Excitonic insulator and novel trion states in twisted MoSe2/WSe2 bilayers

Twisted bilayer transition metal dichalcogenides have emerged as one of the most active research fields in recent years. At the full filling of the first moiré valence band, the electron–hole pairs formed in the moiré energy bands can combine to form moiré excitons. We investigate the phase diagram of twisted ${\text{MoSe}}_2/{\text{WSe}}_2$ bilayers with varying twist angle and moiré potential strength. It is found that a strong moiré potential is beneficial for the excitonic insulator (EI) state at large twist angles. Moreover, the condensation of moiré excitons can further induce a ground state with non-conserved trion number. The emergence of this state might provide novel electrical properties. A toy model is proposed to describe the occurrence of this state and a rigorous proof is given to the critical conditions through the application of the Feynman diagram technique.