with nonpolarized light. The size of the superconducting gap obtained at several points on the Fermi surface shows a linear dependence with respect to the gap function of the (Formula presented) wave defined as (Formula presented). This suggests that the anomaly in the superconducting gap observed with polarized synchrotron-radiation light is not intrinsic, being probably due to the superstructure of the BiO plane, and rather the strong anisotropy of the superconducting gap, like the (Formula presented) symmetry, is intrinsic. © 1996 The American Physical Society.

DOI： 10.1103/PhysRevB.53.14055

Extended van-Hove singularity in Sr2RuO4 observed by angle-resolved photoemission

T. Yokoya, A. Chainani, T. Takahashi, H. Katayama-Yoshida, M. Kasai, Y. Tokura

Physica C: Superconductivity and its Applications 263 ( 1-4 ) 505 - 509 1996

Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier

We have performed low-temperature high-resolution angle-resolved photoemission spectroscopy on a non-cuprate superconductor Sr2RuO4 (Tc ∼ 1 K) which has the same crystal structure as La2CuO4 but with RuO2 layers replacing CuO2. We found that Sr2RuO4 has an extended van-Hove singularity (VHS) close to EF(20 ± 2 meV) like high-Tc cuprates, regardless of the character of the electronic states at EF, suggesting that the extended VHS is a general feature of a two-dimensional d-electron metal. This demands re-examination of the VHS scenario for the superconducting properties of cuprates, while T-linear resistivity seems to be consistent with this result.

DOI： 10.1016/0921-4534(96)00073-1

Electronic excitations in Bi2Sr2 CaCu2O8: Fermi surface, dispersion, and absence of bilayer splitting

H. Ding, A. F. Bellman, J. C. Campuzano, M. Randeria, M. R. Norman, T. Yokoya, T. Takahashi, H. Katayama-Yoshida, T. Mochiku, K. Kadowaki, G. Jennings, G. P. Brivio

Physical Review Letters 76 ( 9 ) 1533 - 1536 1996

Language：English Publishing type：Research paper (scientific journal)

From a detailed study, including polarization dependence, of the normal state angle-resolved photoemission spectra for Bi2Sr2CaCu2O8, we find only one CuO2 band related feature. All other spectral features can be ascribed either to umklapps from the superlattice or to “shadow bands.” Even though the dispersion of the peaks looks like band theory, the line shape is anomalously broad and no evidence is found for bilayer splitting. We argue that the “dip feature” in the spectrum below Tc arises not from bilayer splitting, but rather from many-body effects. © 1996 The American Physical Society.

DOI： 10.1103/PhysRevLett.76.1533

High-Resolution Photoemission Study of UNi2Al3 and URu2Si2

See-Hun Yang, Hiroshi Kumigashira, Takayoshi Yokoya, Ashishi Chainani, Takashi Takahashi, Se-Jung Oh, Noriaki Sato, Takemi Komatsubara

Journal of the Physical Society of Japan 65 ( 8 ) 2685 - 2689 1996

Language：English Publishing type：Research paper (scientific journal) Publisher：Physical Society of Japan

High-resolution low-temperature photoemission spectroscopy has been performed for UNi2Al3 and URu2Si2 to study the nature of 5f electrons in U-based heavy fermion materials. Photoemission spectra of both compounds have a sharp peak at EF and a large broad peak around 2 eV, while only UNi2Al3 exhibits an additional small broad feature at 0.6 eV. The two features in the vicinity of EF (the EF-peak and the small structure at 0.6 eV) are found to have a dominant U 5f character while the prominent peak around 2 eV is due to the Ni 3d or Ru 4d states. Comparison with the band structure calculations and experimental results on UPd2Al3 suggests that the EF-peak is ascribed to itinerant U 5f electrons while the following broad feature at 0.6 eV represents localized f electrons.

DOI： 10.1143/JPSJ.65.2685

MOMENTUM DEPENDENCE OF THE SUPERCONDUCTING GAP IN BI2SR2CACU2O8 (VOL 74, PG 2784, 1995) Reviewed

H DING, JC CAMPUZANO, AF BELLMAN, T YOKOYA, MR NORMAN, M RANDERIA, T TAKAHASHI, H KATAYAMAYOSHIDA, T MOCHIKU, K KADOWAKI, G JENNINGS

PHYSICAL REVIEW LETTERS 75 ( 7 ) 1425 - 1425 1995.8

Angle-resolved and resonant photoemission study of Sr2RuO4

T. Yokoya, A. Chainani, T. Takahashi, H. Katayama-Yoshida, M. Kasai, Y. Tokura

Journal of Physics and Chemistry of Solids 56 ( 12 ) 1885 - 1886 1995

Language：English Publishing type：Research paper (scientific journal)

Resonant and angle-resolved photoemission measurements were performed for the non-cuprate layered superconductor Sr2RuO4 (Tc = 0.93 K). Resonant valence band photoemission spectroscopy shows the existence of a correlation induced satellite to the main d band across the Ru 4p-4d threshold. Angle-resolved measurements show a surprisingly narrow band of about 50 meV width and positioned at 26 meV below the Fermi level, along the ΓX and ΓZ directions. This band, arising fom Ru 4d-O 2p antibonding states, is nearly dispersionless over a wide range of k points about the zone boundary along the Γ-Z direction (Ru-O direction), reminiscent of the extended Van-Hove singularity observed in the cuprates. The results emphasize the presence of strong correlations, substantial charge-transfer character to states at the Fermi level and the existence of a singular flat band near the Fermi-level in this low Tc layered superconductor, just like the cuprates. © 1995.

DOI： 10.1016/0022-3697(95)00193-X

High-resolution photoemission spectroscopy of the Verwey transition in Fe3O4

A. Chainani, T. Yokoya, T. Morimoto, T. Takahashi, S. Todo

Physical Review B 51 ( 24 ) 17976 - 17979 1995

Language：English Publishing type：Research paper (scientific journal)

We study the first-order Verwey transition in single-crystal Fe3O4 using high-resolution temperature-dependent (100-300 K) photoemission spectroscopy. The near-Fermi-level (EF) spectrum exhibits a clear gap of ∼70 meV in the occupied part of the density of states (DOS) in the low-temperature semiconducting phase. The gap is closed above the transition temperature TV=122 K, establishing a metal-semiconductor transition. The Fe 3d derived features arising from Fe3+ (eg2) majority- and Fe2+ (t2g1) minority-spin states responsible for the transition retain their character just above the transition but merge into a single feature by 300 K. Simultaneously, the DOS at EF increases systematically in the metallic phase. The results indicate the existence of, and a gradual change in, the short-range order for T&gt
TV, which causes the nonmetalliclike electrical conductivity observed just above TV. © 1995 The American Physical Society.

DOI： 10.1103/PhysRevB.51.17976

Crystal-field splitting in CeB6 as observed by high-resolution photoemission

T. Takahashi, T. Morimoto, T. Yokoya, S. Kunii, T. Komatsubara, O. Sakai

Physical Review B 52 ( 13 ) 9140 - 9142 1995

Language：English Publishing type：Research paper (scientific journal)

Comparative high-resolution photoemission measurement was performed for CeB6 and CeCu6 to study the electronic structure near EF. The photoemission spectra near EF of both compounds show a characteristic satellite at 270 meV from EF due to the spin-orbit splitting of a Ce 4f electron (4f7/2-4f5/2), while the spectrum of CeB6 exhibits an additional small feature around 70 meV. Model calculations based on the noncrossing approximation show that this satellite is due to the crystal-field splitting (7-8) of the Ce 4f5/2 state in CeB6. © 1995 The American Physical Society.

DOI： 10.1103/PhysRevB.52.9140

Electronic structure of the quasi-one-dimensional organic conductors DCNQI (N,N′-dicyanoquinonediimine)-Cu salts

Akinori Tanaka, Ashish Chainani, Takayoshi Yokoya, Takashi Takahashi, Takahumi Miyazaki, Shinji Hasegawa, Takehiko Mori

Physical Review B 52 ( 11 ) 7951 - 7959 1995

Language：English Publishing type：Research paper (scientific journal)

A comparative study of the electronic structure of in situ synthesized quasi-one-dimensional organic conductors (DMe-DCNQI)2Cu and (MeBr-DCNQI)2Cu has been carried out using various techniques of electron spectroscopy, where DMe-DCNQI and MeBr-DCNQI are 2,5-dimethyl-N,N′-dicyanoquinonediimine and 2,5-methylbromine-N,N′-dicyanoquinonediimine, respectively. From the photon-energy dependence of the valence-band photoemission spectra obtained using synchrotron radiation, the origins of each observed feature are unambiguously characterized. While the feature at the Fermi level is primarily derived from π-bonded C and N 2p states, the contribution of Cu 3d states at the Fermi level is larger in the (MeBr-DCNQI)2Cu compared to the (DMe-DCNQI)2Cu. X-ray photoemission spectra of the valence band imply extensive hybridization of the Cu 3d states with C and N 2p states near the Fermi level. Line-shape analyses of the Cu 2p core-level spectra show that the ratio of Cu2+ to Cu+ is higher in (MeBr-DCNQI)2Cu compared to (DMe-DCNQI)2Cu, with the ratio being closer to 1:2 for (MeBr-DCNQI)2Cu. From a comparison of C KVV and Cu LVV Auger spectra with the self-convolution of the valence-band spectra, it is found that the effective on-site Coulomb correlation energies between the valence electrons are high on C sites as well as Cu sites in both salts, with U(pp)=6.5 eV and U(dd)=8.0 eV, respectively. In conjunction with core-level spectra, the spectra indicate that the on-site Coulomb correlation, the hybridization strength, and the charge-transfer energy between the Cu 3d and N 2p ligands are very similar in the two salts. The metal-insulator transition in (MeBr-DCNQI)2Cu at 160 K is then facilitated by the proximity of the Cu2+-to-Cu+ ratio to 1:2 supporting charge disproportion, while deviation from it stabilizes the metallic phase in (DMe-DCNQI)2Cu down to very low temperatures. © 1995 The American Physical Society.

DOI： 10.1103/PhysRevB.52.7951

Momentum distribution sum rule for angle-resolved photoemission

Mohit Randeria, Hong Ding, J. C. Campuzano, A. Bellman, G. Jennings, T. Yokoya, T. Takahashi, H. Katayama-Yoshida, T. Mochiku, K. Kadowaki

Physical Review Letters 74 ( 24 ) 4951 - 4954 1995

Language：English Publishing type：Research paper (scientific journal)

Within a spectral function A(k,ω) interpretation of angle-resolved photoemission (ARPES), the intensity obeys the sum rule dω fωA(k,ω)=nk, where f is the Fermi function and nk the momentum distribution. We show the usefulness of this sum rule for analyzing ARPES data. The integrated intensity in Bi2Sr2CaCu2O8 at kF is independent of temperature even though the spectrum is strongly T dependent. We also demonstrate the possibility of measuring nk using ARPES for YBa2Cu4O8. © 1995 The American Physical Society.

DOI： 10.1103/PhysRevLett.74.4951

Anisotropy of the electronic structure and superconducting gap in Bi2Sr2CaCu2O8

T. Yokoya, T. Takahashi, T. Mochiku, K. Kadowaki

Physical Review B 51 ( 6 ) 3945 - 3948 1995

Language：English Publishing type：Research paper (scientific journal)

Existence of a large anisotropy of the superconducting energy gap in Bi2Sr2CaCu2O8 was confirmed by angle-resolved photoemission. The gap size, Δ (the full superconducting energy gap being 2Δ), is 0-4 meV for the ΓX and ΓY directions while that of the ΓM̄ direction, 45°away from the former two, is 17-19 meV. Comparison with previous reports was made with regard to the symmetry of the superconducting order parameter. It was also found that the electronic structure of the normal state in the ΓY direction is considerably modified by the incommensurate superstructure in the BiO plane. In contrast, the size of superconducting gap is identical within the present accuracy for both ΓX and ΓY directions, suggesting that the superstructure has a negligible effect on the size of the superconducting energy gap. © 1995 The American Physical Society.

DOI： 10.1103/PhysRevB.51.3945

Arpes studies in the normal and superconducting state of Bi2Sr2CaCu2O8

J. C. Campuzano, H. Ding, A. Bellman, M. R. Norman, M. Randeria, G. Jennings, T. Yokoya, T. Takahashi, H. Katayama-Yoshida, T. Mochiku, K. Kadowaki

Journal of Physics and Chemistry of Solids 56 ( 12 ) 1863 - 1864 1995

High-energy resolution photoemission study of CsxC60. Comparative study with KxC60

T. Takahashi, T. Morimoto, T. Yokoya

Physica C: Superconductivity and its applications 232 ( 3-4 ) 227 - 232 1994.11

Language：English Publishing type：Research paper (scientific journal)

Comparative high-energy resolution (ΔE ∼ 30 meV) photoemission spectroscopy has been performed for CsxC60 and KxC60 to study the difference and similarity in the electronic structure near the Fermi level (EF) between the superconducting and the non-superconducting alkali-doped C60. It was found that CsxC60 (0≤x≤6) is a semiconductor or an insulator in the whole composition range except for 0&lt
x≤1 where the Cs doped C60 appears to be metallic with a finite density of states at EF. This shows a sharp contrast with the case of KxC60 which exhibits a relatively large density of states at EF in the composition of 0&lt
x&lt
4 with the largest intensity around x = 3. © 1994.

DOI： 10.1016/0921-4534(94)90780-3

HIGH-ENERGY-RESOLUTION PHOTOEMISSION-STUDY OF SINGLE-CRYSTAL CESIX (X=1.86 AND 1.7) - A TEST OF THE KONDO MODEL Reviewed

T TAKAHASHI, T MORIMOTO, T YOKOYA, M KOHGI, T SATOH

PHYSICAL REVIEW B 49 ( 22 ) 15688 - 15691 1994.6

Language：English Publishing type：Research paper (scientific journal) Publisher：AMERICAN PHYSICAL SOC

Photoemission spectra of single crystal CeSi(x) (x = 1.86; nonmagnetic, T(K) approximately 20 K, and x = 1.7; ferromagnetic, T(c) = 13.7 K) were measured at very high-energy resolution and low temperature. The Fermi-edge band in the photoemission spectrum of CeSi1.86 is narrower and closer to E(F) Compared with the previous reports on polycrystals, showing closer resemblance to the calculation based on the Kondo model. The intensity of the Fermi-level peak is drastically reduced in going from CeSi1.86 to CeSi1.7, which is in good qualitative agreement with the Kondo model.

Photoemission study of phase transition of solid C60

T. Takahashi, T. Morimoto, A. Ito, T. Yokoya

Solid State Communications 92 ( 4 ) 331 - 335 1994

Language：English Publishing type：Research paper (scientific journal)

The change of electronic structure due to the phase transition in solid C60 from face-centered-cubic to simple-cubic was observed by photoemission spectroscopy. The effect of disorder in solid C60 to the electronic structure was also studied by comparing the photoemission spectra of a highly disordered C60 film prepared by vapor deposition on a cold (≈20K) substrate and a single crystal C60 film. © 1994.

DOI： 10.1016/0038-1098(94)90711-0

dx2-y2-like superconducting gap in Bi2Sr2CaCu2O6 observed by angle-resolved photoemission

T. Yokoya, T. Takahashi, T. Mochiku, K. Kadowaki

Physica C: Superconductivity and its applications 235-240 ( 2 ) 1013 - 1014 1994

Language：English Publishing type：Research paper (scientific journal)

A large anisotropy of superconducting energy gap in Bi2Sr2CaCu2O8 was directly observed by high-energy resolution angle-resolved photoemission spectroscopy with a high-quality twin-free single crystal. The superconducting gap (Δ) observed in the ΓM direction is 17-19 meV while that in the ΓX (or ΓY) direction is 0-2 meV, suggesting the dx2-y2 superconducting order parameter. © 1994.

DOI： 10.1016/0921-4534(94)91731-0

Anomalous band folding due to the BiO superstructure in Bi2Sr2CaCu2O8 studied by angle-resolved photoemission

T. Yokoya, T. Takahashi, T. Mochiku, K. Kadowaki

Physical Review B 50 ( 14 ) 10225 - 10229 1994

Language：English Publishing type：Research paper (scientific journal)

Angle-resolved photoemission measurement has been performed on a twin-free Bi2Sr2CaCu2O8 single crystal to study the band-folding effect due to the BiO superstructure. It was found that the superstructure running along the b axis considerably modulates the electronic structure near the Fermi level (EF) in that direction, almost smearing out the dispersive feature of a band which crosses the Fermi level. In contrast with the energy region near EF, the main body of the valence band (1 to 7 eV in binding energy) shows a comparable but different energy dispersion between the a- and b-axis directions. This indicates that the band-folding effect may be different between the near-EF region and the whole valence band in Bi2Sr2CaCu2O8. © 1994 The American Physical Society.

DOI： 10.1103/PhysRevB.50.10225

High-resolution temperature-dependent photoemission spectroscopy of FeSi: Evidence for localized states

A. Chainani, T. Yokoya, T. Morimoto, T. Takahashi, S. Yoshii, M. Kasaya

Physical Review B 50 ( 12 ) 8915 - 8917 1994

Language：English Publishing type：Research paper (scientific journal)

We report the high-resolution temperature-dependent photoemission spectra of FeSi. We measure an extremely small gap of less than 5 meV in contrast to electrical-resistivity, magnetic-susceptibility, and optical-conductivity measurements, which show an activation energy of about 60 meV. The difference in the gap value is attributed to localized states near the Fermi level, which effectively increase the activation energy associated with a charge or spin excitation as observed in conductivity and susceptibility studies. The density of states at the Fermi level increase systematically on increasing temperature and result in a transition to a metallic phase at 300 K. The data suggest that localization effects play an important role in the ground-state properties of FeSi. © 1994 The American Physical Society.

DOI： 10.1103/PhysRevB.50.8915

SUPERCONDUCTING ENERGY-GAP IN BI2SR2CACU2O8 OBSERVED BY HIGH-RESOLUTION PHOTOEMISSION SPECTROSCOPY Reviewed

T YOKOYA, T TAKAHASHI, A FUJIWARA, Y KOIKE

SOLID STATE COMMUNICATIONS 87 ( 6 ) 553 - 556 1993.8

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

A high-energy resolved low-temperature photoemission spectrometer of angle-integrated mode was constructed to study the detailed electronic structure near the Fermi level of novel materials, in particular, cuprate high-Tc superconductors. Photoemission measurement with this spectrometer on a Bi2Sr2CaCu2O8 polycrystal clearly showed a transfer of spectral. weight from the vicinity of the Fermi level to the higher binding-energy region below the superconducting transition temperature, indicating opening of the superconducting gap. Numerical simulation for both normal and superconducting states showed that the superconducting gap parameter (DELTA) of 18 meV with dx2-y2 symmetry gives a most reasonable fitting.

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