Fluxoid-induced pairing suppression and near-zero modes in quantum dots coupled to full-shell nanowires

TL;DR

This paper investigates how fluxoids in a full-shell nanowire influence the induced pairing and subgap states in a coupled quantum dot, revealing mechanisms for near-zero energy modes and parity crossing modifications.

Contribution

It introduces a model showing fluxoid effects can suppress pairing and stabilize near-zero modes in quantum dot-nanowire systems, expanding understanding of topological and trivial zero-energy states.

Findings

Fluxoids cause the induced pairing to vanish under symmetric coupling.

Subgap levels are pushed closer to zero energy by fluxoid-induced level renormalization.

Near-zero modes are stabilized and weakly dispersing in certain fluxoid lobes.

Abstract

We analyze the subgap excitations and phase diagram of a quantum dot (QD) coupled to a semiconducting nanowire fully wrapped by a superconducting (S) shell. We take into account how a Little-Parks (LP) pairing fluxoid (a winding in the S phase around the shell) influences the proximity effect on the dot. We find that under axially symmetric QD-S coupling, shell fluxoids cause the induced pairing to vanish, producing instead a level renormalization that pushes subgap levels closer to zero energy and flattens fermionic parity crossings as the coupling strength increases. This fluxoid-induced stabilization mechanism has analoges in symmetric S-QD-S Josephson junctions at phase $\pi$, and can naturally lead to patterns of near-zero modes weakly dispersing with parameters in all but the zero-th lobe of the LP spectrum.