Quantum theory of an atom in proximity to a superconductor*

The impact of superconductivity on localized atomic states is important for a wide range of experiments. This includes scanning tunneling microscopy (STM) of atoms at the surface of superconductors, superconducting-ion-chip spectroscopy of Rydberg states, and the role of atom-like centers as a source of noise in superconducting qubits. We present a theory of the proximity effect on many-electron atoms beyond the classical spin (Shiba) approximation [1]. We demonstrate that the emerging orbitally-degenerate Yu-Shiba-Rusinov (YSR) subgap bound states are split by the presence of Coulomb repulsion, suggesting a novel interpretation for the peak splittings recently observed in STM measurements. Moreover, we demonstrate that the combination of orbital degeneracy and particle number admixture due to Cooper-pair formation opens up many forbidden channels for electric and magnetic transitions. As a result, atoms in proximity to superconductors have much more optical and noise activity than anticipated by simple models.

[1] M. Le Dall, I. Diniz, L. G. G. V. Dias da Silva, and R. de Sousa, arXiv:1710.07297.