Faculty Profiles - KOBAYASHI Tatsuo

Papers - KOBAYASHI Tatsuo

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Resistance Anomalies Accompanying Crossover from Heavy-Fermion Regime to Intermediate-Valence Regime: A Study of Cu-Ni Substitution and Pressure Effects on CeCu2Si2 Reviewed

Yoichi Ikeda, Yuzo Ito, Shingo Araki, Tatsuo C. Kobayashi, Hideki Yoshizawa

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 84 ( 2 ) 024702-1-024702-9 2015.2

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

We present detailed investigations on the effects of Ni substitution (x) on Ce(Cu1-xNix)(2)Si-2 with x < 0.3, and a pressure (p) study for x = 0.04(2), 0.05(2), and 0.09(2) by resistivity measurements. The Cu-Ni substitution suppresses both antiferromagnetic fluctuations and superconductivity in CeCu2Si2, and leads to a dramatic crossover from the heavy-fermion (HF) regime to the intermediate-valence(IV) regime with progressive Ni doping. The crossover can be characterized by a marked increase in Kondo temperature evaluated from resistivity, specific heat, and susceptibility measurements. In high-pressure resistivity measurements, the Ni-doped system exhibits similar anomalies to those observed in CeCu2Si2, namely, T-linear dependence, an enhancement of residual resistivity, and a marked decrease in the A coefficient of the T-2-resistivity. Furthermore, we observed an unexpected upturn anomaly in resistivity in the Ni-doped system under high pressure. By constructing the x-p phase diagram of Ce(Cu1-xNix)(2)Si-2, we demonstrate that the crossover from the HF regime to the IV regime can be associated with these characteristic anomalies in resistivity.

DOI： 10.7566/JPSJ.84.024702

Characterization of Ferromagnetic Order in CePd2P2 Reviewed

Y. Ikeda, H. Yoshizawa, S. Konishi, S. Araki, T. C. Kobayashi, T. Yokoo, S. Ito

INTERNATIONAL CONFERENCE ON STRONGLY CORRELATED ELECTRON SYSTEMS 2014 (SCES2014) 592 012013-1-012013-6 2015

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

We examine magnetic ordering in CePd2P2 by neutron-scattering experiment and confirmed a ferromagnetic order at low temperatures. From the neutron-scattering data, the ordered moment is evaluated to be similar to 1.3(3) mu(B) per Ce ion at 3 K and is considered to be parallel to the c-axis. In addition, no evidence of the change in the magnetic structure was observed below T-C within the experimental accuracy. In DC magnetization measurement with an as-grown powder sample and a magnetically aligned sample, a clear magnetic anisotropy in CePd2P2 was observed in both ferromagnetic and paramagnetic phases. To obtain further insight to the large Curie temperature T-C of CePd2P2, we have examined the magnetic properties of GdPd2P2 as a reference material. In DC magnetization measurement, successive magnetic transitions at T-I = 10.6 K and T-II = 7.4 K were observed in GdPd2P2. By comparing the transition temperatures of the Ce and Gd compounds, we argue the origin of the large Curie temperature of CePd2P2.

DOI： 10.1088/1742-6596/592/1/012013

Simultaneous suppression of superconductivity and structural phase transition under pressure in Ca-10(Ir4As8)(Fe2-xIrxAs2)(5) Reviewed

Shunsaku Kitagawa, Shingo Araki, Tatsuo C. Kobayashi, Hiroyuki Ishii, Kazunori Fujimura, Daisuke Mitsuoka, Kazutaka Kudo, Minoru Nohara

PHYSICAL REVIEW B 90 ( 22 ) 224513-1-224513-4 2014.12

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

We measured the pressure dependence of in-plane resistivity rho(ab) in the recently discovered iron-based superconductor Ca-10(Ir4As8)(Fe2-xIrxAs2)(5), which shows a unique structural phase transition in the absence of magnetic ordering, with a superconducting transition temperature T-c = 16 K and a structural phase transition temperature T-s similar or equal to 100 K at ambient pressure. T-c and T-s are suppressed on applying pressure and disappear at approximately 0.5 GPa, suggesting a relationship between superconductivity and structure. Ca-10(Ir4As8)(Fe2-xIrxAs2)(5) is a rather rare example in which the superconductivity appears only in a low-temperature ordered phase. The fact that the change in the crystal structure is directly linked with superconductivity suggests that the crystal structure as well as magnetism are important factors governing superconductivity in iron pnictides.

DOI： 10.1103/PhysRevB.90.224513

Role of Valence Fluctuations in the Superconductivity of Ce122 Compounds Reviewed

H. Yamaoka, Y. Ikeda, I. Jarrige, N. Tsujii, Y. Zekko, Y. Yamamoto, J. Mizuki, J. -F. Lin, N. Hiraoka, H. Ishii, K. -D. Tsuei, T. C. Kobayashi, F. Honda, Y. Onuki

PHYSICAL REVIEW LETTERS 113 ( 8 ) 86403-1-86403-6 2014.8

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

Pressure dependence of the Ce valence in CeCu2Ge2 has been measured up to 24 GPa at 300 K and to 17 GPa at 18-20Kusing x-ray absorption spectroscopy in the partial fluorescence yield. Asmooth increase of the Ce valence with pressure is observed across the two superconducting (SC) regions without any noticeable irregularity. The chemical pressure dependence of the Ce valence was also measured in Ce(Cu1-xNix)(2)Si-2 at 20 K. Avery weak, monotonic increase of the valence with x was observed, without any significant change in the two SC regions. Within experimental uncertainties, our results showno evidence for the valence transition with an abrupt change in the valence state near the SC II region, challenging the valence-fluctuation mediated superconductivity model in these compounds at high pressure and low temperature.

DOI： 10.1103/PhysRevLett.113.086403

Novel Phase of Solid Oxygen Induced by Ultrahigh Magnetic Fields Reviewed

T. Nomura, Y. H. Matsuda, S. Takeyama, A. Matsuo, K. Kindo, J. L. Her, T. C. Kobayashi

PHYSICAL REVIEW LETTERS 112 ( 24 ) 247201-1-247201-5 2014.6

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

Magnetization measurements and magnetotransmission spectroscopy of the solid oxygen alpha phase were performed in ultrahigh magnetic fields of up to 193 T. An abrupt increase in magnetization with large hysteresis was observed when pulsed magnetic fields greater than 120 T were applied. Moreover, the transmission of light significantly increased in the visible range. These experimental findings indicate that a first-order phase transition occurs in solid oxygen in ultrahigh magnetic fields, and that it is not just a magnetic transition. Considering the molecular rearrangement mechanism found in the O-2-O-2 dimer system, we conclude that the observed field-induced transition is caused by the antiferromagnetic phase collapsing and a change in the crystal structure.

DOI： 10.1103/PhysRevLett.112.247201

Orbital Zeeman Effect of Liquid Oxygen in High Magnetic Fields Reviewed

Toshihiro Nomura, Yasuhiro H. Matsuda, Shojiro Takeyama, Akira Matsuo, Koichi Kindo, Tatsuo C. Kobayashi

Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013) 2014.6

Superconducting phases in (NH3)(y)MxFeSe1-zTez (M = Li, Na, and Ca) Reviewed

Yusuke Sakai, Lu Zheng, Masanari Izumi, Kazuya Teranishi, Ritsuko Eguchi, Hidenori Goto, Taiki Onji, Shingo Araki, Tatsuo C. Kobayashi, Yoshihiro Kubozono

PHYSICAL REVIEW B 89 ( 14 ) 144509-1-144509-6 2014.4

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

Superconducting phases of (NH3)(y)MxFeSe1-z Te-z have been synthesized by the intercalation of metal atoms (M: Li, Na, Ca) into FeSe and FeSe0.5Te0.5 using a low-temperature liquid NH3 technique. The superconducting transition temperature (T-c) is 31.5 K for Na-doped FeSe, and for Li-, Na-, and Ca-doped FeSe0.5Te0.5 it is 26, 22, and 17 K, respectively. The 31.5 K superconducting is the superconducting phase in ammoniated Na-doped FeSe. The T-c is lower than that (onset T-c = 46 K) of the superconducting phase reported previously. The reason why the T-c of this phase is lower is discussed based on the structure. The pressure dependences of T-c in the (NH3)(y)Na0.5FeSe and (NH3)(y)Na0.4FeSe0.5Te0.5 samples have been measured and a negative pressure dependence is observed; i.e., a decrease in lattice constant c leads to a decrease in Tc, consistent with the behavior of (NH3)(y)Cs0.4FeSe reported previously by our group. Furthermore, the magnetic behavior of (NH3)(y)Na0.4FeSe0.5Te0.5 has been fully investigated at different applied magnetic fields (H) to determine the critical magnetic This is a successful metal intercalation into FeSe1-z Te-z (z not equal 0) and an observation of superconductivity.

DOI： 10.1103/PhysRevB.89.144509

Charge Density Wave Transition in EuAl4 Reviewed

Shingo Araki, Yoichi Ikeda, Tatsuo C. Kobayashi, Ai Nakamura, Yuichi Hiranaka, Masato Hedo, Takao Nakama, Yoshichika Onuki

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 83 ( 1 ) 015001-1-015001-2 2014.1

Emergence of novel phase of solid oxygen in ultrahigh magnetic field Reviewed

T. C. Kobayashi, T. Nomura, Y. H. Matsuda, J. L. Her, S. Takeyama, A. Matsuo, K. Kindo

27TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT27), PTS 1-5 568 42018-1-42018-6 2014

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

Faraday discovered the magnetism of molecular oxygen in 1850, and since then, oxygen has attracted significant interests as a ubiquitous but exotic molecular magnet. In solid oxygen, the magnetic energy through the exchange interaction is comparable to the van der Waals cohesive energy, resulting in a strong spin-lattice correlation. We reported the discovery of a magnetic-field-induced phase transition of solid oxygen using ultrahigh magnetic fields up to 186 T. This is the first experiments demonstrating the phase control through the spin by applying a magnetic field. Magneto-optical absorption measurements of the a phase at low temperatures revealed that visible light absorption suddenly ceases and the background transparency dramatically improves above the critical field. The large hysteresis in these data indicates a first-order transition with a relaxation time on the order of microseconds. The results strongly suggest the reconstruction of the O-2 molecular geometry by the magnetic field, which causes that the antiferromagnetic phase collapses and the field-induced phase should have an isotropic crystal structure. Since all of the known phases of O-2 are induced in high pressure, the discovery of a field-induced phase is noteworthy in the long history of solid O-2 studies.

DOI： 10.1088/1742-6596/568/4/042018

Pressure-Induced Magnetic Transition and Disappearance of Superconductivity in CeNiGe3 Reviewed

Yoichi Ikeda, Takeshi Tomijima, Shingo Araki, Tatsuo C. Kobayashi, Keizo Murata

JPS Conference Proceedings 3 015044-1-015044-6 2014

Metamagnetic Transition of Itinerant Ferromagnet U₃P₄under High Pressure Reviewed

Shingo Araki, Minami Hayashida, Naoto Nishiumi, Hiroki Manabe, Yoichi Ikeda, Tatsuo C. Kobayashi, Keizo Murata, Yoshihiko Inada, Piotr Wiśniewski, Dai Aoki, Yoshichika Ōnuki, Etsuji Yamamoto, Yoshinori Haga

Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013) 3 011081-1-011081-6 2014

Valence Crossover of Ce Ions in CeCu2Si2 under High Pressure - Pressure Dependence of the Unit Cell Volume and the NQR Frequency Reviewed

Tatsuo C. Kobayashi, Kenji Fujiwara, Keiki Takeda, Hisatomo Harima, Yoichi Ikeda, Takafumi Adachi, Yasuo Ohishi, Christoph Geibel, Frank Steglich

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 82 ( 11 ) 114701-1-6 2013.11

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

The pressure dependence of the valence of Ce ions in CeCu2Si2 has been investigated by means of X-ray powder diffraction (XRPD) and nuclear quadrupole resonance (NQR) measurements. The high-pressure XRPD measurements were carried out at 12K for CeCu2Si2 and CeCu2Ge2. Precise measurements under smoothly-applied pressure were achieved through the selection of the gasket material of a diamond anvil cell. The jump of the unit cell volume, which was reported previously in CeCu2Ge2, was not observed within the experimental accuracy, indicating the absence of a first-order valance transition. The pressure dependence of Cu-63-NQR frequency nu(Q) measured in CeCu2Si2 was compared with that calculated from the measured lattice parameters. The nu(Q)'s were calculated for the two cases of LaCu2Si2 and CeCu2Si2, corresponding to localized and itinerant cases for 4f electrons, respectively. The measured and calculated slopes d nu(Q)/dP show good agreement. A small but significant deviation from the pressure-linear dependence in nu(Q) was observed in the narrow pressure range of 3.9-4.5 GPa, indicating a valence crossover adjacent to the critical end point.

DOI： 10.7566/JPSJ.82.114701

Superconductivity in (NH3)yCs0.4FeSe Reviewed

Lu Zheng, Masanari Izumi, Yusuke Sakai, Ritsuko Eguchi, Hidenori Goto, Yasuhiro Takabayashi, Takashi Kambe, Taiki Onji, Shingo Araki, Tatsuo C. Kobayashi, Jungeun Kim, Akihiko Fujiwara, Yoshihiro Kubozono

Physical Review B - Condensed Matter and Materials Physics 88 ( 9 ) 094521-094525 2013.9

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

Alkali-metal-intercalated FeSe materials, (NH3) yM0.4FeSe (M: K, Rb, and Cs), have been synthesized using the liquid NH3 technique. (NH3)yCs 0.4FeSe shows a superconducting transition temperature (T c) as high as 31.2 K, which is higher by 3.8 K than the Tc of nonammoniated Cs0.4FeSe. The Tcs of (NH 3)yK0.4FeSe and (NH3) yRb0.4FeSe are almost the same as those of nonammoniated K0.4FeSe and Rb0.4FeSe. The Tc of (NH 3)yCs0.4FeSe shows a negative pressure dependence. A clear correlation between Tc and lattice constant c is found for ammoniated metal-intercalated FeSe materials, suggesting a correlation between Fermi-surface nesting and superconductivity. © 2013 American Physical Society.

DOI： 10.1103/PhysRevB.88.094521

Longitudinal magnetoresistance of Ce(Cu1-xNix)2Si2 Reviewed

Yoichi Ikeda, Shingo Araki, Tatsuo C. Kobayashi

Journal of the Korean Physical Society 63 ( 3 ) 766 - 767 2013.8

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

The longitudinal magnetoresistance (LMR) of polycrystalline Ce(Cu1-xNix)2Si2 samples is investigated between T = 2 and 15 K up to H = 80 kOe. The application of magnetic field induces anomalous temperature dependence of the resistivity with the power index n &lt
2 for x = 0.12 and 0.14, in which ρ(T) obeys a T 2-law at H = 0 Oe. A sign-change of the LMR from positive (x &lt
0.03) to negative (x &gt
0.03) is observed at 2 K. The negative LMR at 80 kOe is maximized (∼ -4 % at T = 2 K) around x ∼ 0.10 and subsequently disappears at higher Ni concentrations. Disappearance of the positive LMR in lower x region and the negative LMR at higher Ni concentrations implies a collapse of a coherent Kondo lattice state, and a rapid enhancement of Kondo temperature, respectively. Anomalous quantum fluctuations are probably developed around x ∼ 0.10. © 2013 The Korean Physical Society.

DOI： 10.3938/jkps.63.766

Spin-Dependent Molecular Orientation of O-2-O-2 Dimer Formed in the Nanoporous Coordination Polymer Reviewed

Akihiro Hori, Tatsuo C. Kobayashi, Yoshiki Kubota, Akira Matsuo, Koichi Kindo, Jungeun Kim, Kenichi Kato, Masaki Takata, Hirotoshi Sakamoto, Ryotaro Matsuda, Susumu Kitagawa

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 82 ( 8 ) 084703-1-6 2013.8

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

In a model system of an O-2-O-2 dimer confined in the nanopores of Cu-1,4-cyclohexanedicarboxylic acid, the molecular orientation associated with the spin state was observed by precise synchrotron radiation structure analysis. The obtained charge density level structures revealed that the molecular orientation varied with increasing temperature, coupling with the spin states of the O-2-O-2 dimer. The magnetic properties previously reported were consistently explained by the S = 1 dimer model taking into account the molecular arrangements. The obtained gap parameters are different from those of the O-2-O-2 dimer confined in the nanopores of Cu-2,3-pyrazinedicarboxylate-pyrazine, which is explained by the pore deformation due to the adsorption, depending on the pore size and/or flexibility of the coordination polymer.

DOI： 10.7566/JPSJ.82.084703

Pressure effects on rattling and superconductivity in the einstein solids Reviewed

Yoichi Ikeda, Yasuhiro Kawasaki, Takafumi Shinohara, Shingo Araki, Tatsuo C. Kobayashi, Atsushi Onosaka, Yoshihiko Okamoto, Jun-Ichi Yamaura, Zenji Hiroi

Journal of the Physical Society of Japan 82 ( 6 ) 063707-1-4 2013.6

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

Pressure effects on the rattling and superconductivity in the Einstein solids
AxV2Al20 (Ax = Ga0:2, Al0:3, and Y) are investigated by means of electrical resistivity measurements under high pressure. We observed bumps in resistivity at low temperatures only for Ga0:2 and Al0:3, which can be explained by scattering of conduction electrons by local phonons with Einstein temperatures of 8.5 and 22 K, respectively. The bumps gradually fade out with increasing pressure, and the temperature variations of resistivity for Ga0:2 and Al0:3 approach those of Y. This indicates that the low-energy local phonons in the Ga and Al compounds disappear at high pressures. We also observed T2 behavior with large A coefficients in the low-temperature limit for Ga and Al compounds, but not for Y. With increasing pressure, the A coefficient is largely suppressed, while H' c2 ≡ jdHc2=dTj Tc that also reflects the density of states at Fermi energy decreases more slowly. The T2 behavior in Ga and Al compounds probably originates from el-ph scatterings enhanced by rattling of Ga and Al atoms rather than el-el scatterings. An enhancement in the superconducting transition temperatures by the rattling is also demonstrated by comparing the pressure responses of T c between the three compounds. The enhancements of Tc and el-ph interaction λ is relatively small for these Einstein solids in contrast to those of the β-pyrochlore oxides. © 2013 The Physical Society of Japan.

DOI： 10.7566/JPSJ.82.063707

Magneto-absorption in the α phase of solid oxygen at megagauss magnetic fields Reviewed

T. Nomura, Y. H. Matsuda, J. L. Her, S. Takeyama, A. Matsuo, K. Kindo, T. C. Kobayashi

Journal of Low Temperature Physics 170 ( 5-6 ) 372 - 376 2013.3

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

Optical absorption spectra of solid oxygen α phase and liquid oxygen were measured in high magnetic fields over 100 T using a single turn coil method. The absorption originates from the bimolecular transition was observed. The spectra strongly reflect the local magnetic interaction between two oxygen molecules. The shape of the absorption spectrum of the α oxygen changed dramatically with increasing magnetic fields, even though the absorption intensity of liquid oxygen simply declined. This difference cannot be explained by only the local magnetic interaction, and we speculate that the field induced lattice distortion plays an important role in the solid oxygen α phase. © 2012 Springer Science+Business Media, LLC.

DOI： 10.1007/s10909-012-0699-z

Indenter Type of Pressure Cell and Its Applications

KOTEGAWA Hisashi, KOBAYASHI Tatsuo C.

The Review of High Pressure Science and Technology 22 ( 3 ) 214 - 221 2012.8

Language：Japanese Publisher：The Japan Society of High Pressure Science and Technology

We review an indenter-type pressure cell, which possesses a maximum pressure of 5 GPa and a sample space of φ1.6×1.6 mm. This pressure cell is available not only for transport measurements but also for ac-susceptibility and NMR measurements. An advantage is the small size being able to change the direction of the pressure cell in the conventional superconducting magnet, which makes it possible to measure the anisotropy against magnetic field.<br>

DOI： 10.4131/jshpreview.22.214

Other Link： https://jlc.jst.go.jp/DN/JALC/10008377707?from=CiNii

A Study of Ni-Substitution Effects on Heavy-Fermion CeCu2Si2 - Similarities between Ni Substitution and High-Pressure Effects - Reviewed

Yoichi Ikeda, Shingo Araki, Tatsuo C. Kobayashi, Yusei Shimizu, Tatsuya Yanagisawa, Hiroshi Amitsuka

81 ( 8 ) 083701-1-083701-4 2012.6

The effects of Ni substitution on Ce(Cu1-xNix)2Si2 have been studied by
specific heat and electrical resistivity measurements. The specific heat
measurement has revealed that the enhanced quantum fluctuations around an
antiferromagnetic quantum critical point are markedly suppressed by Ni
substitution, and that the Fermi liquid state recovers in the Nirich region (x
> 0.12). The characteristic T-linear dependence of the resistivity has been
observed at approximately x ~ 0.10 together with a sign of superconductivity.
The variation of n in the form of rho - rho0 = aT^n against Tmax^1, at which
the resistivity peaks, coincides with the case of high-pressure experiments on
pure CeCu2Si2. The anomalous T-linear behavior appears to occur in the
crossover region from the Kondo regime to the valence fluctuation regime rather
than in the conventional antiferromagnetic quantum critical region.

DOI： 10.1143/JPSJ.81.083701

Iron-platinum-arsenide superconductors Ca10(PtnAs8)(Fe2−xPtxAs2)5 Reviewed

Minoru,Nohara, Satomi Kakiya, Kazutaka Kudoa, Yoshihiro Oshiro, Shingo Araki, Tatsuo C. Kobayashi, Kenta Oku, Eiji Nishibori, Hi

Solid State Communications. 152 ( 8 ) 635 - 639 2012.4

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