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

TL;DR

This study investigates the superconducting gap structure of SrFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$ using microwave measurements, revealing evidence of both nodal and nodeless gaps, which is crucial for understanding iron-based superconductivity.

Contribution

The paper provides new experimental evidence of mixed gap structures in SrFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$, highlighting differences from similar compounds and advancing knowledge of superconducting mechanisms.

Findings

Superfluid density follows a power law with an exponent of 1.5-1.6.

Flux flow resistivity is enhanced at low magnetic fields.

Presence of both line nodes and deep minima in the superconducting gap.

Abstract

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.