Excitonic topological order in imbalanced electron-hole bilayers

TL;DR

This paper reports the discovery of an excitonic topological order in imbalanced electron-hole bilayers, demonstrating a new quantum phase with long-range entanglement and unique edge transport properties in InAs/GaSb quantum wells.

Contribution

It introduces the realization of moat-band physics and excitonic topological order in solid-state systems, supported by experimental observations and theoretical modeling.

Findings

Observation of a bulk gap with edge channels in InAs/GaSb quantum wells

Persistence of the gap and Hall plateau under high magnetic fields

Theoretical explanation of topological order from moat-band excitons

Abstract

Correlation and frustration play essential roles in physics, giving rise to novel quantum phases [1-6]. A typical frustrated system is correlated bosons on moat bands, which could host topological orders with long-range quantum entanglement [4]. However, the realization of moat-band physics is still challenging. Here, we explore moat-band phenomena in shallowly-inverted InAs/GaSb quantum wells, where we observe an unconventional time-reversal-symmetry breaking excitonic ground state under imbalanced electron and hole densities. We find a large bulk gap exists encompassing a broad range of density imbalance at zero magnetic field (B), accompanied by edge channels that resemble helical transport. Under an increasing perpendicular B, the bulk gap persists, and an anomalous plateau of Hall signals appears, which demonstrates an evolution from helical-like to chiral-like edge transport with a Hall conductance ~e2/h at 35 Tesla. Theoretically, we show that strong frustration from density imbalance leads to a moat band for excitons, resulting in a time-reversal-symmetry breaking excitonic topological order, which explains all our experimental observations. Our work opens up a new direction for research on topological and correlated bosonic systems in solid states beyond the framework of symmetry-protected topological phases, including but not limited to the bosonic fractional quantum Hall effect.