Pairing symmetry of interacting fermions on a twisted bilayer graphene superlattice

The pairing symmetry of the Hubbard Hamiltonian on a triangle lattice with a nearly flat low-energy band is studied with the determinant quantum Monte Carlo method (DQMC). We show that the low-temperature phase is insulating at half-filling, even for relatively weak interactions. The natures of the spin and pairing correlations upon doping are determined, and they exhibit an electron-hole asymmetry. Among the pairing symmetries allowed, we demonstrate that the dominating channels are d wave, opening the possibility of condensation into an unconventional d(x)2_(y)2 + id(xy) phase, which is characterized by an integer topological invariant and gapless edge states. The results are closely related to the correlated insulating phase and unconventional superconductivity discovered recently in twisted bilayer graphene.