Non-contact Andreev reflection as a direct probe of superconductivity on the atomic scale

TL;DR

This paper introduces a novel tunneling spectroscopy method for atomic-scale imaging of superconductivity via Andreev reflection, overcoming mechanical contact limitations and enabling detailed analysis of superconducting properties at the nanoscale.

Contribution

A new tunneling spectroscopy technique is developed to detect Andreev reflection at the atomic scale, allowing for precise identification of superconducting features and defect sensitivities.

Findings

Enabled atomic-scale imaging of superconducting properties.

Detected higher order Andreev processes in nanoscale junctions.

Revealed sensitivity of Andreev reflection to natural defects.

Abstract

Direct detection of superconductivity has long been a key strength of point-contact Andreev reflection. However, its applicability to atomic-scale imaging is limited by the mechanical contact of the Andreev probe. To this end, we present a new method to probe Andreev reflection in a tunnel junction, leveraging tunneling spectroscopy and junction tunability to achieve quantitative detection of Andreev scattering. This method enables unambiguous assignment of superconducting origins of current-carrying excitations as well as detection of higher order Andreev processes in atomic-scale junctions. We furthermore revealed distinct sensitivity of Andreev reflection to natural defects, such as step edges, even in classical superconductors. The methodology opens a new path to nanoand atomic-scale imaging of superconducting properties, including disordered superconductors and proximity to phase transitions.