Unconventional topological superconductivity and phase diagram for an effective two-orbital model as applied to twisted bilayer graphene

We consider the superconducting and Mott-insulating states for twisted bilayer graphene, modeled as a two-narrow-band system of electrons with appreciable intra-atomic Coulomb interactions. The interaction induces kinetic exchange which leads to real space, either triplet- or singlet-spin pairing, in direct analogy to heavy fermions and high-temperature superconductors. By employing the statistically consistent Gutzwiller method, we construct explicitly the phase diagram as a function of electron concentration for the spin-triplet d_x²-y²+idxy paired case, as well as determine the topological edge states. The model reproduces principal features observed experimentally in a semiquantitative manner. The essential role of electronic correlations in driving both the Mott-insulating and superconducting transitions is emphasized. The transformation of the spin-triplet state into its spin-singlet analog is also analyzed, as well as the appearance of the phase-separated superconducting+Mott-insulating state close to the half-filling.