Details of the disorder-induced transition between $s_\pm$ and $s_{++}$ states in two-band model for Fe-based superconductors

TL;DR

This study investigates how disorder affects the transition between $s_$ and $s_{++}$ states in Fe-based superconductor models, revealing that the transition's nature depends on impurity scattering parameters and temperature.

Contribution

It provides a detailed analysis of the disorder-induced transition between superconducting states, highlighting the influence of impurity scattering strength and temperature on the transition behavior.

Findings

Transition behavior can be steep or smooth depending on impurity scattering parameters.

Steep gap changes occur at low temperatures ($T < 0.1 T_{c0}$).

Critical temperature remains smooth despite gap behavior changes.

Abstract

Irradiation of superconductors with different particles is one of many ways to investigate effects of disorder. Here we study the disorder-induced transition between $s_\pm$ and $s_{++}$ states in two-band model for Fe-based superconductors with nonmagnetic impurities. Specifically, the important question of whether the superconducting gaps during the transition change smoothly or steeply? We show that the behavior can be of either type and is controlled by the ratio of intra- and interband impurity scattering and a parameter $\sigma$ that represents a scattering strength and changes from zero (Born approximation) to one (unitary limit). For the pure interband scattering potential and $\sigma \lesssim 0.11$, the $s_\pm \to s_{++}$ transition is accompanied by the steep behavior of gaps, while for larger values of $\sigma$, gaps change smoothly. The steep behavior of the gaps occurs at low temperatures, $T < 0.1 T_{c0}$, otherwise it is smooth. The critical temperature $T_c$ is always a smooth function of the scattering rate in spite of the steep changes in the behavior of the gaps.