Even-odd effect and Majorana states in full-shell nanowires

TL;DR

This paper provides a theoretical analysis of full-shell nanowires, demonstrating how their unique properties can host Majorana zero modes under specific conditions, with implications for topological quantum computing.

Contribution

It introduces a criterion based on radial subband occupation for the emergence of Majorana zero modes in full-shell nanowires, accounting for realistic device effects.

Findings

Majorana zero modes appear under odd radial subband occupation.

Zero modes cause zero-bias anomalies in tunneling transport.

Subband mixing influences the stability and protection of zero modes.

Abstract

Full-shell nanowires (semiconducting nanowires fully coated with a superconducting shell) have been recently presented as a novel means to create Majorana zero modes. In contrast to partially coated nanowires, it has been argued that full-shell nanowires do not require high magnetic fields and low densities to reach a putative topological regime. Here we present a theoretical study of these devices taking into account all the basic ingredients, including a charge distribution spread across the section of the nanowire, required to qualitatively explain the first experimental results (Vaitiekenas et al., arXiv:1809.05513). We derive a criterion, dependent on the even-odd occupation of the radial subbands with zero angular momentum, for the appearance of Majorana zero modes. In the absence of angular subband mixing, these give rise to strong zero-bias anomalies in tunneling transport in roughly half of the system's parameter space under an odd number of flux quanta. Due to their coexistence with gapless subbands, the zero modes do not enjoy generic topological protection. Depending on the details of subband mixing in realistic devices, they can develop a topological minigap, acquire a finite lifetime or even be destroyed.