Exceptional Suppression of Flux-Flow Resistivity in FeSe$_{0.4}$Te$_{0.6}$ by Back-Flow from Excess Fe Atoms and Se/Te Substitutions

TL;DR

This study reveals an unusually suppressed flux-flow resistivity in FeSe$_{0.4}$Te$_{0.6}$ due to back-flow effects from excess Fe atoms and substitutions, highlighting a novel back-flow phenomenon in disordered multi-band superconductors.

Contribution

It reports the first observation of back-flow caused by non-pinning origins in a multi-band superconductor, explaining the suppressed flux-flow resistivity.

Findings

Flux-flow resistivity is significantly lower than in other Fe-based superconductors.

Back-flow effects are attributed to disorder from excess Fe and Se/Te substitutions.

Superfluid density shows a quadratic temperature dependence indicating strong pair-breaking.

Abstract

We measured the microwave surface impedance of FeSe$_{0.4}$Te$_{0.6}$ single crystals with- and without external magnetic fields. The superfluid density exhibited a quadratic temperature dependence, indicating a strong pair-breaking effect. The flux-flow resistivity behaved as $\rho_f(B\ll B_{\rm c2})/\rho_n=\alpha B/B_{\rm c2}$. The observed $\alpha$ value of $\approx0.66$ was considerably smaller than that of other Fe-based materials ($\alpha\geq1$) and was attributed to a back-flow of superfluids remarkable in disordered superconductors. This is the first-time observation of the back-flow phenomenon caused by an origin other than the vortex pinning in multiple-band systems.