Tunneling processes into localized subgap states in superconductors

TL;DR

This paper investigates tunneling mechanisms into localized subgap states in superconductors, revealing how single-electron tunneling and Andreev reflections dominate at different distances, and how relaxation processes influence current flow.

Contribution

It combines experimental spectroscopy with theoretical analysis to elucidate tunneling processes and relaxation mechanisms into Shiba bound states in superconductors.

Findings

Single-electron tunneling occurs at large tip-substrate distances.

Andreev reflections dominate at smaller distances.

Relaxation processes are essential for current flow into Shiba states.

Abstract

We combine scanning-tunneling-spectroscopy experiments probing magnetic impurities on a superconducting surface with a theoretical analysis of the tunneling processes between (superconducting) tip and substrate. We show that the current is carried by single-electron tunneling at large tip-substrate distances and Andreev reflections at smaller distances. The single-electron current requires relaxation processes between the impurity-induced Shiba bound state and the quasiparticle continuum, allowing us to extract information on such relaxation processes from our analysis.