Electronic phases in twisted bilayer graphene at magic angles as a result of Van Hove singularities and interactions

The discovery of different phases as a result of correlations, especially in low-dimensional materials, has been always an exciting and fundamental subject of research. Recent experiments on twisted bilayer graphene have revealed reentrant unconventional superconductivity as a function of doping as well as a Mott-like insulating phase when the two layers are twisted with respect to each other at certain magic angles for doping, corresponding to two particles per moire unit cell. In this paper, we propose a microscopic model that takes into account interactions and the Van Hove singularities in the density of states of twisted bilayer graphene at doping corresponding to one particle (nu = 1) per moire unit cell and study how superconductivity emerges. We identify the possible symmetry of the order parameter as s(+/-), while, if the intervalley coupling is negligible, the symmetry is s(++). In addition, we find and characterize the insulating region of the system as a region with a uniform charge instability where there is coexistence of the metallic and insulating phases.