Evidence for FFLO superconducting states in the BCS-BEC crossover superconductor FeSe

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 [1,2]. 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 distinct high-field superconducting phases 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 [3]. For H || c, the presence of a high-field phase is shown by an anomalous kink of the heat capacity, which occurs well below the irreversibility field. We attribute these high-field superconducting phases to the FFLO superconducting states. We also point out the importance of the multi-band nature and the orbital dependent pairing for the formation of the FFLO phase in FeSe.
This work has been done in collaboration with Y. Sato, Y. Suzuki, Y. Masuda (Kyoto Univ.), T. Shibauchi (Univ. of Tokyo), T. Hanaguri, T. Machida (RIKEN), S. Licciardello, M. Čulo, N. E. Hussey (HFML-Nijmegen), S. Arsenijević, J. Wosnitza (HLD-Dresden), J. Böker, I. Eremin (Ruhr-Universität Bochum).
[1] S. Kasahara et al., Proc. Natl. Acad. Sci. USA 111, 16309 (2014).
[2] T. Shibauchi, T. Hanaguri, Y. Matsuda, J. Phys. Soc. Jpn. 89, 102002 (2020).
[3] S. Kasahara et al., Phys. Rev. Lett. 124, 107001 (2020).