Impurity Effects in Sign Reversing Fully-Gapped Superconductors: Analysis of FeAs Superconductors

TL;DR

This paper analyzes how impurities affect the superconducting transition temperature in FeAs superconductors, revealing conditions under which superconductivity remains robust despite impurity scattering.

Contribution

It provides a theoretical analysis of impurity effects in a two-band BCS model, explaining the robustness of sign-reversing fully-gapped superconductivity in FeAs compounds.

Findings

Superconductivity is suppressed when interband impurity scattering equals intraband scattering.

Superconductivity remains robust when interband scattering is much weaker than intraband.

The natural ratio of impurity scattering strengths explains the observed robustness in FeAs superconductors.

Abstract

To understand the impurity effect on Tc in FeAs superconductors, we analyze a simple two-band BCS model with repulsive interband interaction. The realized fully-gapped superconducting state with sign reversal, which is predicted by spin fluctuation theories in this compound, is suppressed by impurities due to the interband hopping of Cooper pairs, if the interband impurity scattering $I'$ is equal to the intraband one $I$. When $|I'/I| < 1$, in highly contrast, Tc is almost unchanged by strong impurity scattering since the interband scattering is almost prohibited by the multiple scattering effect. Since $|I'/I| \sim 0.5$ is expected, the robustness of superconductivity against impurities in FeAs superconductors is naturally understood in term of the sign reversing fully-gapped state.