Equilibrium exciton insulator 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. Atomic double layers of transition metal dichalcogenide (TMD) semiconductor, which support large interlayer exciton binding energy, provide an ideal platform to realize exciton insulators with high ionization temperatures. We provide 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 the exciton binding energy. Compressibility measurements show that the fluid is exciton-compressible but charge-imcompressible, directly demonstrating its exciton insulating character. We have further obtained a phase diagram for the strongly interacting exciton fluid.