Violation of Anderson's Theorem for Sign-Reversing s-Wave State of Iron-Pnictide Superconductors

TL;DR

This study reveals that the sign-reversing s-wave state in iron-pnictide superconductors is highly sensitive to impurities due to orbital effects, challenging previous assumptions of its robustness.

Contribution

It demonstrates that impurity effects in iron pnictides are significantly enhanced by orbital degrees of freedom, leading to fragility of the sign-reversing s-wave state.

Findings

Interband impurity scattering is promoted by d-orbital degrees of freedom.

Small impurity concentrations cause large pair-breaking effects.

The sign-reversing s-wave state is fragile against impurities.

Abstract

Based on the five-orbital model, we study the effect of local impurity in iron pnictides, and find that the interband impurity scattering is promoted by the d-orbital degree of freedom. This fact means that the fully-gapped sign-reversing s-wave state, which is predicted by spin fluctuation theories, is very fragile against impurities. In the BCS theory, only 1% impurities with intermediate strength induce huge pair-breaking, resulting in the large in-gap state and prominent reduction in Tc, contrary to the prediction based on simple orbital-less models. The present study provides a stringent constraint on the pairing symmetry and the electronic states in iron pnictides.