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

摘要

We measured the microwave surface impedances and obtained the superfluiddensity and flux flow resistivity in single crystals of a phosphor-dopediron-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 afractional exponent of $n=1.5$-1.6. The flux flow resistivity was significantlyenhanced at low magnetic fields. These features are consistent with thepresences of both a gap with line nodes and nodeless gaps with a deep minimum.The remarkable difference observed in the superconducting gap structure betweenSrFe$_2$(As$_{1-x}$P$_{x}$)$_2$ and BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$ in ourexperiments is important for clarifying the mechanism of iron-basedsuperconductivity.