Using controlled disorder to distinguish $s_\pm$ and $s_{++}$ gap structure in Fe-based superconductors

TL;DR

This paper investigates how disorder affects superconductors with different gap structures in Fe-based materials, proposing experimental signatures to distinguish between $s_$ and $s_{++}$ types.

Contribution

It provides a comparative analysis of disorder effects on $T_c$, residual resistivity, and density of states in a two-band model, suggesting practical experimental signatures to identify gap structures.

Findings

$T_c$ suppression varies with impurity model details.

Disorder-induced changes in penetration depth, NMR, and thermal conductivity can distinguish gap types.

Proposes experimental methods to identify $s_$ versus $s_{++}$ gap structures.

Abstract

We reconsider the effect of disorder on the properties of a superconductor characterized by a sign-changing order parameter appropriate for Fe-based materials. Within a simple two band model, we calculate simultaneously $T_c$, the change in residual resistivity $\Delta \rho_0$, and the zero-energy density of states, and show how these results change for various types of gap structures and assumptions regarding the impurity scattering. The rate of $T_c$ suppression is shown to vary dramatically according to details of the impurity model considered. We search therefore for a practical, experimentally oriented signature of a gap of the $s_\pm$ type, and propose that observation of particular evolution of the penetration depth, nuclear magnetic resonance relaxation rate, or thermal conductivity temperature dependence with disorder would suffice.