Anomalous density of states in a metallic film in proximity with a superconductor

TL;DR

This study uses low-temperature STM to examine the local electronic density of states in Nb-Au bilayers, revealing a proximity effect with sub-gap features that challenge existing theoretical models.

Contribution

It provides experimental evidence of anomalous density of states in superconductor-normal metal bilayers that cannot be explained by standard Usadel equations.

Findings

Observation of a reduced energy gap in the normal metal surface

Presence of sub-gap states within the mini-gap

Discrepancy between experimental spectra and theoretical predictions

Abstract

We investigated the local electronic density of states in superconductor-normal metal (Nb-Au) bilayers using a very low temperature (60 mK) STM. High resolution tunneling spectra measured on the normal metal (Au) surface show a clear proximity effect with an energy gap of reduced amplitude compared to the bulk superconductor (Nb) gap. Within this mini-gap, the density of states does not reach zero and shows clear sub-gap features. We show that the experimental spectra cannot be described with the well-established Usadel equations from the quasi-classical theory.