研究者詳細 - 笠原　成

Majorana quantization and half-integer thermal quantum Hall effect in a Kitaev spin liquid 査読

Kasahara Y, Ohnishi T, Mizukami Y, Tanaka O, Ma Sixiao, Sugii K, Kurita N, Tanaka H, Nasu J, Motome Y, Shibauchi T, Matsuda Y

NATURE 559 ( 7713 ) 227 - + 2018年7月

Two distinct superconducting pairing states divided by the nematic end point in FeSe1-xSx 査読

Hanaguri, Tetsuo, Iwaya, Katsuya, Kohsaka, Yuhki, Machida, Tadashi, Watashige, Tatsuya, Kasahara, Shigeru, Shibauchi, Takasada, Matsuda, Yuji

SCIENCE ADVANCES 4 ( 5 ) eaar6419 2018年5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 ASSOC ADVANCEMENT SCIENCE

Unconventional superconductivity often competes or coexists with other electronic orders. In iron-based superconductors, a central issue has been the relationship between superconductivity and electronic nematicity, spontaneous breaking of the lattice rotational symmetry. Using spectroscopic-imaging scanning tunneling microscopy, we simultaneously investigated the electronic structure and the superconducting gap in FeSe1-xSx, where the nematicity diminishes above the nematic end point (NEP) at x = 0.17. The nematic band structure appears as anisotropic quasiparticleinterference patterns that gradually become isotropic with increasing x without anomalies at the NEP. By contrast, the superconducting gap, which is intact in the nematic phase, discontinuously shrinks above the NEP. This implies that the presence or absence of nematicity results in two distinct pairing states, whereas the pairing interaction is insensitive to the strength of nematicity.

DOI： 10.1126/sciadv.aar6419

Abrupt change of the superconducting gap structure at the nematic critical point in FeSe1-xSx 査読

Yuki Sato, Shigeru Kasahara, Tomoya Taniguchi, Xiangzhuo Xing, Yuichi Kasahara, Yoshifumi Tokiwa, Youichi Yamakawa, Hiroshi Kontani, Takasada Shibauchi, Yuji Matsuda

PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA 115 ( 6 ) 1227 - 1231 2018年2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：NATL ACAD SCIENCES

The emergence of the nematic electronic state that breaks rotational symmetry is one of the most fascinating properties of the iron-based superconductors, and has relevance to cuprates as well. FeSe has a unique ground state in which superconductivity coexists with a nematic order without long-range magnetic ordering, providing a significant opportunity to investigate the role of nematicity in the superconducting pairing interaction. Here, to reveal how the superconducting gap evolves with nematicity, we measure the thermal conductivity and specific heat of FeSe1-xSx, in which the nematicity is suppressed by isoelectronic sulfur substitution and a nematic critical point (NCP) appears at x(c) approximate to 0.17. We find that, in the whole nematic regime (0 <= x <= 0.17), the field dependence of two quantities consistently shows two-gap behavior; one gap is small but highly anisotropic with deep minima or line nodes, and the other is larger and more isotropic. In stark contrast, in the tetragonal regime (x = 0.20), the larger gap becomes strongly anisotropic, demonstrating an abrupt change in the superconducting gap structure at the NCP. Near the NCP, charge fluctuations of d(xz) and d(yz) orbitals are enhanced equally in the tetragonal side, whereas they develop differently in the orthorhombic side. Our observation therefore directly implies that the orbital-dependent nature of the nematic fluctuations has a strong impact on the superconducting gap structure and hence on the pairing interaction.

DOI： 10.1073/pnas.1717331115

In situ STM observation of nonmagnetic impurity effect in MBE-grown CeCoIn5 films 査読

Masahiro Haze, Yohei Torii, Robert Peters, Shigeru Kasahara, Yuichi Kasahara, Takasada Shibauchi, Takahito Terashima, Yuji Matsuda

Journal of the Physical Society of Japan 87 ( 3 ) 2018年

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

Local electronic effects in the vicinity of an impurity provide pivotal insight into the origin of unconventional superconductivity, especially when the materials are located on the edge of magnetic instability. In high-temperature cuprate superconductors, a strong suppression of superconductivity and appearance of low-energy bound states are clearly observed near nonmagnetic impurities. However, whether these features are common to other strongly correlated superconductors has not been established experimentally. Here, we report the in situ scanning tunneling microscopy observation of electronic structure around a nonmagnetic Zn impurity in heavy-fermion CeCo(In1-xZnx)5 films, which are epitaxially grown by the state-of-the-art molecular beam epitaxy technique. The films have very wide atomically flat terraces and Zn atoms residing on two different In sites are clearly resolved. Remarkably, no discernible change is observed for the superconducting gap at and around the Zn atoms. Moreover, the local density of states around Zn atoms shows little change inside the c-f hybridization gap, which is consistent with calculations for a periodic Anderson model without local magnetic order. These results indicate that no nonsuperconducting region is induced around a Zn impurity and do not support the scenario of antiferromagnetic droplet formation suggested by indirect measurements in Cd-doped CeCoIn5. These results also highlight a significant difference of the impurity effect between cuprates and CeCoIn5, in both of which d-wave superconductivity arises from the non-Fermi liquid normal state near antiferromagnetic instabilities.

DOI： 10.7566/JPSJ.87.034702

Scopus

FeSe_1-xS_x(x = 0.12) の磁気秩序相近傍における高圧下⁷⁷Se-NMR測定

桑山昂典, 松浦康平, 水上雄太, 笠原成, 松田祐司, 芝内孝禎, 上床美也, 藤原直樹

日本物理学会講演概要集 73 ( 0 ) 1390 - 1390 2018年

Thermodynamic evidence for a nematic phase transition at the onset of the pseudogap in YBa2Cu3Oy 査読

Y. Sato, S. Kasahara, H. Murayama, Y. Kasahara, E. -G. Moon, T. Nishizaki, T. Loew, J. Porras, B. Keimer, T. Shibauchi, Y. Matsuda

NATURE PHYSICS 13 ( 11 ) 1074 - + 2017年11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 PUBLISHING GROUP

A long-standing controversial issue in the quest to understand the superconductivity in cuprates is the nature of the enigmatic pseudogap region of the phase diagram(1). Especially important is whether the pseudogap state is a distinct thermodynamic phase characterized by broken symmetries below the onset temperature T*. Here we report torque-magnetometry measurements of anisotropic susceptibility within the ab planes in orthorhombic YBa2Cu3Oy with exceptionally high precision. The in-plane anisotropy displays a significant increase with a distinct kink at the pseudogap onset temperature T*, showing a remarkable scaling behaviour with respect to T/T* in a wide doping range. Our systematic analysis reveals that the rotational symmetry breaking sets in at T* in the limit where the effect of orthorhombicity is eliminated. These results provide thermodynamic evidence that the pseudogap onset is associated with a second-order nematic phase transition, which differs from the recently reported charge-density-wave transition that accompanies translational symmetry breaking(2-10).

DOI： 10.1038/nphys4205

Maximizing T-c by tuning nematicity and magnetism in FeSe1-xSx superconductors 査読

K. Matsuura, Y. Mizukami, Y. Arai, Y. Sugimura, N. Maejima, A. Machida, T. Watanuki, T. Fukuda, T. Yajima, Z. Hiroi, K. Y. Yip, Y. C. Chan, Q. Niu, S. Hosoi, K. Ishida, K. Mukasa, S. Kasahara, J. -G. Cheng, S. K. Goh, Y. Matsuda, Y. Uwatoko, T. Shibauchi

NATURE COMMUNICATIONS 8 ( 1 ) 1143 2017年10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 PUBLISHING GROUP

A fundamental issue concerning iron-based superconductivity is the roles of electronic nematicity and magnetism in realising high transition temperature (T-c). To address this issue, FeSe is a key material, as it exhibits a unique pressure phase diagram involving non-magnetic nematic and pressure-induced antiferromagnetic ordered phases. However, as these two phases in FeSe have considerable overlap, how each order affects superconductivity remains perplexing. Here we construct the three-dimensional electronic phase diagram, temperature (T) against pressure (P) and isovalent S-substitution (x), for FeSe1-xSx. By simultaneously tuning chemical and physical pressures, against which the chalcogen height shows a contrasting variation, we achieve a complete separation of nematic and antiferromagnetic phases. In between, an extended non-magnetic tetragonal phase emerges, where T-c shows a striking enhancement. The completed phase diagram uncovers that high-T-c superconductivity lies near both ends of the dome-shaped antiferromagnetic phase, whereas T-c remains low near the nematic critical point.

DOI： 10.1038/s41467-017-01277-x

Impact of Disorder on the Superconducting Phase Diagram in BaFe2(As1-xPx)(2) 査読

Yuta Mizukami, Marcin Konczykowski, Kohei Matsuura, Tatsuya Watashige, Shigeru Kasahara, Yuji Matsuda, Takasada Shibauchi

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 86 ( 8 ) 2017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：PHYSICAL SOC JAPAN

In many classes of unconventional superconductors, the question of whether the superconductivity is enhanced by the quantum-critical fluctuations on the verge of an ordered phase remains elusive. One of the most direct ways of addressing this issue is to investigate how the superconducting dome traces a shift of the ordered phase. Here, we study how the phase diagram of the iron-based superconductor BaFe2(As1-xPx) 2 changes with disorder via electron irradiation, which keeps the carrier concentrations intact. With increasing disorder, we find that the magneto-structural transition is suppressed, indicating that the critical concentration is shifted to the lower side. Although the superconducting transition temperature T-c is depressed at high concentrations (x greater than or similar to 0.28), it shows an initial increase at lower x. This implies that the superconducting dome tracks the shift of the antiferromagnetic phase, supporting the view of the crucial role played by quantum-critical fluctuations in enhancing superconductivity in this iron-based high-T-c family.

DOI： 10.7566/JPSJ.86.083706

Weakening of the diamagnetic shielding in FeSe1-xSx at high pressures 査読

K. Y. Yip, Y. C. Chan, Q. Niu, K. Matsuura, Y. Mizukami, S. Kasahara, Y. Matsuda, T. Shibauchi, Swee K. Goh

PHYSICAL REVIEW B 96 ( 2 ) 2017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：AMER PHYSICAL SOC

The superconducting transition of FeSe1-xSx with three distinct sulfur concentrations x was studied under hydrostatic pressures up to similar to 70 kbar via bulk ac susceptibility. The pressure dependence of the superconducting transition temperature (T-c) features a small dome-shaped variation at low pressures for x = 0.04 and x = 0.12, followed by a more substantial T-c enhancement to a value of around 30 K at moderate pressures. In x = 0.21, a similar overall pressure dependence of T-c is observed, except that the small dome at low pressures is flattened. For all three concentrations, a significant weakening of the diamagnetic shielding is observed beyond the pressure around which the maximum T-c of 30 K is reached near the verge of the pressure-induced magnetic phase. This observation points to a strong competition between the magnetic and high-T-c superconducting states at high pressure in this system.

DOI： 10.1103/PhysRevB.96.020502

Quasiparticle Excitations in the Superconducting State of FeSe Probed by Thermal Hall Conductivity in the Vicinity of the BCS-BEC Crossover 査読

Tatsuya Watashige, Stevan Arsenijevic, Takuya Yamashita, Daiki Terazawa, Takafumi Onishi, Lars Opherden, Shigeru Kasahara, Yoshifumi Tokiwa, Yuichi Kasahara, Takasada Shibauchi, Hilbert von Loehneysen, Jochen Wosnitza, Yuji Matsuda

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 86 ( 1 ) 2017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：PHYSICAL SOC JAPAN

There is growing evidence that the superconducting semimetal FeSe (T-c similar to 8K) is in the crossover regime between weak-coupling Bardeen-Cooper-Schrieffer (BCS) and strong-coupling Bose-Einstein-condensate (BEC) limits. We report on longitudinal and transverse thermal conductivities,kappa(xx) and kappa(xy), respectively, in magnetic fields up to 20 T. The field dependences of kappa(xx) and kappa(xy) imply that a highly anisotropic small superconducting gap forms at the electron Fermisurface pocket whereas a more isotropic and larger gap forms at the hole pocket. Below similar to 1.0 K, both kappa(xx) and kappa(xy) exhibit distinct anomalies (kinks) at the upper critical field H-c2 and at a field H* slightly below H-c2. The analysis of the thermal Hall angle (kappa(xy)/kappa(xx)) indicates a change of the quasiparticle scattering rate at H*. These results provide strong support to the previous suggestion that above H* a distinct field-induced superconducting phase emerges with an unprecedented large spin imbalance.

DOI： 10.7566/JPSJ.86.014707

Giant superconducting fluctuations in the compensated semimetal FeSe at the BCS-BEC crossover 査読

S. Kasahara, T. Yamashita, A. Shi, R. Kobayashi, Y. Shimoyama, T. Watashige, K. Ishida, T. Terashima, T. Wolf, F. Hardy, C. Meingast, H. Von Loehneysen, A. Levchenko, T. Shibauchi, Y. Matsuda

NATURE COMMUNICATIONS 7 12843 2016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：NATURE PUBLISHING GROUP

The physics of the crossover between weak-coupling Bardeen-Cooper-Schrieffer (BCS) and strong-coupling Bose-Einstein condensate (BEC) limits gives a unified framework of quantum-bound (superfluid) states of interacting fermions. This crossover has been studied in the ultracold atomic systems, but is extremely difficult to be realized for electrons in solids. Recently, the superconducting semimetal FeSe with a transition temperature T-c = 8.5 K has been found to be deep inside the BCS-BEC crossover regime. Here we report experimental signatures of preformed Cooper pairing in FeSe, whose energy scale is comparable to the Fermi energies. In stark contrast to usual superconductors, large non-linear diamagnetism by far exceeding the standard Gaussian superconducting fluctuations is observed below T-star similar to 20 K, providing thermodynamic evidence for prevailing phase fluctuations of superconductivity. Nuclear magnetic resonance and transport data give evidence of pseudogap formation at similar to T-star. The multiband superconductivity along with electron-hole compensation in FeSe may highlight a novel aspect of the BCS-BEC crossover physics.

DOI： 10.1038/ncomms12843

Diamagnetic vortex barrier stripes in underdoped BaFe2(As1-xPx)(2)

A. Yagil, Y. Lamhot, A. Almoalem, S. Kasahara, T. Watashige, T. Shibauchi, Y. Matsuda, O. M. Auslaender

PHYSICAL REVIEW B 94 ( 6 ) 2016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

We report magnetic force microscopy (MFM) measurements on underdoped BaFe2(As1-xPx)(2) (x = 0.26) that showenhanced superconductivity along stripes parallel to twin boundaries. These stripes of enhanced diamagnetic response repel superconducting vortices and act as barriers for them to cross. The width of the stripes is hundreds of nanometers, on the scale of the penetration depth, well within the inherent spatial resolution of MFM and implying that the width is set by the interaction of the superconductor with the MFM's magnetic tip. Unlike similar stripes observed previously by scanning SQUID in the electron doped Ba(Fe1-xCox)(2)As-2, the stripes in the isovalently doped BaFe2(As1-xPx)(2) disappear gradually when we warm the sample towards the superconducting transition temperature. Moreover, we find that the stripes move well below the reported structural transition temperature in BaFe2(As1-xPx)(2) and that they can be much denser than in the Ba(Fe1-xCox)(2)As-2 study. When we cool in finite magnetic field we find that some vortices appear in the middle of stripes, suggesting that the stripes may have an inner structure, which we cannot resolve. Finally, we use both vortex decoration at higher magnetic field and deliberate vortex dragging by the MFM magnetic tip to obtain bounds on the strength of the interaction between the stripes and vortices. We find that this interaction is strong enough to play a significant role in determining the critical current in underdoped BaFe2(As1-xPx)(2).

DOI： 10.1103/PhysRevB.94.064510

arXiv

Charge-induced nematicity in FeSe 査読

Pierre Massat, Donato Farina, Indranil Paul, Sandra Karlsson, Pierre Strobel, Pierre Toulemonde, Marie-Aude Measson, Maximilien Cazayous, Alain Sacuto, Shigeru Kasahara, Takasada Shibauchi, Yuji Matsuda, Yann Gallais

PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA 113 ( 33 ) 9177 - 9181 2016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：NATL ACAD SCIENCES

The spontaneous appearance of nematicity, a state of matter that breaks rotation but not translation symmetry, is one of the most intriguing properties of the iron-based superconductors (Fe SC), and has relevance for the cuprates as well. Establishing the critical electronic modes behind nematicity remains a challenge, however, because their associated susceptibilities are not easily accessible by conventional probes. Here, using FeSe as a model system, and symmetry-resolved electronic Raman scattering as a probe, we unravel the presence of critical charge nematic fluctuations near the structural/ nematic transition temperature, T-S similar to 90 K. The diverging behavior of the associated nematic susceptibility foretells the presence of a Pomeranchuk instability of the Fermi surface with d-wave symmetry. The excellent scaling between the observed nematic susceptibility and elastic modulus data demonstrates that the structural distortion is driven by this d-wave Pomeranchuk transition. Our results make a strong case for charge-induced nematicity in FeSe.

DOI： 10.1073/pnas.1606562113