Excitations in a superconducting Coulombic energy gap

TL;DR

This paper investigates how strong Coulomb repulsion in a superconductor affects Yu-Shiba-Rusinov excitations when coupled to a quantum dot, revealing a transition from many-body to two-body states and loss of electron-hole symmetry.

Contribution

It demonstrates that strong Coulomb repulsion transforms singlet states into two-body states and breaks electron-hole symmetry of excitations.

Findings

Strong Coulomb repulsion alters the many-body state to a two-body state.

It breaks the electron-hole symmetry of Yu-Shiba-Rusinov excitations.

The excitations lose their original Yu-Shiba-Rusinov character.

Abstract

Cooper pairing and Coulomb repulsion are antagonists, producing distinct energy gaps in superconductors and Mott insulators. When a superconductor exchanges unpaired electrons with a quantum dot, its gap is populated by a pair of electron-hole symmetric Yu-Shiba-Rusinov excitations between doublet and singlet many-body states. The fate of these excitations in the presence of a strong Coulomb repulsion in the superconductor is unknown, but of importance in applications such as topological superconducting qubits and multi-channel impurity models. Here we couple a quantum dot to a superconducting island with a tunable Coulomb repulsion. We show that a strong Coulomb repulsion changes the singlet many-body state into a two-body state. It also breaks the electron-hole energy symmetry of the excitations, which thereby lose their Yu-Shiba-Rusinov character.