Impurity states in multiband s-wave superconductors: analysis of iron pnictides

TL;DR

This paper investigates how non-magnetic impurities affect multiband s-wave superconductors, especially iron pnictides, revealing that band structure mismatches influence impurity state formation and providing analytic and numerical insights.

Contribution

The study derives analytic expressions for impurity-induced states in multiband superconductors and demonstrates the impact of band structure mismatch on impurity state formation.

Findings

Band structure mismatch reduces impurity state formation likelihood.

Analytic expressions for Green's functions in multiband superconductors.

Numerical confirmation using tight-binding models relevant to pnictides.

Abstract

We examine the effect of a single, non-magnetic impurity in a multiband, extended s-wave superconductor allowing for anisotropy of the gaps on the Fermi surfaces. We derive analytic expressions for the Green's functions in the continuum and analyse the conditions for the existence of sharp impurity-induced resonant states. Underlying band structure is more relevant for the multiband than for single band case, and mismatch between the bands generically makes the formation of the impurity states less likely in the physical regime of parameters. We confirm these conclusions by numerically solving the impurity problem in a tight-binding parameterization of the bands relevant to pnictide superconductors.