Excitonic topological order in imbalanced electron–hole bilayers

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 that a large bulk gap exists, encompassing a broad range of density imbalances 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 approximately equal to e 2 / h at 35 tesla, where e is the elementary charge and h is Planck’s constant. 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.