研究者詳細 - 笠原　成

論文 - 笠原　成

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Magnetothermal transport in ultraclean single crystals of Kitaev magnet α-RuCl3

Y. Xing, R. Namba, K. Imamura, K. Ishihara, S. Suetsugu, T. Asaba, K. Hashimoto, T. Shibauchi, Y. Matsuda, Y. Kasahara

npj Quantum Materials 10 ( 1 ) 2025年3月

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

Abstract

The layered honeycomb magnet α-RuCl₃ has emerged as a promising candidate for realizing a Kitaev quantum spin liquid. Previous studies have reported oscillation-like anomalies in the longitudinal thermal conductivity and half-integer quantized thermal Hall conductivity above the antiferromagnetic critical field H _c, generating significant interest. However, the origins of these phenomena remain contentious due to strong sample dependence. Here we re-examine the magnetothermal transport properties using recently available ultra-pure α-RuCl₃ single crystals to further elucidate potential signatures of the spin liquid state. Our findings reveal that while anomalies in thermal conductivity above H _c persist even in ultraclean crystals, their magnitude is significantly attenuated, contrary to the quantum oscillations hypothesis. This suggests that the anomalies are likely attributable to localized stacking faults inadvertently introduced during magnetothermal transport measurements. The thermal Hall conductivity exhibits a half-quantized plateau, albeit with a narrower width than previously reported. This width reduction can be understood through two distinct mechanisms: sample-dependent magnetic critical fields that influence the lower boundary of the plateau region, and the decoupling between chiral Majorana edge currents and phononic thermal transport that determines the upper boundary. These results indicate that structural imperfections exert a substantial influence on both the oscillation-like anomalies and quantization effects observed in magnetothermal transport measurements of α-RuCl₃.

DOI： 10.1038/s41535-025-00749-4

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その他リンク： https://www.nature.com/articles/s41535-025-00749-4

Apparent nonreciprocal transport in FeSe bulk crystals

Taichi Terashima, Shinya Uji, Yuji Matsuda, Takasada Shibauchi, Shigeru Kasahara

Physical Review B 2025年2月

Transport evidence for twin-boundary pinning of superconducting vortices in FeSe 査読

Taichi Terashima, Hideaki Fujii, Yoshitaka Matsushita, Shinya Uji, Yuji Matsuda, Takasada Shibauchi, Shigeru Kasahara

Physical Review B 2024年1月

Evidence for an odd-parity nematic phase above the charge-density-wave transition in a kagome metal

T. Asaba, A. Onishi, Y. Kageyama, T. Kiyosue, K. Ohtsuka, S. Suetsugu, Y. Kohsaka, T. Gaggl, Y. Kasahara, H. Murayama, K. Hashimoto, R. Tazai, H. Kontani, B. R. Ortiz, S. D. Wilson, Q. Li, H. -H. Wen, T. Shibauchi, Y. Matsuda

Nature Physics 20 ( 1 ) 40 - 46 2024年1月

Unusual crossover from Bardeen-Cooper-Schrieffer to Bose-Einstein-condensate superconductivity in iron chalcogenides 査読

Yuta Mizukami, Masahiro Haze, Ohei Tanaka, Kohei Matsuura, Daiki Sano, Jakob Böker, Ilya Eremin, Shigeru Kasahara, Yuji Matsuda, Takasada Shibauchi

Communications Physics 2023年7月

Spin fluctuations from Bogoliubov Fermi surfaces in the superconducting state of S-substituted FeSe 査読

Zhongyu Yu, Koya Nakamura, Kazuya Inomata, Xiaoling Shen, Taketora Mikuri, Kohei Matsuura, Yuta Mizukami, Shigeru Kasahara, Yuji Matsuda, Takasada Shibauchi, Yoshiya Uwatoko, Naoki Fujiwara

Communications Physics 6 ( 1 ) 2023年7月

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

Abstract

The study of the iron-based superconductor, FeSe, has resulted in various topics, such as the interplay among superconductivity, nematicity, and magnetism, Bardeen-Cooper-Schrieffer Bose-Einstein-condensation (BCS-BEC) crossover, and Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductivity. Recently, topologically protected nodal Fermi surfaces, referred to as Bogoliubov Fermi surfaces (BFSs), have garnered much attention. A theoretical model for the S-substituted FeSe system demonstrated that BFSs can manifest under the conditions of spin-orbit coupling, multi-band systems, and superconductivity with time-reversal symmetry breaking. Here we report the observation of spin fluctuations originating from BFSs in the superconducting (SC) state via ⁷⁷Se-nuclear magnetic resonance measurements to 100 mK. In a heavily S-substituted FeSe, we found an anomalous enhancement of low-energy spin fluctuations deep in the SC state, which cannot be explained by an impurity effect. Such unusual behavior implies the presence of significant spin fluctuations of Bogoliubov quasiparticles, which are associated with possible nesting properties between BFSs.

DOI： 10.1038/s42005-023-01286-x

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その他リンク： https://www.nature.com/articles/s42005-023-01286-x

Expanded quantum vortex liquid regimes in the electron nematic superconductors FeSe1−xSx and FeSe1−xTex 査読

M. Čulo, S. Licciardello, K. Ishida, K. Mukasa, J. Ayres, J. Buhot, Y.-T. Hsu, S. Imajo, M. W. Qiu, M. Saito, Y. Uezono, T. Otsuka, T. Watanabe, K. Kindo, T. Shibauchi, S. Kasahara, Y. Matsuda, N. E. Hussey

Nature Communications 14 ( 1 ) 4150-1 - 4150-9 2023年7月

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

Abstract

The quantum vortex liquid (QVL) is an intriguing state of type-II superconductors in which intense quantum fluctuations of the superconducting (SC) order parameter destroy the Abrikosov lattice even at very low temperatures. Such a state has only rarely been observed, however, and remains poorly understood. One of the key questions is the precise origin of such intense quantum fluctuations and the role of nearby non-SC phases or quantum critical points in amplifying these effects. Here we report a high-field magnetotransport study of FeSe_1−xS_x and FeSe_1−xTe_x which show a broad QVL regime both within and beyond their respective electron nematic phases. A clear correlation is found between the extent of the QVL and the strength of the superconductivity. This comparative study enables us to identify the essential elements that promote the QVL regime in unconventional superconductors and to demonstrate that the QVL regime itself is most extended wherever superconductivity is weakest.

DOI： 10.1038/s41467-023-39730-9

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その他リンク： https://www.nature.com/articles/s41467-023-39730-9

Enhanced Superconducting Pairing Strength near a Pure Nematic Quantum Critical Point 査読

Kiyotaka Mukasa, Kousuke Ishida, Shusaku Imajo, Mingwei Qiu, Mikihiko Saito, Kohei Matsuura, Yuichi Sugimura, Supeng Liu, Yu Uezono, Takumi Otsuka, Matija Čulo, Shigeru Kasahara, Yuji Matsuda, Nigel E. Hussey, Takao Watanabe, Koichi Kindo, Takasada Shibauchi

Physical Review X 13 ( 1 ) 2023年3月

Resistivity and thermal conductivity of an organic insulator β′–EtMe3Sb[Pd(dmit)2]2 査読

Minoru Yamashita, Yuki Sato, Yuichi Kasahara, Shigeru Kasahara, Takasada Shibauchi, Yuji Matsuda

Scientific Reports 12 ( 1 ) 2022年12月

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

Abstract

A finite residual linear term in the thermal conductivity at zero temperature in insulating magnets indicates the presence of gapless excitations of itinerant quasiparticles, which has been observed in some candidate materials of quantum spin liquids (QSLs). In the organic triangular insulator β′–EtMe₃Sb[Pd(dmit)₂]₂, a QSL candidate material, the low-temperature thermal conductivity depends on the cooling process and the finite residual term is observed only in samples with large thermal conductivity. Moreover, the cooling rate dependence is largely sample dependent. Here we find that, while the low-temperature thermal conductivity significantly depends on the cooling rate, the high-temperature resistivity is almost perfectly independent of the cooling rate. These results indicate that in the samples with the finite residual term, the mean free path of the quasiparticles that carry the heat at low temperatures is governed by disorders, whose characteristic length scale of the distribution is much longer than the electron mean free path that determines the high-temperature resistivity. This explains why recent X-ray diffraction and nuclear magnetic resonance measurements show no cooling rate dependence. Naturally, these measurements are unsuitable for detecting disorders of the length scale relevant for the thermal conductivity, just as they cannot determine the residual resistivity of metals. Present results indicate that very careful experiments are needed when discussing itinerant spin excitations in β′–EtMe₃Sb[Pd(dmit)₂]₂.

DOI： 10.1038/s41598-022-13155-8

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その他リンク： https://www.nature.com/articles/s41598-022-13155-8

Quantized and unquantized thermal Hall conductance of the Kitaev spin liquid candidate ?-RuCl3 査読

Y. Kasahara, S. Suetsugu, T. Asaba, S. Kasahara, T. Shibauchi, N. Kurita, H. Tanaka, Y. Matsuda

PHYSICAL REVIEW B 106 ( 6 ) 2022年8月

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

Despite extensive investigations, a topological state that hosts Majorana edge modes in the magnetic fieldinduced quantum disordered state of the Kitaev candidate material alpha-RuCl3 has been hotly debated. To gain more insight into this issue, we measured the thermal Hall conductivity kappa xy of various samples grown by the Bridgman method. The results show that the half-integer quantum thermal Hall effect is intimately related to the magnitude of longitudinal thermal conductivity and the Neel temperature at zero field, both of which are sample dependent. Samples exhibiting the half-integer quantum thermal Hall effect have larger zero-field thermal conductivity values than a threshold value, implying that a long mean free path of heat carriers is an important prerequisite. In addition, we find that samples with a higher Neel temperature exhibit a higher magnetic field at which quantization starts to occur. These results indicate that the quantization phenomenon is significantly affected by the impurity scatterings and the non-Kitaev interactions.

DOI： 10.1103/PhysRevB.106.L060410

Web of Science

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Transport evidence for decoupled nematic and magnetic criticality in iron chalcogenides 査読

Jake Ayres, Matija Čulo, Jonathan Buhot, Bence Bernáth, Shigeru Kasahara, Yuji Matsuda, Takasada Shibauchi, Antony Carrington, Sven Friedemann, Nigel E. Hussey

Communications Physics 2022年4月

Intrinsic suppression of the topological thermal Hall effect in an exactly solvable quantum magnet 査読

S. Suetsugu, T. Yokoi, K. Totsuka, T. Ono, I. Tanaka, S. Kasahara, Y. Kasahara, Z. Chengchao, H. Kageyama, Y. Matsuda

Physical Review B 105 ( 2 ) 2022年1月

Charge-neutral fermions and magnetic field-driven instability in insulating YbIr3Si7 査読

Y. Sato, S. Suetsugu, T. Tominaga, Y. Kasahara, S. Kasahara, T. Kobayashi, S. Kitagawa, K. Ishida, R. Peters, T. Shibauchi, A. H. Nevidomskyy, L. Qian, E. Morosan, Y. Matsuda

NATURE COMMUNICATIONS 13 ( 1 ) 2022年1月

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

Charge-neutral excitations have been proposed to explain metal-like thermal transport in Kondo insulators. Here, the authors demonstrate the coupling between charge-neutral excitations and spin degrees of freedom in a Kondo insulator YbIr3Si7, which puts restrictions on current theories.Kondo lattice materials, where localized magnetic moments couple to itinerant electrons, provide a very rich backdrop for strong electron correlations. They are known to realize many exotic phenomena, with a dramatic example being recent observations of quantum oscillations and metallic thermal conduction in insulators, implying the emergence of enigmatic charge-neutral fermions. Here, we show that thermal conductivity and specific heat measurements in insulating YbIr3Si7 reveal emergent neutral excitations, whose properties are sensitively changed by a field-driven transition between two antiferromagnetic phases. In the low-field phase, a significant violation of the Wiedemann-Franz law demonstrates that YbIr3Si7 is a charge insulator but a thermal metal. In the high-field phase, thermal conductivity exhibits a sharp drop below 300 mK, indicating a transition from a thermal metal into an insulator/semimetal driven by the magnetic transition. These results suggest that spin degrees of freedom directly couple to the neutral fermions, whose emergent Fermi surface undergoes a field-driven instability at low temperatures.

DOI： 10.1038/s41467-021-27541-9

Web of Science

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Quasiparticle Nodal Plane in the Fulde-Ferrell-Larkin-Ovchinnikov State of FeSe 査読

S. Kasahara, H. Suzuki, T. Machida, Y. Sato, Y. Ukai, H. Murayama, S. Suetsugu, Y. Kasahara, T. Shibauchi, T. Hanaguri, Y. Matsuda

Physical Review Letters 127 ( 25 ) 2021年12月

掲載種別：研究論文（学術雑誌）

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, characterized by Cooper pairs condensed at finite momentum, has been a long-sought state that remains unresolved in many classes of fermionic systems, including superconductors and ultracold atoms. A fascinating aspect of the FFLO state is the emergence of periodic nodal planes in real space, but its observation is still lacking. Here we investigate the superconducting order parameter at high magnetic fields H applied perpendicular to the ab plane in a high-purity single crystal of FeSe. The heat capacity and magnetic torque provide thermodynamic evidence for a distinct superconducting phase at the low-temperature/high-field corner of the phase diagram. Despite the bulk superconductivity, spectroscopic-imaging scanning tunneling microscopy performed on the same crystal demonstrates that the order parameter vanishes at the surface upon entering the high-field phase. These results provide the first demonstration of a pinned planar node perpendicular to H, which is consistent with a putative FFLO state.

DOI： 10.1103/PhysRevLett.127.257001

Scopus

PubMed

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Pressure-induced reconstitution of Fermi surfaces and spin fluctuations in S-substituted FeSe 査読

T. Kuwayama, K. Matsuura, J. Gouchi, Y. Yamakawa, Y. Mizukami, S. Kasahara, Y. Matsuda, T. Shibauchi, H. Kontani, Y. Uwatoko, N. Fujiwara

Scientific Reports 11 ( 1 ) 2021年12月

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

Abstract

FeSe is a unique high-$$T_c$$ iron-based superconductor in which nematicity, superconductivity, and magnetism are entangled with each other in the P-T phase diagram. We performed $$^{77}$$Se-nuclear magnetic resonance measurements under pressures of up to 3.9 GPa on 12% S-substituted FeSe, in which the complex overlap between the nematicity and magnetism are resolved. A pressure-induced Lifshitz transition was observed at 1.0 GPa as an anomaly of the density of states and as double superconducting (SC) domes accompanied by different types of antiferromagnetic (AF) fluctuations. The low-$$T_{\mathrm{c } }$$ SC dome below 1 GPa is accompanied by strong AF fluctuations, whereas the high-$$T_{\mathrm{c } }$$ SC dome develops above 1 GPa, where AF fluctuations are fairly weak. These results suggest the importance of the $$d_{xy}$$ orbital and its intra-orbital coupling for the high-$$T_{\mathrm{c } }$$ superconductivity.

DOI： 10.1038/s41598-021-96277-9

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その他リンク： https://www.nature.com/articles/s41598-021-96277-9

Discovery of mesoscopic nematicity wave in iron-based superconductors 査読

T. Shimojima, Y. Motoyui, T. Taniuchi, C. Bareille, S. Onari, H. Kontani, M. Nakajima, S. Kasahara, T. Shibauchi, Y. Matsuda, S. Shin

Science 373 ( 6559 ) 1122 - 1125 2021年9月

Observation of a superconducting state of a topological superconductor candidate, FeTe0.6Se0.4, equipping ferromagnetic electrodes with perpendicular magnetic anisotropy 査読

Kosuke Ohnishi, Sachin Gupta, Shigeru Kasahara, Yuichi Kasahara, Yuji Matsuda, Ei Shigematsu, Ryo Ohshima, Yuichiro Ando, Masashi Shiraishi

Applied Physics Express 14 ( 9 ) 093002 - 093002 2021年9月

Description of Resonant Inelastic X-Ray Scattering in Correlated Metals 査読

Keith Gilmore, Jonathan Pelliciari, Yaobo Huang, Joshua J. Kas, Marcus Dantz, Vladimir N. Strocov, Shigeru Kasahara, Yuji Matsuda, Tanmoy Das, Takasada Shibauchi, Thorsten Schmitt

Physical Review X 11 ( 3 ) 2021年9月

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

To fully capitalize on the potential and versatility of resonant inelastic x-ray scattering (RIXS), it is essential to develop the capability to interpret different RIXS contributions through calculations, including the dependence on momentum transfer, from first principles for correlated materials. Toward that objective, we present new methodology for calculating the full RIXS response of a correlated metal in an unbiased fashion. Through comparison of measurements and calculations that tune the incident photon energy over a wide portion of the Fe absorption resonance of the benchmark material , we show that the RIXS response in is dominated by the direct-channel contribution, including the Raman-like response below threshold. Calculations are initially performed within the first-principles Bethe-Salpeter equation (BSE) framework, which we then significantly improve by invoking a quasiboson model to describe the secondary excitations within the intermediate state. This enhancement allows the many-electron RIXS signal to be approximated as a convolution of BSE-calculated spectra with effective spectral functions. We construct these spectral functions, also from first principles, by employing the cumulant expansion of the Green’s function and performing a real-time time-dependent density functional theory calculation of the response of the electronic system to the perturbation of the intermediate-state excitation. Importantly, this process allows us to evaluate the indirect RIXS response from first principles, accounting for the full periodicity of the crystal structure and with full dependence on the momentum transfer.

DOI： 10.1103/PhysRevX.11.031013

Scopus

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Half-integer quantized anomalous thermal Hall effect in the Kitaev material candidate α-RuCl 3 査読

T. Yokoi, S. Ma, Y. Kasahara, S. Kasahara, T. Shibauchi, N. Kurita, H. Tanaka, J. Nasu, Y. Motome, C. Hickey, S. Trebst, Y. Matsuda

Science 373 ( 6554 ) 568 - 572 2021年7月

Quadrupolar charge dynamics in the nonmagnetic FeSe1−xSx superconductors 査読

Weilu Zhang, Shangfei Wu, Shigeru Kasahara, Takasada Shibauchi, Yuji Matsuda, Girsh Blumberg

Proceedings of the National Academy of Sciences 118 ( 20 ) e2020585118 - e2020585118 2021年5月

掲載種別：研究論文（学術雑誌）出版者・発行元：Proceedings of the National Academy of Sciences

We use polarization-resolved electronic Raman spectroscopy to study quadrupolar charge dynamics in a nonmagnetic <inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi mathvariant="normal">F</m:mi><m:mi mathvariant="normal">e</m:mi><m:mi mathvariant="normal">S</m:mi><m:mi mathvariant="normal">e</m:mi></m:mrow><m:mrow><m:mn>1</m:mn><m:mo>−</m:mo><m:mi>x</m:mi></m:mrow></m:msub><m:msub><m:mrow><m:mi mathvariant="normal">S</m:mi></m:mrow><m:mrow><m:mi>x</m:mi></m:mrow></m:msub></m:math></inline-formula> superconductor. We observe two types of long-wavelength <inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:mi>X</m:mi><m:mi>Y</m:mi></m:math></inline-formula> symmetry excitations: 1) a low-energy quasi-elastic scattering peak (QEP) and 2) a broad electronic continuum with a maximum at 55 meV. Below the tetragonal-to-orthorhombic structural transition at <inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi>T</m:mi></m:mrow><m:mrow><m:mi>S</m:mi></m:mrow></m:msub><m:mrow><m:mo>(</m:mo><m:mrow><m:mi>x</m:mi></m:mrow><m:mo>)</m:mo></m:mrow></m:math></inline-formula>, a pseudogap suppression with temperature dependence reminiscent of the nematic order parameter develops in the <inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:mi>X</m:mi><m:mi>Y</m:mi></m:math></inline-formula> symmetry spectra of the electronic excitation continuum. The QEP exhibits critical enhancement upon cooling toward <inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi>T</m:mi></m:mrow><m:mrow><m:mi>S</m:mi></m:mrow></m:msub><m:mrow><m:mo>(</m:mo><m:mrow><m:mi>x</m:mi></m:mrow><m:mo>)</m:mo></m:mrow></m:math></inline-formula>. The intensity of the QEP grows with increasing sulfur concentration x and maximizes near critical concentration <inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi>x</m:mi></m:mrow><m:mrow><m:mi>c</m:mi><m:mi>r</m:mi></m:mrow></m:msub><m:mo>≈</m:mo><m:mn>0.16</m:mn></m:math></inline-formula>, while the pseudogap size decreases with the suppression of <inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi>T</m:mi></m:mrow><m:mrow><m:mi>S</m:mi></m:mrow></m:msub><m:mrow><m:mo>(</m:mo><m:mrow><m:mi>x</m:mi></m:mrow><m:mo>)</m:mo></m:mrow></m:math></inline-formula>. We interpret the development of the pseudogap in the quadrupole scattering channel as a manifestation of transition from the non-Fermi liquid regime, dominated by strong Pomeranchuk-like fluctuations giving rise to intense electronic continuum of excitations in the fourfold symmetric high-temperature phase, to the Fermi liquid regime in the broken-symmetry nematic phase where the quadrupole fluctuations are suppressed.

DOI： 10.1073/pnas.2020585118

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その他リンク： https://syndication.highwire.org/content/doi/10.1073/pnas.2020585118

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