Strongly correlated excitonic charge transfer insulators and selective Mott transition in twisted bilayers transition metal dichalcogenides (TMDs)

In the last years, a novel class of materials, namely twisted layered heterostructures of transition metal dichalcogenides (TMDs), has emerged as a new experimental platform for the study of strongly correlated phases. Among other reasons, there is a great interest in TMDs because they are good candidates to host the so long sought excitonic insulating phase, that is stabilized by condensation of excitons, i.e. bound pairs of electrons and holes. The forbidden hybridization between two different AB stacked homobilayers could facilitate the condensation of inter-layer excitons, that could explain the non-magnetic insulating phases obtained in twisted homobilayers of WSe₂. Also a Curie-Weiss 1/T behavior of the magnetic susceptibility has been reported for this material pointing to the formation of local moments, a hallmark of strong correlations. However, the precise nature of such an insulating phase has not yet been determined for the excitonic order could coexist with a charge-transfer insulator, Mott insulator or it could lead to a supersolid phase.

Here we present a study of the SU(4)-Hubbard model in the triangular lattice, that should capture the salient features of this material, using Dynamical Mean Field Theory (DMFT).

Our DMFT results show that strong correlations stabilizes an exitonic insulating phase via a selective Mott transition occurring in one of two effective orbitals.

Since the orbital unaffected by the Mott transition already lies below the Fermi level before and after the transition, the system's spectral function displays the typical feautres of a charge transfer insulator. Furthermore, our calculations show a strong enhancement of the local spin/valley susceptibility approaching the transition, in agreement with the experimental evidence of local moments formation.