The Yu-Shiba-Rusinov state, arising from exchange coupling between a magnetic impurity and a superconductor, undergoes a quantum phase transition at a critical coupling. In a scanning tunnelling microscopy experiment, Karan et al. reveal distinct tunnelling spectra on each side of the transition in a magnetic field, which allows them to distinguish the free spin regime from the screened spin regime.
- Sujoy Karan
- Haonan Huang
- Christian R. Ast