Excitonic insulator in semiconducting TMD moiré heterojunction (Part 1)

The moiré superlattices of 2D semiconductors exhibit many novel physics phenomena including moiré excitons and correlated insulating states. In this work, we report the evidence of an excitonic insulator in angle-aligned 1L/2L WS₂/WSe₂ moiré heterostructure. First, we investigate reflectance contrast spectra of WS₂/WSe₂ moiré heterostructure with varying WSe₂ layer number. The layer degree of freedom strongly modifies the resonant energies of the moiré excitons. In addition, the 1L/2L and 1L/3L WS₂/WSe₂ show an extra excitonic peak close to the original WSe₂ A-exciton resonance, a result of the interfacial nature of the moiré coupling. Further, we show that the valence bands of the 1^st and the 2^nd WSe₂ layer in 1L/2L WS₂/WSe₂ can be tuned to overlap by applying electric field. Holes can partially populate the 2^nd WSe₂ layer at a total filling of one hole per moiré supercell, which is evidenced by the emergence of exciton-polarons from the 2^nd WSe₂ layer. The abrupt changes of peak positions and intensities of moiré excitons suggest that the system is in an insulating state at n=-1. From particle-hole transformation, electron-like carriers reside in the moiré miniband of the 1^st WSe₂ layer and bound with holes in the 2^nd WSe₂ layer due to strong Coulomb interaction, forming an excitonic insulator phase.