Closing of the Induced Gap in a Hybrid Superconductor-Semiconductor Nanowire

TL;DR

This study uses nonlocal conductance measurements to investigate the gap closure in superconductor-semiconductor nanowires, challenging simple topological interpretations and revealing complex extended states and uncorrelated zero-bias peaks.

Contribution

It introduces a novel experimental approach to probe the spatial extent of states in Majorana wires, providing new insights into the gap closure and topological transition.

Findings

Gap closes in the bulk before zero-bias peaks appear

Nonlocal signals fluctuate independently of local zero-bias peaks

Results are incompatible with simple topological superconductor models

Abstract

Hybrid superconductor-semiconductor nanowires are predicted to undergo a field-induced phase transition from a trivial to a topological superconductor, marked by the closure and re-opening of the excitation gap, followed by the emergence of Majorana bound states at the nanowire ends. Many local density-of-states measurements have reported signatures of the topological phase, however this interpretation has been challenged by alternative explanations. Here, by measuring nonlocal conductance, we identify the closure of the excitation gap in the bulk of the semiconductor before the emergence of zero-bias peaks. This observation is inconsistent with scenarios where zero-bias peaks occur due to end-states with a trivially gapped bulk, which have been extensively considered in the theoretical and experimental literature. We observe that after the gap closes, nonlocal signals fluctuate strongly and persist irrespective of the presence of local-conductance zero-bias peaks. Thus, our observations are also incompatible with a simple picture of clean topological superconductivity. This work presents a new experimental approach for probing the spatial extent of states in Majorana wires, and reveals the presence of a regime with a continuum of spatially extended states and uncorrelated zero-bias peaks.