Photon-assisted resonant Andreev reflections: Yu-Shiba-Rusinov and Majorana states

TL;DR

This paper provides a detailed analytical study of photon-assisted tunneling into subgap states in superconductors, revealing how it can accurately measure small energies of Yu-Shiba-Rusinov and Majorana states, with implications for quantum state identification.

Contribution

The authors develop a systematic Keldysh approach to analyze photon-assisted tunneling into subgap states, extending beyond simple Tien-Gordon theory, and demonstrate its application to Yu-Shiba-Rusinov and Majorana states.

Findings

Analytical understanding of photon-assisted tunneling into subgap states.

Agreement with recent experimental results.

Method to distinguish Majorana states from conventional subgap states.

Abstract

Photon-assisted tunneling frequently provides detailed information on the underlying charge-transfer process. In particular, the Tien-Gordon approach and its extensions predict that the sideband spacing in bias voltage is a direct fingerprint of the number of electrons transferred in a single tunneling event. Here, we analyze photon-assisted tunneling into subgap states in superconductors in the limit of small temperatures and bias voltages where tunneling is dominated by resonant Andreev processes and does not conform to the predictions of simple Tien-Gordon theory. Our analysis is based on a systematic Keldysh calculation of the subgap conductance and provides a detailed analytical understanding of photon-assisted tunneling into subgap states, in excellent agreement with a recent experiment. We focus on tunneling from superconducting electrodes and into Yu-Shiba-Rusinov states associated with magnetic impurities or adatoms, but we also explicitly extend our results to include normal-metal electrodes or other types of subgap states in superconductors. In particular, we argue that photon-assisted Andreev reflections provide a high-accuracy method to measure small, but nonzero energies of subgap states which can be important for distinguishing conventional subgap states from Majorana bound states.