Graduate student Kohei Matsuura, Masaki Roppongi, Associate Professor Kenichiro Hashimoto, Professor Takasada Shibauchi of the Department of Advanced Materials Science in the Graduate School of Frontier Sciences led the research project.
Iron-chalcogenide superconductors FeSe1−xSx possess unique electronic properties such as nonmagnetic nematic order and its quantum critical point. The nature of superconductivity with such nematicity is important for understanding the mechanism of unconventional superconductivity. A recent theory suggested the possible emergence of a fundamentally new class of superconductivity with the so-called Bogoliubov Fermi surfaces (BFSs) in this system. However, such an ultranodal pair state requires broken time-reversal symmetry (TRS) in the superconducting state, which has not been observed experimentally. Here, we report muon spin relaxation (μSR) measurements in FeSe1−xSx superconductors for 0 ≤ x ≤ 0.22 covering both orthorhombic (nematic) and tetragonal phases. We find that the zero-field muon relaxation rate is enhanced below the superconducting transition temperature Tc for all compositions, indicating that the superconducting state breaks TRS both in the nematic and tetragonal phases. Moreover, the transverse-field μSR measurements reveal that the superfluid density shows an unexpected and substantial reduction in the tetragonal phase (x > 0.17). This implies that a significant fraction of electrons remain unpaired in the zero-temperature limit, which cannot be explained by the known unconventional superconducting states with point or line nodes. The TRS breaking and the suppressed superfluid density in the tetragonal phase, together with the reported enhanced zero-energy excitations, are consistent with the ultranodal pair state with BFSs. The present results reveal two different superconducting states with broken TRS separated by the nematic critical point in FeSe1−xSx, which calls for the theory of microscopic origins that account for the relation between nematicity and superconductivity.
Publication: Proceeding of the National Academy of Sciences USA (PNAS)
Title: Two superconducting states with broken time-reversal symmetry in FeSe1-xSx
Authors: Kohei Matsuura, Masaki Roppongi, Mingwei Qiu, Qi Sheng, Yipeng Cai, Kohtaro Yamakawa, Zurab Guguchia, Ryan P. Day, Kenji M.Kojima, Andrea Damascelli, Yuichi Sugimura, Mikihiko Saito, Takaaki Takenaka, Kota Ishihara, Yuta Mizukami, Kenichiro Hashimoto, Yilun Gu, Shengli Guo, Licheng Fu, Zheneng Zhang, Fanlong Ning, Guoqiang Zhao, Guangyang Dai, Changqing Jin, James W. Beare, Graeme M. Luke, Yasutomo J. Uemura, and Takasada Shibauchi*