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 $rho_f(Bll B_{rm c2})/rho_n=alpha B/B_{rm c2}$. The observed $alpha$ value of $approx0.66$ was considerably smaller than that of other Fe-based materials ($alphageq1$) 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.

Investigation of the Superconducting Gap Structure in SrFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$ by Magnetic Penetration Depth and Flux Flow Resistivity Analysis

We measured the microwave surface impedances and obtained the superfluid density and flux flow resistivity in single crystals of a phosphor-doped iron-based superconductor SrFe$_2$(As$_{1-x}$P$_{x}$)$_2$ single crystals ($x=0.30$, $T_c=25 mathrm{K}$). At low temperatures, the superfluid density, $n_s (T)/n_s(0)$, obeys a power law, $n_s (T)/n_s (0)=1-C(T/T_c)^n$, with a fractional exponent of $n=1.5$-1.6. The flux flow resistivity was significantly enhanced at low magnetic fields. These features are consistent with the presences of both a gap with line nodes and nodeless gaps with a deep minimum. The remarkable difference observed in the superconducting gap structure between SrFe$_2$(As$_{1-x}$P$_{x}$)$_2$ and BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$ in our experiments is important for clarifying the mechanism of iron-based superconductivity.