Strongly correlated excitonic insulator in atomic double layers

Excitonic insulators (EIs) arise from the formation of bound electron–hole pairs (excitons) 1 , 2 in semiconductors and provide a solid-state platform for quantum many-boson physics 3 – 8 . Strong exciton–exciton repulsion is expected to stabilize condensed superfluid and crystalline phases by suppressing both density and phase fluctuations 8 – 11 . Although spectroscopic signatures of EIs have been reported 6 , 12 – 14 , conclusive evidence for strongly correlated EI states has remained elusive. Here we demonstrate a strongly correlated two-dimensional (2D) EI ground state formed in transition metal dichalcogenide (TMD) semiconductor double layers. A quasi-equilibrium spatially indirect exciton fluid is created when the bias voltage applied between the two electrically isolated TMD layers is tuned to a range that populates bound electron–hole pairs, but not free electrons or holes 15 – 17 . Capacitance measurements show that the fluid is exciton-compressible but charge-incompressible—direct thermodynamic evidence of the EI. The fluid is also strongly correlated with a dimensionless exciton coupling constant exceeding 10. We construct an exciton phase diagram that reveals both the Mott transition and interaction-stabilized quasi-condensation. Our experiment paves the path for realizing exotic quantum phases of excitons 8 , as well as multi-terminal exciton circuitry for applications 18 – 20 .