高橋 秀光

@article{H.Takahashi_JPSJ_2025,

title = {Subtle Structural Anomaly under Compression in Line-Nodal CaSb2},

author = {H Takahashi and A Ikeda and S Kitagawa and H Kadobayashi and N Hirao and K Ishida and T Imazu and Y Maeno},

url = {https://arxiv.org/abs/2503.21340},

doi = {10.7566/JPSJ.94.044605},

year  = {2025},

date = {2025-03-27},

urldate = {2025-03-27},

journal = {Journal of the Physical Society of Japan},

volume = {94},

pages = {044605},

abstract = {We report X-ray diffraction patterns and calculated electronic band structures of the Dirac line-nodal material CaSb2 under pressure. Its superconducting transition temperature (Tc = 1.7 K) increases under pressure and reaches a maximum at 3.4 K at around 3 GPa. We observed subtle anomalies in lattice parameters accompanied by a jump in bulk modulus without any change in crystal symmetry at around 3 GPa. First-principles calculations revealed that the distorted lattice of Sb(1) site deforms in the pressure range of 0–3 GPa. Those results suggest the existence of a first-order structural transition and arouse expectations for unusual phononic properties affecting the superconducting state. The calculated pressure dependence of the electronic density of states (DOS) confirms that it is not the change in the DOS that governs the variations in Tc.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{H.Takahashi_PRBL_2024,

title = {Pressure evolution of the normal- and superconducting-state properties of the line-nodal material CaSb2 revealed by 123Sb nuclear quadrupole resonance},

author = {H Takahashi and S Kitagawa and K Ishida and A Ikeda and S R Saha and S Yonezawa and J Paglione and Y Maeno},

url = {https://arxiv.org/abs/2402.12783},

doi = {10.1103/PhysRevB.109.L100501},

year  = {2024},

date = {2024-03-04},

urldate = {2024-03-04},

journal = {Physical Review B},

volume = {109},

pages = {L100501},

abstract = {CaSb 2 is the Dirac line-nodal material that exhibits a superconducting (SC) transition at 1.7 K. In spite of its conventional SC state at ambient pressure, the transition temperature T c shows a peak structure against hydrostatic pressure. We performed ac magnetic susceptibility and 123 Sb nuclear quadrupole resonance (NQR) measurements on single-crystalline CaSb 2 under pressures up to 2.08 GPa. T c monotonically increased in this pressure region, which is consistent with a previous study. We observed continuous broadening of the NQR spectrum against pressure, which is a sign of unique compression behavior of the lattice. In the normal state, the nuclear spin-lattice relaxation rate 1 / T 1 is proportional to temperature in all pressure values, typical of a metal. However, 1 / T 1 T in the normal state is independent of pressure, indicating that the density of states at the Fermi energy N ( E F ) , which is one of the parameters governing T c , is insensitive to pressure. From these results, we conclude that N ( E F ) does not govern the origin of the enhancement in T c . This is unusual for a weak electron-phonon coupling superconductor. In the SC state, we revealed that the SC gap becomes larger and more isotropic under pressure.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{CaSb2ARPES,

title = {Fermiology of a topological line-nodal compound  CaSb2  and its implication to superconductivity: Angle-resolved photoemission study},

author = {C-Wen Chuang and S Souma and A Moriya and K Nakayama and A Ikeda and M Kawaguchi and K Obata and S Ranjan Saha and H Takahashi and S Kitagawa and K Ishida and K Tanaka and M Kitamura and K Horiba and H Kumigashira and T Takahashi and S Yonezawa and J Paglione and Y Maeno and T Sato},

url = {https://arxiv.org/abs/2211.15083},

doi = {10.1103/PhysRevMaterials.6.104203},

year  = {2022},

date = {2022-10-24},

urldate = {2022-10-24},

journal = {Physical Review Materials},

volume = {6},

issue = {10},

pages = {104203},

abstract = {We performed angle-resolved photoemission spectroscopy with microfocused beam on a topological line-nodal compound CaSb2 which undergoes a superconducting transition at the onset Tc ∼ 1.8 K, to clarify the Fermi-surface topology relevant to the occurrence of superconductivity. We found that a three-dimensional hole pocket at the Γ point is commonly seen for two types of single-crystalline samples fabricated by different growth conditions. On the other hand, the carrier-doping level estimated from the position of the chemical potential was found to be sensitive to the sample fabrication condition. The cylindrical electron pocket at the Y(C) point predicted by the calculations is absent in one of the two samples, despite the fact that both samples commonly show superconductivity with similar Tc's. This suggests a key role of the three-dimensional hole pocket to the occurrence of superconductivity, and further points to an intriguing possibility to control the topological nature of superconductivity by carrier tuning in CaSb2.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{A.Ikeda_PRB_2022,

title = {Quasi-two-dimensional Fermi surface of superconducting line-nodal metal  CaSb2},

author = {A Ikeda and S Ranjan Saha and D Graf and P Saraf and D Sergeevich Sokratov and Y Hu and H Takahashi and S Yamane and A Jayaraj and J Sławińska and M Buongiorno Nardelli and S Yonezawa and Y Maeno and J Paglione},

url = {https://arxiv.org/abs/2206.15346},

doi = {10.1103/PhysRevB.106.075151},

year  = {2022},

date = {2022-08-29},

urldate = {2022-08-29},

journal = {Physical Review B},

volume = {106},

issue = {7},

pages = {075151},

abstract = {We report on the Fermi surfaces and superconducting parameters of CaSb2 single crystals (superconducting below Tc ∼ 1.8 K) grown by the self-flux method. The frequency of de Haas–van Alphen and Shubnikov–de Haas oscillations evidences a quasi-two-dimensional (quasi-2D) Fermi surface, consistent with one of the Fermi surfaces forming Dirac lines predicted by first-principles calculations. Measurements in the superconducting state reveal that CaSb2 is close to a type-I superconductor with the Ginzburg-Landau parameter of around unity. The temperature dependence of the upper critical field Hc2 is well described by a model considering two superconducting bands, and the enhancement of the effective mass estimated from Hc2 (0 K) is consistent with the quasi-2D band observed by the quantum oscillations. Our results indicate that a quasi-2D band forming Dirac lines contributes to the superconductivity in CaSb2.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{H.Takahashi_JPSJ_2021,

title = {S-Wave Superconductivity in the Dirac Line-Nodal Material CaSb2},

author = {H Takahashi and S Kitagawa and K Ishida and M Kawaguchi and A Ikeda and S Yonezawa and Y Maeno},

url = {https://arxiv.org/abs/2105.13614},

doi = {10.7566/JPSJ.90.073702},

year  = {2021},

date = {2021-06-11},

urldate = {2021-06-11},

journal = {Journal of the Physical Society of Japan},

volume = {90},

pages = {073702},

abstract = {We performed 121/123Sb-nuclear quadrupole resonance (NQR) measurements on the superconducting (SC) line-nodal material CaSb2 in order to investigate electronic properties in the normal and SC states from a microscopic point of view. In the normal state, the nuclear spin–lattice relaxation rate 1/T1 for the Sb(1) site, which is responsible for the line-nodal parts, is approximately proportional to temperature, indicating the conventional Fermi liquid state. From comparison with band structure calculations, it is considered that the NQR properties related to the line-nodal character are hidden because the conventional behavior originating from Fermi-surface parts away from the nodes is dominant. In the SC state, a clear coherence peak just below the transition temperature and an exponential decrease at lower temperatures were observed in 1/T1. These results strongly suggest that conventional s-wave superconductivity with a full gap is realized in CaSb2.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}