Impurity-induced in-gap state and Tc in sign-reversing s-wave superconductors: analysis of iron oxypnictide superconductors

TL;DR

This paper investigates how nonmagnetic impurities affect the superconducting transition temperature and density of states in sign-reversing s-wave superconductors, specifically iron oxypnictides, using a two-band BCS model.

Contribution

It provides a detailed theoretical analysis of impurity effects on Tc and DOS in oxypnictides, highlighting the importance of interband scattering and anisotropy in the gap function.

Findings

Interband impurity scattering significantly suppresses Tc in the Born regime.

Impurity-induced DOS can mimic power-law behavior in relaxation rates.

In the unitary regime, impurity effects are minimal unless interband and intraband scattering are equal.

Abstract

The sign-reversing fully gapped superconducting state, which is expected to be realized in oxypnictide superconductors, can be prominently affected by nonmagnetic impurities due to the interband scattering of Cooper pairs. We study this problem based on the isotropic two-band BCS model: In oxypnictide superconductors, the interband impurity scattering $I'$ is not equal to the intraband one $I$. In the Born scattering regime, the reduction in Tc is sizable and the impurity-induced density of states (DOS) is prominent if $I\sim I'$, due to the interband scattering. Although impurity-induced DOS can yield a power-law temperature dependence in $1/T_1$, a sizable suppression in Tc is inevitably accompanied. In the unitary scattering regime, in contrast, impurity effect is very small for both Tc and DOS except at $I=I'$. By comparing theory and experiments, we expect that the degree of anisotropy in the $s_\pm$-wave gap function strongly depends on compounds.