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 ($T_c\sim8$ 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, $\kappa_{xx}$ and $\kappa_{xy}$, respectively, in magnetic fields up to 20 T. The field dependences of $\kappa_{xx}$ and $\kappa_{xy}$ 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 $\sim1.0$ K, both $\kappa_{xx}$ and $\kappa_{xy}$ 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 ($\kappa_{xy}/\kappa_{xx}$) 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.