UC Berkeley

Electron-hole bilayers provide a versatile platform for realizing strongly correlated quantum matter composed of both fermionic and bosonic degrees of freedom. In this dissertation, we investigate correlated exciton physics in van der Waals heterostructures, where spatially separated electrons and holes form long-lived, equilibrium many-body states. Using transition metal dichalcogenide heterobilayers with atomically thin hexagonal boron nitride spacers, we demonstrate precise electrostatic control of electron and hole densities and develop optical spectroscopy techniques to probe their thermodynamic properties. At charge neutrality and strong interlayer coupling, we establish the formation of a dipolar excitonic insulator, characterized by spontaneous binding of electrons and holes into interlayer excitons. Through combined optical and transport measurements, we reveal a thermodynamic charge gap, extract exciton binding energies, and observe charge-insulating behavior and perfect Coulomb drag. When a large external magnetic field is applied, the excitonic insulator exhibits unconventional quantum oscillations and competes with quantum Hall states, revealing a rich interplay between Coulomb correlations and Landau quantization. Beyond two-body excitons, we explore higher-order bound states, including three-body interlayer trions and four-body interlayer biexcitons, which are in thermal equilibrium and electrically tunable. We investigate their spectroscopic responses, extract their binding energies, and probe their internal spin structures. These results establish electrically controlled composite bosonic and fermionic systems and continuously tunable Bose-Fermi mixtures. We further investigate the many-body collective phases of the stable exciton fluid in the quantum degenerate regime. We report optical signatures of equilibrium exciton condensation, characterized by tunable spinor condensate order with multiple competing condensate states. The ground state at zero magnetic field is a coherent superposition of two simultaneously condensed exciton flavors, with a condensate transition temperature up to 1.8 K. Finally, by introducing a moiré superlattice potential in the bilayer, we demonstrate the emergence of exciton crystals at fractional filling, realizing a strongly interacting lattice boson system. These results establish van der Waals electron-hole bilayers as a powerful platform for engineering correlated excitonic matter.