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

摘要

There is growing evidence that the superconducting semimetal FeSe (T-c similar to 8K) 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 Fermisurface pocket whereas a more isotropic and larger gap forms at the hole pocket. Below similar to 1.0 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.