Quasiparticle Excitations in the Superconducting State of FeSe Probed by Thermal Hall Conductivity in the Vicinity of the BCS–BEC Crossover

There is growing evidence that the superconducting semimetal FeSe (T_c ∼ 8 K) is in the crossover regime between weak-coupling Bardeen–Cooper–Schrieffer (BCS) and strong-coupling Bose–Einstein-condensate (BEC) limits. We report on longitudinal and transverse thermal conductivities, κ_xx and κ_xy, respectively, in magnetic fields up to 20 T. The field dependences of κ_xx and κ_xy imply that a highly anisotropic small superconducting gap forms at the electron Fermi-surface pocket whereas a more isotropic and larger gap forms at the hole pocket. Below ∼1.0 K, both κ_xx and κ_xy exhibit distinct anomalies (kinks) at the upper critical field H_c2 and at a field H* slightly below H_c2. The analysis of the thermal Hall angle (κ_xy/κ_xx) indicates a change of the quasiparticle scattering rate at H*. These results provide strong support to the previous suggestion that above H* a distinct field-induced superconducting phase emerges with an unprecedented large spin imbalance.