Andreev reflection at the interface with an oxide in the quantum Hall regime

TL;DR

This study explores Andreev reflection at the interface between a ZnO-based 2DES and a superconductor in the quantum Hall regime, revealing suppression of Andreev reflection due to multiple reflections at high magnetic fields.

Contribution

It introduces ZnO heterostructures as a new platform for 2DES/superconductor junctions and demonstrates the suppression of Andreev reflection in the quantum Hall regime.

Findings

Andreev reflection observed at low temperatures in ZnO 2DES.

Conductance oscillates with quantum Hall filling factors.

Suppression of Andreev reflection in well-developed quantum Hall states.

Abstract

Quantum Hall/superconductor junctions have been an attractive topic as the two macroscopically quantum states join at the interface. Despite longstanding efforts, however, experimental understanding of this system has not been settled yet. One of the reasons is that most semiconductors hosting high-mobility two-dimensional electron systems (2DES) usually form Schottky barriers at the metal contacts, preventing efficient proximity between the quantum Hall edge states and Cooper pairs. Only recently have relatively transparent 2DES/superconductor junctions been investigated in graphene. In this study, we propose another material system for investigating 2DES/superconductor junctions, that is ZnO-based heterostrcuture. Due to the ionic nature of ZnO, a Schottky barrier is not effectively formed at the contact with a superconductor MoGe, as evidenced by the appearance of Andreev reflection at low temperatures. With applying magnetic field, while clear quantum Hall effect is observed for ZnO 2DES, conductance across the junction oscillates with the filling factor of the quantum Hall states. We find that Andreev reflection is suppressed in the well developed quantum Hall regimes, which we interpret as a result of equal probabilities of normal and Andreev reflections as a result of multiple Andreev reflection at the 2DES/superconductor interface.