# Magnetic disorder in superconductors: Enhancement by mesoscopic fluctuations

## TL;DR

This paper investigates how mesoscopic fluctuations in disordered superconducting films with magnetic impurities lead to a non-zero density of states within the superconducting gap, affecting the transition temperature and gap filling.

## Contribution

It introduces a model accounting for mesoscopic fluctuations that enhances understanding of magnetic disorder effects beyond mean-field theory in superconductors.

## Key findings

- Mesoscopic fluctuations induce a non-zero DOS within the superconducting gap.
- The effect can surpass potential disorder fluctuations in filling the gap.
- The suppression of $T_c$ varies depending on magnetic impurity strength.

## Abstract

We study the density of states (DOS) and the transition temperature $T_c$ in a dirty superconducting film with rare classical magnetic impurities of an arbitrary strength described by the Poissonian statistics. We take into account that the potential disorder is a source for mesoscopic fluctuations of the local DOS, and, consequently, for the effective strength of magnetic impurities. We find that these mesoscopic fluctuations result in a non-zero DOS for all energies in the region of the phase diagram where without this effect the DOS is zero within the standard mean-field theory. This mechanism can be more efficient in filling the mean-field superconducting gap than rare fluctuations of the potential disorder (instantons). Depending on the magnetic impurity strength, the suppression of $T_c$ by spin-flip scattering can be faster or slower than in the standard mean-field theory.