Magnetic disorder in superconductors: Enhancement by mesoscopic fluctuations

Abstract

We study the density of states (DOS) and the transition temperature Tc 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 Tc by spin-flip scattering can be faster or slower than in the standard mean-field theory. arXiv:1703.02126v2 [cond-mat.supr-con]

Broadened Yu-Shiba-Rusinov states in dirty superconducting films and heterostructures

“…The broadening of the YSR state seems to be of the order of the variance for the YSR energy due to the potential disorder found in Refs. [23,36]. We find that the effect of a magnetic impurity on the LDoS is significantly affected by the mesoscopic fluctuations of the dimensionless strength of the magnetic impurity caused by potential disorder.…”

“…This indicates that the nonzero LDoS around 𝐸 YSR can be thought, physically, as a result of mesoscopic fluctuations of the YSR energy (more precisely, of 𝛼, see Ref. [23]) due to potential disorder. A few remarks are in order here.…”

“…Here we employ an alternative idea introduced in Ref. [23] for the case of dilute concentration of magnetic impurities, 𝑛…”

“…However, as it was shown in Ref. [23], since we are interested in physics at the length scale 𝜉 (alternatively, at the energy scale 𝐸), we need to renormalize the NLSM action from the mean free path up to 𝜉 (or from elastic scattering rate 1/𝜏 down to 𝐸). Upon this renormalization, the term describing scattering by magnetic impurities is strongly renormalized.…”

“…[21,22] for a review). Recently, it has been shown [23] that mesoscopic fluctuations caused by non-magnetic disorder result in strong modification of the YSR bands in the av-erage density of states in comparison with the mean-field analysis.…”