Flux flow and orbital upper critical field in multiband FeSe$_{0.5}$Te$_{0.5}$ explored by microwave magnetotransport

Abstract

We measure the flux flow resistivity in FeSe$_{0.5}$Te$_{0.5}$ epitaxial films using a microwave dual-frequency technique (16 GHz and 27 GHz), in the range 5 K-$T_c$, in static magnetic fields up to 1.2 T. By applying a temperature scaling procedure, we extract from flux flow measurements the temperature dependence of the orbital upper critical field, that shows features of multiband superconductors. The reduced orbital upper critical field is then fitted with a two-band model with strong intraband and weak interband coupling, as expected in 11 Fe-based systems. We derive the vortex viscosity and we estimate the bands-averaged vortex core quasiparticle reduced scattering time within the Bardeen-Stephen framework. Our data suggest that, for our epitaxial FeSe$_{0.5}$Te$_{0.5}$ films, the quasiparticle scattering rate values are at the upper edge for the dirty regime. Finally, tentative numerical values of the orbital upper critical field and coherence length are provided.