Comparative study of the effects of electron irradiation and natural disorder in single crystals of SrFe$_{2}$(As$_{1-x}$P$_x$)$_2$ ($x=$0.35) superconductor

TL;DR

This study compares the effects of electron irradiation and natural disorder on the superconducting properties of SrFe$_{2}$(As$_{1-x}$P$_x$)$_2$ crystals, revealing how different disorders influence the superconducting gap structure and transition temperature.

Contribution

It provides new insights into how artificial and natural disorders differently affect the superconducting gap and transition temperature in iron-based superconductors.

Findings

Irradiation increases the power-law exponent n beyond 2, indicating lifting of accidental nodes.

Disorder impacts the transition temperature more strongly in irradiated samples.

The effects of different disorder types on intra- and inter-band scattering are discussed.

Abstract

London penetration depth, $\lambda(T)$, was measured in single crystals of SrFe$_2$(As$_{1-x}$P$_x$)$_2$ ($x=$0.35) iron - based superconductor. The influence of disorder on the transition temperature, $T_c$, and on $\lambda(T)$ was investigated. The effects of scattering controlled by the annealing of as-grown crystals was compared with the effects of artificial disorder introduced by 2.5~MeV electron irradiation. The low temperature behavior of $\lambda(T)$ can be described by a power-law function, $\Delta \lambda (T)=AT^n$, with the exponent $n$ close to one in pristine annealed samples, as expected for superconducting gap with line nodes. Upon $1.2 \times 10^{19}$ \ecm irradiation, the exponent $n$ increases rapidly exceeding a dirty limit value of $n=$ 2 implying that the nodes in the superconducting gap are accidental and can be lifted by the disorder. The variation of the exponent $n$ with $T_c$ is much stronger in the irradiated crystals compared to the crystals in which disorder was controlled by the annealing of the growth defects. We discuss the results in terms of different influence of different types of disorder on intra- and inter- band scattering.