Nodal superconductivity in isovalently substituted SrFe$_2$(As$_{1-x}$P$_{x}$)$_2$ pnictide superconductor at the optimal doping, $x=$0.35

TL;DR

This study investigates the superconducting gap structure in optimally doped SrFe$_2$(As$_{1-x}$P$_{x}$)$_2$ by measuring the London penetration depth, revealing line nodes and differences from charge-doped pnictides.

Contribution

It provides evidence of nodal superconductivity in SrFe$_2$(As$_{1-x}$P$_{x}$)$_2$, highlighting the effects of isovalent substitution on the gap structure.

Findings

Low-temperature λ(T) ∼ T indicating line nodes.

Annealing reduces λ(0) and increases T_c.

Superfluid density analysis supports nodal gap, differing from simple d-wave.

Abstract

Temperature-dependent London penetration depth, $\lambda(T)$, was measured in optimally - doped, $x=$0.35, as-grown ($T_c \approx$25 K, RRR=$\rho(300K)/\rho(T_c)$=4.5) and annealed ($T_c \approx$35 K, RRR=6.4) single crystals of SrFe$_2$(As$_{1-x}$P$_{x}$)$_2$ iron - based superconductor. Annealing increases the RRR and decreases the absolute value of the London penetration depth from $\lambda(0) = 300 \pm 10$ nm in as-grown sample to $\lambda(0) = 275 \pm 10$ nm. At low temperatures, $\lambda(T) \sim T$ indicating superconducting gap with line nodes. Analysis of the full - temperature range superfluid density is consistent with the line nodes, but differs from the simple single - gap $d-$wave. The observed behavior is very similar to that of BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$, showing that isovalently substituted pnictides are inherently different from the charge - doped materials.