Gapless Andreev bound states in the quantum spin Hall insulator HgTe

TL;DR

This paper provides experimental evidence of topological superconductivity in a HgTe quantum well, demonstrating 4π-periodic supercurrents and edge states consistent with Majorana physics, advancing the understanding of topological quantum computation.

Contribution

It reports the first clear experimental observation of topological superconductivity and gapless Andreev bound states in a quantum spin Hall insulator, HgTe.

Findings

Observation of 4π-periodic supercurrent via ac Josephson effect.

Detection of edge-localized states responsible for topological superconductivity.

Enhanced effects when gating towards the quantum spin Hall regime.

Abstract

In recent years, Majorana physics has attracted considerable attention in both theoretical and experimental studies due to exotic new phenomena and its prospects for fault-tolerant topological quantum computation. To this end, one needs to engineer the interplay between superconductivity and electronic properties in a topological insulator, but experimental work remains scarce and ambiguous. Here we report experimental evidence for topological superconductivity induced in a HgTe quantum well, a two-dimensional topological insulator that exhibits the quantum spin Hall effect. The ac Josephson effect demonstrates that the supercurrent has a $4\pi$-periodicity with the superconducting phase difference as indicated by a doubling of the voltage step for multiple Shapiro steps. In addition, an anomalous SQUID-like response to a perpendicular magnetic field shows that this $4\pi$-periodic supercurrent originates from states located on the edges of the junction. Both features appear strongest when the sample is gated towards the quantum spin Hall regime, thus providing evidence for induced topological superconductivity in the quantum spin Hall edge states.