Superconducting atomic contacts under microwave irradiation

TL;DR

This paper investigates how microwave irradiation affects superconducting atomic contacts, revealing fractional resonances and supporting a quantum theory based on Andreev bound states.

Contribution

It provides experimental evidence of fractional Shapiro resonances and photon-assisted multiple Andreev reflections, confirming the full quantum transport theory.

Findings

Observation of fractional Shapiro resonances

Modification of background current due to photon-assisted processes

Confirmation of Andreev bound state-based quantum transport theory

Abstract

We have measured the effect of microwave irradiation on the dc current-voltage characteristics of superconducting atomic contacts. The interaction of the external field with the ac supercurrents leads to replicas of the supercurrent peak, the well known Shapiro resonances. The observation of supplementary fractional resonances for contacts containing highly transmitting conduction channels reveals their non-sinusoidal current-phase relation. The resonances sit on a background current which is itself deeply modified, as a result of photon assisted multiple Andreev reflections. The results provide firm support for the full quantum theory of transport between two superconductors based on the concept of Andreev bound states.