Phenomenological theory of correlated insulators and superconductivity in twisted bilayer graphene

We introduce and analyze a model that sheds light on the interplay between correlated insulating states, superconductivity, and flavor-symmetry breaking in magic-angle twisted bilayer graphene. Using a variational mean-field theory, we determine the normal-state phase diagram of our model as a function of the band filling. The phase diagram includes robust insulators at even integer fillings, occasional weaker insulators at odd integer fillings, and a pattern of flavor-symmetry breaking at non-integer fillings. Adding a phonon-mediated inter-valley retarded attractive interaction, we obtain strong-coupling superconducting domes, whose structure is in qualitative agreement with experiments. Our model elucidates how the intricate form of the interactions and the particle-hole asymmetry of the electronic spectrum determine the phase diagram. It also explains how subtle differences between devices may lead to experimentally observing different behaviors. A similar model with minor modifications is suitable for describing other systems, such as untwisted and twisted trilayer Graphene.

zoom link: https://technion.zoom.us/j/98101735152