Unexpected impact of magnetic disorder on multiband superconductivity

TL;DR

This paper investigates how magnetic disorder influences multiband superconductors, revealing scenarios where the critical temperature is not fully suppressed and the gap structure can change or remain stable.

Contribution

It demonstrates that magnetic impurities do not always suppress superconductivity fully and can cause state transformations or preserve gap structures in multiband models.

Findings

Transition temperature saturation occurs in strong disorder regimes.

The $s_ ext{pm}$ gap remains intact under certain impurity scattering.

The $s_{++}$ state can transform into the $s_ ext{pm}$ state with magnetic disorder.

Abstract

We analyze how the magnetic disorder affects the properties of the two-band $s_\pm$ and $s_{++}$ models, which are subject of hot discussions regarding iron-based superconductors and other multiband systems like MgB$_2$. We show that there are several cases when the transition temperature $T_c$ is not fully suppressed by magnetic impurities in contrast to the Abrikosov-Gor'kov theory, but a saturation of $T_c$ takes place in the regime of strong disorder. These cases are: (1) the purely interband impurity scattering, (2) the unitary scattering limit. We show that in the former case the $s_\pm$ gap is preserved, while the $s_{++}$ state transforms into the $s_\pm$ state with increasing magnetic disorder. For the case (2), the gap structure remains intact.