Metallization of Rashba wire by superconducting layer in the strong-proximity regime

TL;DR

This paper numerically investigates a semiconductor Rashba wire strongly coupled to a superconductor, revealing significant band shifts and metallization effects that challenge the realization of topological phases in such systems.

Contribution

It provides a detailed numerical analysis of the strong-coupling regime, highlighting the impact of superconductor-induced band shifts on topological phase realization.

Findings

Strong coupling causes large band shifts in the wire.

Disorder and width have minimal impact on the induced gap.

Metallization of the semiconductor complicates topological phase realization.

Abstract

Semiconducting quantum wires defined within two-dimensional electron gases and strongly coupled to thin superconducting layers have been extensively explored in recent experiments as promising platforms to host Majorana bound states. We study numerically such a geometry, consisting of a quasi-one-dimensional wire coupled to a disordered three-dimensional superconducting layer. We find that, in the strong-coupling limit of a sizable proximity-induced superconducting gap, all transverse subbands of the wire are significantly shifted in energy relative to the chemical potential of the wire. For the lowest subband, this band shift is comparable in magnitude to the spacing between quantized levels that arise due to the finite thickness of the superconductor (which typically is $\sim500$ meV for a 10-nm-thick layer of Aluminum); in higher subbands, the band shift is much larger. Additionally, we show that the width of the system, which is usually much larger than the thickness, and moderate disorder within the superconductor have almost no impact on the induced gap or band shift. We provide a detailed discussion of the ramifications of our results, arguing that a huge band shift and significant renormalization of semiconducting material parameters in the strong-coupling limit make it challenging to realize a topological phase in such a setup, as the strong coupling to the superconductor essentially metallizes the semiconductor. This metallization of the semiconductor can be tested experimentally through the measurement of the band shift.