Majorana bound states in non-homogeneous semiconductor nanowires

TL;DR

This paper shows that non-homogeneous semiconductor nanowires can host low-energy Majorana-like states without topological phase transitions, complicating their experimental identification through charge transport measurements.

Contribution

It reveals that overlapping Majorana bound states can appear in trivial regimes, challenging the uniqueness of topological Majorana zero modes in experiments.

Findings

Overlapping MBSs are common in non-homogeneous nanowires.

Charge transport cannot reliably distinguish trivial MBSs from topological MZMs.

Smooth potential variations lead to a continuum from ABSs to weakly overlapping MBSs.

Abstract

We demonstrate that partially overlapping Majorana bound states (MBSs) represent a generic low-energy feature that emerges in non-homogeneous semiconductor nanowires coupled to superconductors in the presence of a Zeeman field. The emergence of these low-energy modes is not correlated with any topological quantum phase transition that the system may undergo as the Zeeman field and other control parameters are varied. Increasing the characteristic length scale of the variations in the potential leads to a continuous evolution from strongly overlapping MBSs, which can be viewed as "regular" Andreev bound states (ABSs) that cross zero energy, to well separated weakly overlapping MBSs, which have nearly zero energy in a significant range of parameters and generate signatures similar to the non-degenerate zero-energy Majorana zero modes (MZMs) that emerge in the topological superconducting phase. We show that using charge (or spin) transport measurements it is virtually impossible to distinguish MZMs from weakly overlapping MBSs emerging in the topologically-trivial regime.