Equilibrium exciton insulator and exciton quasi-condensation in semiconductor atomic double layers

Exciton insulator, a conductor for excitons but an insulator for charges, is expected to emerge in a material when its energy gap for charge excitations becomes smaller than the exciton binding energy, beyond which an exciton gas is spontaneously formed. Although the concept is known to the physics community for nearly seventy years, the thermodynamic evidence of an exciton insulator remains elusive. Atomic double layers of transition metal dichalcogenide (TMD) semiconductor, which support large interlayer exciton binding energy, provide an ideal platform to realize an exciton insulator. In this talk, I will present direct thermodynamic evidence for an equilibrium exciton insulator in TMD double layers. Using a new exciton-contact device design, we have created a charge-free equilibrium exciton fluid by continuously reducing the charge gap of the system below its exciton binding energy. Compressibility measurements show that the fluid is exciton-compressible but charge-imcompressible, directly demonstrating its exciton insulating character. I will further discuss the phase diagram for the strongly interacting exciton fluid and provide direct thermodynamic evidence of 2D quasi-condensation of excitons.