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

Abstract

There is growing evidence that the superconducting semimetal FeSe ($T_c\sim 8$ 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, ${\kappa }_{xx}$ 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 $\sim 1.0$ K, both ${\kappa }_{xx}$ and ${\kappa }_{xy}$ exhibit distinct anomalies (kinks) at the upper critical field $H_{c2}$ and at a field $H^{\ast }$ slightly below $H_{c2}$. The analysis of the thermal Hall angle (${\kappa }_{xy}/{\kappa }_{xx}$) indicates a change of the quasiparticle scattering rate at $H^{\ast }$. These results provide strong support to the previous suggestion that above $H^{\ast }$ a distinct field-induced superconducting phase emerges with an unprecedented large spin imbalance.

arXiv comment7 pages, 3 figures, to be published in JPSJ

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• The key ingredients of the electronic structure of FeSe
• On the Remarkable Superconductivity of FeSe and its Close Cousins
• Exotic Superconducting States in FeSe-based Materials
• Evidence for an FFLO state with segmented vortices in the BCS-BEC-crossover superconductor FeSe
• Highly Anisotropic Superconducting Gap in Nematically Ordered and Tetragonal Phases of FeSe${}_{1-x}$S${}_x$
• Hydrostatic and uniaxial pressure tuning of iron-based superconductors: Insights into superconductivity, magnetism, nematicity and collapsed tetragonal transitions
• Pseudogap Behavior of the Nuclear Spin-lattice Relaxation Rate in FeSe Probed by ${}^{77}$Se-NMR
• Bulk superconductivity and role of fluctuations in the iron-based superconductor FeSe at high pressures
• Imaging anisotropic vortex dynamics in FeSe
• Microscopic mechanism for fluctuating pair density wave
• Evidence of nodal gap structure in the basal plane of the FeSe superconductor
• Quasiparticle Nodal Plane in the Fulde-Ferrell-Larkin-Ovchinnikov State of FeSe
• Quantum FFLO state in clean layered superconductors
• Vortex-lattice melting and paramagnetic depairing in the nematic superconductor FeSe
• Multi-band description of the upper critical field of bulk FeSe
• Superconductivity of the FeSe/SrTiO3 Interface in the View of BCS-BEC Crossover
• Superconductivity of highly spin-polarized electrons in FeSe probed by ${}^{77}$Se NMR
• Intrinsic pinning of FeSe${}_1$${}_{-}$${}_x$S${}_x$ single crystals probed by torque magnetometry