Spin-orbit splitting of Andreev states revealed by microwave spectroscopy

The Josephson supercurrent that flows through a weak link between two superconductors is mediated by fermionic quasiparticle states localized at the weak link: the Andreev bound states. To explore the role of the spin of these states, we have performed their microwave absorption spectroscopy in superconducting weak links with strong spin-orbit coupling: an InAs-Al (core-full shell) epitaxially-grown nanowire. The spectra present distinctive features that we interpret as arising from zero-field spin-split Andreev states. A simple empirical model, which takes into account the Rashba spin-orbit interaction in a multichannel nanowire, explains these features and their evolution with magnetic field. Our results show that the spin of quasiparticles can be a relevant degree of freedom in Josephson weak links.