2022年07月01日（金） 10:30~ 理学館506

　The recent discovery of superconductivity in CeRh₂As₂ clarified an unusual H-T phase diagram with two superconducting phases [1]. In CeRh₂As₂, the inversion symmetry is locally broken at the Ce sites, although the global inversion center exists in the middle of the two Ce sites. Surprisingly, the phase diagram was predicted in theoretical work about locally noncentrosymmetric superconductors [2]. The similarity of these phase diagrams between the experiment [1] and theory [2] suggests that the local inversion symmetry breaking plays an essential role in CeRh₂As₂, and the superconducting phase in the high magnetic field region is the pair-density-wave (PDW) state. The most crucial property of the PDW state is the odd-parity superconducting state with spin-singlet pairing. The character of the PDW state suggests a possibility of topological superconductivity as spin-triplet superconductors are.
　Despite the above success, there are some issues to be resolved. First, the microscopic mechanism of superconductivity in CeRh₂As₂ has not been uncovered. The experimental observations indicate that strong correlation effects impact the electronic state of CeRh₂As₂. Second, although the experimentally observed phase diagrams resemble the theoretical prediction qualitatively, the parity transition field of the experiment is larger than that of the weak-coupling theory by a factor of five. This discrepancy has discouraged conclusive interpretation [1,2].
　In the first half of this talk, using the group theory, we derived the Fermi-surface formula of Z₂invariants specifying the topological crystalline superconductivity protected by the nonsymmorphic glide symmetry [3]. Furthermore, we conducted the first-principles calculation for the electronic structure of CeRh₂As₂. Combining the results, we evaluate the Z₂invariants and found the topological crystalline superconductivity [4].
　In the second half of my talk, we discuss strong correlation effects in CeRh₂As₂. We find XY-type magnetic fluctuation consistent with a recent NMR study [5]. Our theoretical results also show that in the superconducting phase diagram, the parity transition field is significantly enhanced by electron correlation effects [6].

[1] S. Khim et al., Science 373, 1012 (2021).
[2] T. Yoshida, M. Sigrist, and Y. Yanase, Phys. Rev. B 86, 134514 (2012).
[3] M. Sato, Phys. Rev. B 81, 220504(R) (2010).
[4] K. Nogaki, A. Daido, J. Ishizuka, and Y. Yanase, Phys. Rev. Research 3, L032071 (2021).
[5] S. Kitagawa et al., J. Phys. Soc. Jpn. 91, 043702 (2022).
[6] K. Nogaki and Y. Yanase, arXiv:2206.04288.