Magnetic disorder in superconductors: Enhancement by mesoscopic fluctuations

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.

15 pages, 5 figures