Electron-boson-interaction induced particle-hole symmetry breaking of conductance into subgap states in superconductors

TL;DR

This paper demonstrates that electron-boson interactions in superconductors can break particle-hole symmetry in subgap conductance, revealing a new mechanism beyond quasiparticle poisoning that affects superconductor-based qubits.

Contribution

It introduces a novel mechanism showing how electron-boson couplings can break PHS in subgap states, expanding understanding beyond quasiparticle poisoning effects.

Findings

Electron-boson interactions cause particle-hole asymmetry in subgap conductance.

Asymmetry increases with stronger coupling and particle-hole imbalance.

Temperature decrease enhances the PHS breaking effect.

Abstract

Particle-hole symmetry (PHS) of conductance into subgap states in superconductors is a fundamental consequence of a noninteracting mean-field theory of superconductivity. The breaking of this PHS has been attributed to a noninteracting mechanism, i.e., quasiparticle poisoning (QP), a process detrimental to the coherence of superconductor-based qubits.Here, we show that the ubiquitous electron-boson interactions in superconductors can also break the PHS of subgap conductances. We study the effect of such couplings on the PHS of subgap conductances in superconductors using both the rate equation and Keldysh formalism, which have different regimes of validity. In both regimes, we found that such couplings give rise to a particle-hole $asymmetry$ in subgap conductances which increases with increasing coupling strength, increasing subgap-state particle-hole content imbalance and decreasing temperature. Our proposed mechanism is general and applies even for experiments where the subgap-conductance PHS breaking cannot be attributed to QP.