Particle-Hole Asymmetry of Subgap Conductances in Superconductors Without Quasiparticle Poisoning

F. Setiawan, Jay D. Sau

Published 2020-07-30Version 1

Particle-hole symmetry (PHS) of conductance into subgap states in superconductors (SCs) is a fundamental consequence of the mean-field theory of superconductivity. The breaking of this PHS has been attributed to quasiparticle poisoning (QP), a process detrimental to the coherence of SC-based qubits. Here, we propose a generic mechanism without involving QP which can break the PHS of subgap conductances, namely by coupling the subgap state to a bosonic bath. We study the effect of such coupling on the PHS of subgap conductances in SCs using both the rate equation and Keldysh formalism that have different regimes of validity. In both regimes, we found that such coupling gives rise to a particle-hole \textit{asymmetry} in subgap conductances which increases with increasing coupling strength and subgap-state particle-hole content imbalance. Our results show that the PHS breaking of subgap conductances in SCs could be a signature of significant modification of the conductance spectra by coupling to bosonic modes such as phonons instead of QP.