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

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 $\rho_f(B\ll B_{\rm c2})/\rho_n=\alpha B/B_{\rm c2}$\mathrm{.}Theobserved$\alpha$valueof$\approx0.66$wasconsiderablysmallerthanthatofotherFe-basedmaterials($\alpha\geq1$) 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.