Strongly Correlated Excitonic Insulator in Atomic Double Layers

Excitonic insulators arise from the formation of bound electron-hole pairs (excitons) in semiconductors and provide a solid-state platform for quantum many-boson physics. Although the concept of excitonic insulator has been understood for sixty years, it has been challenging to establish distinct experimental signatures of its realization. One problem is that exciton coherence in condensed phases inevitably couples to the crystal Hamiltonian so that condensation does not imply superfluidity. A second problem is that the exciton population of a particular material depends very sensitively on band structure details and cannot be controlled. In this talk, I will discuss a recent experiment in which we solve both problems by establishing electrical control of the chemical potentials of interlayer excitons in semiconducting transition metal dichalcogenide double layers and probing the thermodynamic properties of exciton fluids by capacitance measurements. We construct a phase diagram of strongly correlated excitons that reveals both the Mott transition and interaction-stabilized quasi-condensation. Our experiment paves the path for realizing the exotic quantum phases of excitons, as well as multi-terminal exciton circuitry for applications.