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, K_XX and K_xy, respectively, in magnetic fields up to 20 T. The field dependences of K_XX and K_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 ∼1.0K, both K_XX and K_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 (K_xy/K_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.