American Physical Society (APS) Acta Medica Okayama 2469-9950 104 14 2021 Spin-gap formation due to spin-Peierls instability in π-orbital-ordered NaO2 L140402 EN Mizuki Miyajima Department of Physics, Okayama University Fahmi Astuti Advanced Meson Science Laboratory, RIKEN Nishina Center Takahito Fukuda Department of Physics, Okayama University Masashi Kodani Department of Physics, Okayama University Shinsuke Iida Institute for Solid State Physics, University of Tokyo Shinichiro Asai Institute for Solid State Physics, University of Tokyo Akira Matsuo Institute for Solid State Physics, University of Tokyo Takatsugu Masuda Institute for Solid State Physics, University of Tokyo Koichi Kindo Institute for Solid State Physics, University of Tokyo Takumi Hasegawa Graduate School of Advanced Science and Engineering, Hiroshima University Tatsuo C. Kobayashi Department of Physics, Okayama University Takehito Nakano Institute of Quantum Beam Science, Ibaraki University Isao Watanabe Advanced Meson Science Laboratory, RIKEN Nishina Center Takashi Kambe Department of Physics, Okayama University We have investigated the low-temperature magnetism of sodium superoxide (NaO2), in which spin, orbital, and lattice degrees of freedom are closely entangled. The magnetic susceptibility shows anomalies at T1 = 220 K and T2 = 190 K, which correspond well to the structural phase transition temperatures, and a sudden decrease below T3 = 34 K. At 4.2 K, the magnetization shows a clear stepwise anomaly around 30 T with a large hysteresis. In addition, the muon spin relaxation experiments indicate no magnetic phase transition down to T = 0.3 K. The inelastic neutron scattering spectrum exhibits magnetic excitation with a finite energy gap. These results confirm that the ground state of NaO2 is a spin-singlet state. To understand this ground state in NaO2, we performed Raman scattering experiments. All the Raman-active libration modes expected for the marcasite phase below T2 are observed. Furthermore, we find that several new peaks appear below T3. This directly evidences the low crystal symmetry, namely, the presence of the phase transition at T3.We conclude that the singlet ground state of NaO2 is due to the spin-Peierls instability. No potential conflict of interest relevant to this article was reported.

Royal Society of Chemistry Acta Medica Okayama 2046-2069 10 45 2020 A new protocol for the preparation of superconducting KBi2 26686 26692 EN Huan Li Research Institute for Interdisciplinary Science, Okayama University Yanan Wang Research Institute for Interdisciplinary Science, Okayama University Saki Nishiyama Research Institute for Interdisciplinary Science, Okayama University Xiaofan Yang Research Institute for Interdisciplinary Science, Okayama University Tomoya Taguchi Research Institute for Interdisciplinary Science, Okayama University Akari Miura Research Institute for Interdisciplinary Science, Okayama University Ai Suzuki Research Institute for Interdisciplinary Science, Okayama University Lei Zhi Research Institute for Interdisciplinary Science, Okayama University Hidenori Goto Research Institute for Interdisciplinary Science, Okayama University Ritsuko Eguchi Research Institute for Interdisciplinary Science, Okayama University Takashi Kambe Department of Physics, Okayama University Yen-Fa Liao National Synchrotron Radiation Research Center Hirofumi Ishii National Synchrotron Radiation Research Center Yoshihiro Kubozono Research Institute for Interdisciplinary Science, Okayama University A superconducting KBi2 sample was successfully prepared using a liquid ammonia (NH3) technique. The temperature dependence of the magnetic susceptibility (M/H) showed a superconducting transition temperature (Tc) as high as 3.6 K. In addition, the shielding fraction at 2.0 K was evaluated to be 87%, i.e., a bulk superconductor was realized using the above method. The Tc value was the same as that reported for the KBi2 sample prepared using a high-temperature annealing method. An X-ray diffraction pattern measured based on the synchrotron X-ray radiation was analyzed using the Rietveld method, with a lattice constant, a, of 9.5010(1) Å under the space group of Fd[3 with combining macron]m (face-centered cubic, no. 227). The lattice constant and space group found for the KBi2 sample using a liquid NH3 technique were the same as those reported for KBi2 through a high-temperature annealing method. Thus, the superconducting behavior and crystal structure of the KBi2 sample obtained in this study are almost the same as those for the KBi2 sample reported previously. Strictly speaking, the magnetic behavior of the superconductivity was different from that of a KBi2 sample reported previously, i.e., the KBi2 sample prepared using a liquid NH3 technique was a type-II like superconductor, contrary to that prepared using a high-temperature annealing method, the reason for which is fully discussed. These results indicate that the liquid NH3 technique is effective and simple for the preparation of a superconducting KBi2. In addition, the topological nature of the superconductivity for KBi2 was not confirmed. No potential conflict of interest relevant to this article was reported.

Springer Nature Acta Medica Okayama 2041-1723 11 2020 Direction and symmetry transition of the vector order parameter in topological superconductors CuxBi2Se3 235 EN T. Kawai Department of Physics, Okayama University C. G. Wang Institute of Physics, Chinese Academy of Sciences, and Beijing National Laboratory for Condensed Matter Physics Y. Kandori Department of Physics, Okayama University Y. Honoki Department of Physics, Okayama University K. Matano Department of Physics, Okayama University T. Kambe Department of Physics, Okayama University Guo-qing Zheng Department of Physics, Okayama University Topological superconductors have attracted wide-spreading interests for the bright application perspectives to quantum computing. Cu0.3Bi2Se3 is a rare bulk topological superconductor with an odd-parity wave function, but the details of the vector order parameter d and its pinning mechanism are still unclear. Here, we succeed in growing CuxBi2Se3 single crystals with unprecedented high doping levels. For samples with x = 0.28, 0.36 and 0.37 with similar carrier density as evidenced by the Knight shift, the in-plane upper critical field Hc2 shows a two-fold symmetry. However, the angle at which the Hc2 becomes minimal is different by 90° among them, which indicates that the d-vector direction is different for each crystal likely due to a different local environment. The carrier density for x = 0.46 and 0.54 increases substantially compared to x ≤ 0.37. Surprisingly, the in-plane Hc2 anisotropy disappears, indicating that the gap symmetry undergoes a transition from nematic to isotropic (possibly chiral) as carrier increases. No potential conflict of interest relevant to this article was reported.

The American Physical Society Acta Medica Okayama 1098-0121 88 9 2012 Superconductivity in (NH3)(y)Cs0.4FeSe EN 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 Alkali-metal-intercalated FeSe materials, (NH3)(y)M0.4FeSe (M: K, Rb, and Cs), have been synthesized using the liquid NH3 technique. (NH3)(y)Cs0.4FeSe shows a superconducting transition temperature (T-c) as high as 31.2 K, which is higher by 3.8 K than the T-c of nonammoniated Cs0.4FeSe. The T(c)s of (NH3)(y)K0.4FeSe and (NH3)(y)Rb0.4FeSe are almost the same as those of nonammoniated K0.4FeSe and Rb0.4FeSe. The T-c of (NH3)(y)Cs0.4FeSe shows a negative pressure dependence. A clear correlation between T-c and lattice constant c is found for ammoniated metal-intercalated FeSe materials, suggesting a correlation between Fermi-surface nesting and superconductivity. No potential conflict of interest relevant to this article was reported.

The American Physical Society Acta Medica Okayama 1098-0121 82 19 2010 Electronic structure of pristine and K-doped solid picene: Nonrigid band change and its implication for electron-intramolecular-vibration interaction 195114-1 195114-5 EN 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 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 (EF) while that of K-doped picene has three structures similar to those of pristine picene with new states near EF, 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. No potential conflict of interest relevant to this article was reported.

Acta Medica Okayama 71 22 2005 Metallic phase in the metal-intercalated higher fullerene Rb8.8(7)C84 EN Yoshie Rikiishi Yoko Kashino Haruka Kusai Yasuhiro Takabayashi Eiji Kuwahara Yoshihiro Kubozono Takashi Kambe Taishi Takenobu Yoshihiro Iwasa Naomi Mizorogi Shigeru Nagase Susumu Okada A new material of higher fullerene, RbxC84, was synthesized by intercalating Rb metal into C-84 crystals. The RbxC(84) crystals showed a simple cubic (sc) structure with lattice constant, a, of 16.82 (2) angstrom at 6.5 K, and 16.87 (2) angstrom at 295 K. The Rietveld refinements were achieved with the space group, Pa (3) over bar, based on a model that the C-2 axis of D2d-C84 aligned along [111]. The sample composition was determined to be Rb-8.8(7) C-84. The ESR spectrum at 303 K was composed of a broad peak with peak-to-peak linewidth Delta H-pp of 220 G, and a narrow peak with Delta H-pp of 24 G. Temperature dependence of the broad peak clearly showed a metallic behavior. The metallic behavior was discussed based on a theoretical calculation. This finding of new metallic phase in a higher fullerene is the first step for a development of new types of fullerene materials with novel physical properties such as superconductivity. No potential conflict of interest relevant to this article was reported.

Acta Medica Okayama 61 2 2000 Annealing effects on the magnetic and structural properties of single-crystal TDAE-C-60 EN Takashi Kambe Yoshio Nogami Kokichi Oshima Annealing effects on the magnetic and structural properties of single-crystal TDAE-C-60 are investigated. When a crystal is well-annealed at 350 K, ferromagnetic ordering takes place below 16 K, though no magnetic phase transition is shown in as-grown crystal. The saturated magnetization was obtained to be 0.9+/-0.1 mu(B) per C-60. It was first found that the well-annealed crystal shows a structural phase transition around 180 K, probably associated with the orientational ordering of C-60 molecules. On the other hand, the as-grown crystal undergoes no structural phase transition at least down to 30 K while the motion of C-60 molecules is restricted below around 150 K. The possible relation between the low-temperature structure and the magnetic ordering is discussed. No potential conflict of interest relevant to this article was reported.

Acta Medica Okayama 71 17 2005 Structural differences in two polymorphs of tetra-kis-(dimethylamino)-ethylene-C-60: An x-ray diffraction study EN Motoyasu Fujiwara Takashi Kambe Kokichi Oshima A type of low-temperature structure for ferromagnetic alpha-tetra-kis (dimethylamino) -ethylene (TDAE)-C-60 is proposed on the basis of low-temperature x-ray analysis. We observed that intense superlattice reflections with odd indices successively appeared below T-s = 170 K. The space group symmetry of the low-temperature phase is determined to be P2(1)/n. Two inequivalent C-60 sites exist in the low-temperature phase, which are indispensable to the orbital ordering model Of C-60. The contact configuration for the neighboring C(60)s along the stacking c direction is uniquely determined. The double bond between the hexagons faces the neighboring pentagon. We found that the surrounding TDAE molecules shift along the c axis (similar to 0.07 angstrom) and that these shifts correlate perfectly to the alignment of C-60. This result indicates that the steric effect betwee n C-60 and TDAE molecules plays an important role in the orientational ordering Of C-60, On the other hand, in the alpha' phase, no structural phase transition was observed below 30 K. This indicates that all the C(60)s are crystallographically equivalent. Structural differences separate the magnetic peculiarities of the two polymorphs in TDAE-C-60. No potential conflict of interest relevant to this article was reported.

Acta Medica Okayama 99 17 2007 Antiferromagnetic ordering driven by the molecular orbital order of C-60 in alpha '-tetra-kis-(dimethylamino)-ethylene-C-60 EN Takashi Kambe Koichi Kajiyoshi Motoyasu Fujiwara Kokichi Oshima We have studied the ground state of a fullerene-based magnet, the alpha'-phase tetra-kis-(dimethylamino)ethylene-C-60 (alpha'-TDAE-C-60), by electron spin resonance and magnetic torque measurements. Below T-N = 7 K, nonparamagnetic field dependent resonances with a finite excitation gap ( 1.7 GHz) are observed along the a axis. Strong enhancement in their intensity as temperature is decreased is inconsistent with excitation from a singlet state, which had been proposed for the alpha'-phase ground state. Below T-N, nonquadratic field dependence of the magnetic torque signal is also observed in contrast to quadratic field dependence in the paramagnetic phase. The angle-dependent torque signals below T-N indicate the existence of an anisotropy of the bulk magnetization. From both experiments, we propose an antiferro-magnetic ground state driven by the cooperative orientational ordering of C-60 in the alpha'-TDAE-C-60. No potential conflict of interest relevant to this article was reported.

Acta Medica Okayama 68 14 2003 Polymer phase of the tetrakis(dimethylamino)ethylene-C-60 organic ferromagnet EN Slaven Garaj Takashi Kambe László Forró Andrzej Sienkiewicz Motoyasu Fujiwara Kokichi Oshima High-pressure electron spin resonance (ESR) measurements were performed on tetrakis(dimethylamino) ethylene (TDAE)-C-60 single crystals and stability of the polymeric phase was established in the P-T parameter space. At 7 kbar the system undergoes a ferromagnetic to paramagnetic phase transition due to the pressure-induced polymerization. The polymeric phase remains stable after the pressure release. The depolymerization of the pressure-induced phase was observed at a temperature of 520 K, revealing an unexpectedly high thermal stability of the polymer. Below room temperature, the polymeric phase behaves as a simple Curie-type insulator with one unpaired electron spin per chemical formula. The TDAE(+) donor-related unpaired electron spins, formerly ESR silent, become active above a temperature of 320 K, which demonstrates that the magnetic properties are profoundly defined by miniscule reorientation of TDAE molecules. No potential conflict of interest relevant to this article was reported.