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

Felix von Oppen

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. In this talk, I will focus on 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. I will review recent experiments [1] on photon-assisted tunneling into YuShiba-Rusinov states which exhibit striking deviations from the predictions of simple Tien-Gordon theory and discuss a systematic Keldysh calculation of the subgap conductance, which provides a detailed analytical understanding of these experiment and is in excellent agreement with experiment [2]. I will conclude by extending the analysis to tunneling into Majorana bound states. Here, 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.

[1] O. Peters, N. Bogdanoff, S. Acero González, L. Melischek, J.R. Simon, G. Reecht, C.B. Winkelmann, F. von Oppen, K.J. Franke, Resonant Andreev reflections probed by photon-assisted tunnelling at the atomic scale, Nature Physics (2020)
[2] S. Acero González, L. Melischek, O. Peters, K. Flensberg, K.J. Franke, F. von Oppen, Photon-assisted resonant Andreev reflections: Yu-Shiba-Rusinov and Majorana states, Phys. Rev. B 102, 045413 (2020)