Soft Superconducting Gap in Semiconductor Majorana Nanowires

TL;DR

This paper investigates the causes of the soft superconducting gap in semiconductor-superconductor nanowires, identifying interface inhomogeneity with dissipation as the primary factor, and suggests improving interface quality to achieve a harder gap.

Contribution

It systematically analyzes various disorder and dissipation effects, pinpointing interface inhomogeneity as the key to the soft gap phenomenon in Majorana nanowires.

Findings

Interface inhomogeneity with dissipation explains the soft gap.

Improving interface quality can lead to a harder superconducting gap.

Other factors like magnetic disorder and temperature are less consistent with experiments.

Abstract

We theoretically consider the ubiquitous soft gap measured in the tunneling conductance of semiconductor-superconductor hybrid structures, in which recently observed signatures of elusive Majorana bound states have created much excitement. We systematically study the effects of magnetic and non-magnetic disorder, temperature, dissipative Cooper pair breaking, and interface inhomogeneity, which could lead to a soft gap. We find that interface inhomogeneity with moderate dissipation is the only viable mechanism that is consistent with the experimental observations. Our work indicates that improving the quality of the superconductor-semiconductor interface should result in a harder induced gap.