High-field superconducting phase in FeSe investigated by spectroscopic-imaging scanning tunneling microscopy*

The iron-based superconductor FeSe is characterized by its small Fermi energy that is only several times larger than the superconducting gap amplitude. In such a situation, so-called Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state with periodic nodal planes in the order parameter is expected near the upper critical field at low temperatures. In FeSe, magnetic-field dependence of thermal conductivity exhibits a cusp-like anomaly below the upper critical field, which is argued as a signature of the transition from the low-field normal vortex state to the high-field FFLO state [1]. We performed high-field spectroscopic-imaging scanning tunneling microscopy at an ultra-low temperature below 90 mK to investigate the change in the electronic state across the phase boundary. We found that the vortex image diminishes at about 14 T where the thermal conductivity shows a cusp. This result suggests that the nodal plane is pinned at the surface, if the high-field phase is an FFLO state.
[1] S. Kasahara et al., Proc. Natl. Acad. Sci. U.S.A. 111, 16309 (2014).