Interlayer excitons and trions in electron-hole bilayers

The interplay between attractive and repulsive Coulomb interactions can stabilize a wide variety of quantum multiparticle composites and rich quantum phases. Analogous to atoms, ions and molecules formed by electrons and nuclei, electrons and holes in semiconductors can produce multiparticle states - excitons, trions, biexcitons, and mesoscopic droplets - but these are generally limited to transient excited states under photoexcitation. In this talk, I will present our recent progress on experimental realization of thermodynamically stable exciton and trion fluids in van der Waals heterostructures. By putting a two-dimensional electron gas and a two-dimensional hole gas close to each other while remaining electrically isolated, we achieve spontaneous formation of interlayer excitons and trions in full thermal equilibrium, with tunability via electrostatic gating. Using optical spectroscopy and electrical transport measurements, we investigate thermodynamic properties of such excitons and trions based on MoSe2/hBN/WSe2 heterostructures. Our results include:

1. an excitonic insulator phase at balanced electron and hole densities
2. perfect Coulomb drag behavior in the dipolar exciton fluid;
3. quantum oscillations and quantum Hall phase transitions in the excitonic insulator;
4. spontaneous formation of three-particle trion states at imbalanced electron-hole densities.

These findings provide new insights into strongly correlated electron-hole systems and open pathways for excitonic quantum phases such as exciton condensates and superfluids