Perfect Andreev Reflection of Helical Edge Modes in InAs/GaSb Quantum Wells

TL;DR

This study demonstrates perfect Andreev reflection of helical edge modes in InAs/GaSb quantum wells, confirming their topological nature and robustness against interface barriers, with implications for quantum spin Hall systems.

Contribution

First experimental observation of perfect Andreev reflection in InAs/GaSb quantum wells indicating helical edge modes and their resilience to interface barriers.

Findings

Zero bias dips suggest Andreev reflection dominated transport.

Measured mini-gap excess current aligns with theoretical predictions.

Perfect Andreev reflection is sensitive to time-reversal symmetry breaking.

Abstract

We present an experimental study of inverted InAs/GaSb composite quantum wells in the hybridization regime and contacted by superconducting electrodes. A front gate is used to vary the Fermi level into the mini-gap, where recent experiments indicate existence of helical edge modes [arXiv:1105.0137]. Zero bias dips in differential resistance are observed across the mini-gap, suggesting transport dominated by Andreev reflection processes. Evolution of the mini-gap differential resistance with applied bias as well as measured mini-gap excess current of 150 nA are in good agreement with the prediction of perfect Andreev reflection of the helical edge modes, which is necessitated by the absence of back-scattering channels. The perfect Andreev reflection occurs in spite of a finite barrier at the interface and shows strong sensitivity to time-reversal breaking - hallmarks of the helical nature of quantum spin Hall edges.