Complex state induced by impurities in multiband superconductors

TL;DR

This paper investigates how impurities affect multiband superconductors, revealing a transition from s_{+-} to s_{++} states and the emergence of a time-reversal symmetry-breaking complex state, with implications for iron-based superconductors.

Contribution

It introduces a Ginzburg-Landau framework to describe impurity-induced phase transitions and predicts an intermediate complex state with broken time-reversal symmetry.

Findings

Transition from s_{+-} to s_{++} is direct near T_c

An intermediate complex state appears deeper in the superconducting phase

The complex state breaks time-reversal symmetry

Abstract

We study the role of impurities in a two-band superconductor, and elucidate the nature of the recently predicted transition from s_{+-} state to s_{++} state induced by interband impurity scattering. Using a Ginzburg-Landau theory, derived from microscopic equations, we demonstrate that close to T_c this transition is necessarily a direct one, but deeper in the superconducting state an intermediate complex state appears. This state has a distinct order parameter, which breaks the time-reversal symmetry, and is separated from the s_{+-} and s_{++} states by phase transitions. Based on our results, we suggest a phase diagram for systems with weak repulsive interband pairing, and discuss its relevance to iron-based superconductors.