Abstract Body

The iron-chalcogenide superconductor FeSe, with a critical temperature T_c ~ 9 K, and the series of isovalently substituted FeSe_1-xS_x have garnered significant attention due to the many exotic properties associated with the intertwining of nematicity, magnetism, and unconventional superconductivity [1]. FeSe is argued as a strong candidate superconductor located in the crossover regime between the weakly coupled BCS and the strongly coupled BEC limits [2,3]. Its extremely small and shallow Fermi pockets, large superconducting gap, and consequently a large Maki parameter suggest that FeSe offers an ideal platform to study the potential Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state, in which a new pairing (k, - k + q) with nonzero q is formed. Here, we present several pieces of evidence for the emergence of the FFLO superconducting states in  FeSe [4,5]. In FeSe_1-xS_x the nematic transition is completely suppressed above a critical point at x_c ~ 0.17 [6]. Interestingly, an abrupt change in the superconducting gap has been observed at x = x_c [7, 8], suggesting the presence of an ultra-nodal pairing state with a Bogolubov Fermi surface [9]. Moreover, a strong intertwining of a spin-density-wave (SDW) order and the high-temperature superconductivity has been found in  FeSe_1-xS_x under high pressures, where striking enhancement of T_c is observed in the non-magnetic tetragonal regime [10]. We discuss the superconducting gap structure in the high-T_c tetragonal phase and its connection to the ultra-nodal state.

References

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[10] K. Matsuura et al., Nat. Commun. 8, 1143 (2017).

Acknowledgment

This work has been done in collaboration with various groups, including Y. Matsuda's group (Kyoto University), T. Shibauchi's group (The University of Tokyo), T. Hanaguri's group (RIKEN), and N. E. Hussey's group (Bristol/Radboud University and HFML Nijmegen).