Excitonic insulator in semiconducting TMD moire heterojunction (Part 2)

Here we report the observation of an excitonic insulator, a correlated state with strongly bound electrons and holes, in an angle-aligned 1L-WS₂/2L-WSe₂ moiré superlattice. The moiré coupling induces a flat miniband on the valence-band side only in the 1^st WSe₂ layer interfacing WS₂, barely affecting the 2^nd WSe₂ layer. The electrostatically introduced holes first fill this miniband and form a Mott insulator with a total carrier density corresponding to one hole per moiré supercell. By applying a vertical electric field, the valence band in the 2^nd WSe₂ layer can be tuned to overlap with the moiré miniband in the 1^st WSe₂ layer, realizing the coexistence of electrons and holes at equilibrium, as confirmed by optical spectroscopy measurements. The electron-hole pairs are bound as excitons due to a strong Coulomb interaction. We use microwave impedance microscopy to confirm the insulating nature of this exotic state in a dual-gate device geometry with monolayer graphene as top-gate. The microwave detection of embedded moiré heterojunction is enabled by the reduced conductivity in the graphene top-gate with applied magnetic field. This insulating state has also been observed in a back-gate only device geometry with finite electric field during hole doping. The excitonic insulator has a transition temperature as high as 90 K. Our study demonstrates a moiré system for the study of correlated many-body physics in two dimensions.