Investigation of the superconducting gap structure in SrFe ₂(As _0.7P _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-xP _x) ₂ single crystals (x=0.30, T _c=25K). 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 presence of line nodes on at least one band and modulated nodeless gap with deep minimum on the same bands and/or other bands. The remarkable difference observed in the superconducting gap structure between SrFe ₂(As _1-xP _x) ₂ and BaFe ₂(As _1-xP _x) ₂ in our experiments is important for clarifying the mechanism of iron-based superconductivity in the 122 system.