<研究ノート>FeSeにおけるFulde-Ferrell-Larkin-Ovchinnikov超伝導

Abstract

The iron-chalcogenide FeSe is argued as a strong candidate superconductor located in the crossover regime between the weakly coupled BCS and the strongly coupled BEC limits. 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 a long-sought Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state, in which a new pairing (k, -k+q) with nonzero q vector is formed. Here, we present several pieces of evidence for the emergence of a distinct high-field superconducting phase in FeSe. For H || ab, our state-of-the-art high-field thermal transport up to 33 T shows a discontinuous downward jump within the superconducting state, indicating a first-order phase transition to a distinct high-field superconducting phase, which is attributed to an FFLO state.