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

TL;DR

This paper investigates how potential and magnetic disorder affect Yu-Shiba-Rusinov states in dirty superconducting films, revealing broadening effects, mesoscopic fluctuations, and resonance phenomena near magnetic impurities.

Contribution

It provides a detailed analysis of YSR state broadening and fluctuations due to disorder using the Usadel equation, including effects of measurement techniques and impurity chains.

Findings

Potential disorder broadens YSR states proportional to the square root of resistance.

Mesoscopic fluctuations significantly influence the local density of states profile.

Scanning-tunneling-microscopy can mask YSR features in the local density of states.

Abstract

The interplay of a potential and magnetic disorder in superconductors remains an active field of research for decades. Within the framework of the Usadel equation, we study the local density of states near a solitary classical magnetic impurity in a dirty superconducting film. We find that a potential disorder results in broadening of the delta-function in the local density of states at the Yu-Shiba-Rusinov (YSR) energy. This broadening is proportional to the square root of a normal-state spreading resistance of the film. We demonstrate that mesoscopic fluctuations caused by a potential disorder affect crucially a profile of the local density of states in the vicinity of the YSR energy. In addition, we demonstrate that a scanning-tunneling-microscopy tip can mask an YSR feature in the local density of states. Also, we study the local density of states near a chain of magnetic impurities situated in the normal region of a dirty superconductor/normal-metal junction. We find a resonance in the local density of states near the YSR energy. The energy scale of the resonant peak is controlled by the square root of the film resistance per square in the normal state.