Excitonic insulator in a heterojunction moire superlattice

Two-dimensional moire superlattices provide a highly tunable platform to study strongly correlated physics. In particular, the moire superlattices of two-dimensional semiconductor heterojunctions have been shown to host tunable correlated electronic states such as a Mott insulator and generalized Wigner crystals(1-4). Here we report the observation of an excitonic insulator(5-7), a correlated state with strongly bound electrons and holes, in an angle-aligned monolayer WS2/bilayer WSe2 moire superlattice. The moire coupling induces a flat miniband on the valence-band side only in the first WSe2 layer interfacing WS2. The electrostatically introduced holes first fill this miniband and form a Mott insulator when the carrier density corresponds to one hole per moire supercell. By applying a vertical electric field, we tune the valence band in the second WSe2 layer to overlap with the moire miniband in the first WSe2 layer, realizing the coexistence of electrons and holes at equilibrium, which are bound as excitons due to a strong Coulomb interaction. We show that this new bound state is an excitonic insulator with a transition temperature as high as 90 K. Our study demonstrates a moire system for the study of correlated many-body physics in two dimensions.

Automated summary

This paper reports the observation of an excitonic insulator in an angle-aligned monolayer WS2/bilayer WSe2 moire superlattice. By tuning the valence band in the second WSe2 layer to overlap with the moire miniband in the first WSe2 layer, the coexistence of electrons and holes is achieved, resulting in the formation of an excitonic insulator with a high transition temperature.