Effects of large induced superconducting gap on semiconductor Majorana nanowires

TL;DR

This paper explores the effects of a large induced superconducting gap in semiconductor nanowires with epitaxial superconductor interfaces, highlighting implications for topological superconductivity and Majorana modes.

Contribution

It introduces a minimal theory for the strong-coupling, hard-gap regime in semiconductor-superconductor nanowires, aligning with recent experimental observations and guiding future research.

Findings

Large induced gaps can suppress spin-orbit effects and g-factors.

Strong coupling may require weaker interface tunneling for optimal proximity.

The theory agrees qualitatively with experimental data.

Abstract

With the recent achievement of extremely high-quality epitaxial interfaces between InAs nanowires and superconducting Al shells with strong superconductor-semiconductor tunnel coupling, a new regime of proximity-induced superconductivity in semiconductors can be explored where the induced gap may be similar in value to the bulk Al gap (large gap) with negligible subgap conductance (hard gap). We propose several experimentally relevant consequences of this large-gap strong-coupling regime for tunneling experiments, and we comment on the prospects of this regime for topological superconductivity. In particular, we show that the advantages of having a strong spin-orbit coupling and a large spin g-factor in the semiconductor nanowire may both be compromised in this strongly coupled limit, and somewhat weaker interface tunneling may be necessary for achieving optimal proximity superconductivity in the semiconductor nanowire. We derive a minimal, generic theory for the strong-coupling hard-gap regime obtaining good qualitative agreement with the experiment and pointing out future directions for further progress toward Majorana nanowires in hybrid semiconductor-superconductor structures.