Kekul\'e valence bond order in an extended Hubbard model on the honeycomb lattice, with possible applications to twisted bilayer graphene

Using large-scale quantum Monte Carlo simulations, we exactly solve a model of Fermions hopping on the honeycomb lattice with cluster charge interactions, which has been proposed as an effective model with possible application to twisted bilayer graphene near half-filling. We find an interaction driven semimetal to insulator transition to an insulating phase consisting of a valence bond solid with Kekul'e pattern. Finite size scaling reveals that the phase transition of the semimetal to Kekul'e valence bond solid phase is continuous and belongs to chiral XY universality class.