Germanium-based hybrid semiconductor-superconductor topological quantum computing platforms: Disorder effects

TL;DR

This paper theoretically analyzes disorder effects on Ge-based hybrid superconductor-semiconductor devices for topological quantum computing, showing they can host Majorana zero modes with clear experimental signatures despite disorder.

Contribution

It provides a detailed numerical study of disorder impacts on Ge hole nanowires, demonstrating their robustness for topological Majorana modes even with realistic disorder levels.

Findings

Topological superconductivity persists despite disorder exceeding initial bounds.

Clear zero-bias peaks indicate Majorana modes in weak disorder regimes.

Ge-based devices have higher gap-to-disorder ratios than InAs-based ones.

Abstract

It was recently suggested that proximitized gate-defined Ge hole nanowires could serve as an alternative materials platform for the realization of topological Majorana zero modes (MZMs). Here, we theoretically study the expected experimental signatures of Ge-based MZMs in tunneling conductance measurements, taking into account that unintentional random disorder is unavoidably present in realistic devices. Explicitly, we present numerically calculated local and nonlocal tunneling conductance spectra (as functions of bias voltage and magnetic field) for two different wire lengths, two different disorder models, two different parent superconductors (Al and NbTiN), and various disorder strengths, which we relate to an estimated lower bound for the disorder strength in Ge-based hybrid devices that we extract from the experimentally reported hole mobilities in current state-of-the-art two-dimensional Ge hole gases. We find that even if the actual disorder strength in the Ge-based hybrid device exceeds this theoretical lower bound by an order of magnitude, the system is still in the weak disorder regime, where the topological superconductivity is intact and the local conductance spectra manifest clear end-to-end correlated zero-bias peaks if and only if the system hosts topological end MZMs. This shows that, despite the relatively small pristine topological gaps ($\sim\,$a few tens of $\mu$eV), Ge-based hybrid devices are a promising alternative platform for Majorana experiments due to the extremely high materials quality, which leads to an increased gap-to-disorder ratio and, therefore, to less ambiguity in the experimental tunneling conductance data compared to InAs-based devices.