Disorder-induced half-integer quantized conductance plateau in quantum anomalous Hall insulator-superconductor structures

TL;DR

This paper challenges the interpretation of half-integer conductance plateaus as evidence of topological superconductivity, showing they can arise from disorder-induced percolation effects in quantum anomalous Hall systems, and emphasizes the need for additional signatures.

Contribution

It demonstrates that disorder-induced percolation can produce conductance plateaus similar to those from Majorana modes, complicating the identification of topological superconductivity.

Findings

Half-integer conductance plateaus can result from edge percolation, not just Majorana modes.

Additional signatures like a hard superconducting gap or thermal conductance are necessary.

Disorder effects can mimic topological signatures in transport measurements.

Abstract

A weak superconducting proximity effect in the vicinity of the topological transition of a quantum anomalous Hall system has been proposed as a venue to realize a topological superconductor (TSC) with chiral Majorana edge modes (CMEMs). A recent experiment [Science 357, 294 (2017)] claimed to have observed such CMEMs in the form of a half-integer quantized conductance plateau in the two-terminal transport measurement of a quantum anomalous Hall-superconductor junction. Although the presence of a superconducting proximity effect generically splits the quantum Hall transition into two phase transitions with a gapped TSC in between, in this Rapid Communication we propose that a nearly flat conductance plateau, similar to that expected from CMEMs, can also arise from the percolation of quantum Hall edges well before the onset of the TSC or at temperatures much above the TSC gap. Our Rapid Communication, therefore, suggests that, in order to confirm the TSC, it is necessary to supplement the observation of the half-quantized conductance plateau with a hard superconducting gap (which is unlikely for a disordered system) from the conductance measurements or the heat transport measurement of the transport gap. Alternatively, the half-quantized thermal conductance would also serve as a smoking-gun signature of the TSC.