Investigation of the Superconducting Gap Structure in SrFe₂(As₀.₇P₀.₃)₂ by Magnetic Penetration Depth and Flux Flow Resistivity Analysis

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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 SrFe₂(As₁₋ₓPₓ)₂ single crystals (x=0.30, Tc=25 K). At low temperatures, the superfluid density, nₛ (T)/nₛ(0), obeys a power law, nₛ (T)/nₛ (0)=1-C(T/Tc)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 SrFe₂(As₁₋ₓPₓ)₂ and BaFe₂(As₁₋ₓPₓ)₂ in our experiments is important for clarifying the mechanism of iron-based superconductivity.