  Starting from the strong-coupling limit of an extended Hubbard model, we
develop a spin-fermion theory to study the insulating phase and pairing
symmetry of the superconducting phase in twisted bilayer graphene. Assuming
that the insulating phase is an anti-ferromagnetic insulator, we show that
fluctuations of the anti-ferromagnetic order in the conducting phase can
mediate superconducting pairing. Using a self-consistent mean-field analysis,
we find that the pairing wave function has a chiral d-wave symmetry. Consistent
with this observation, we show explicitly the existence of chiral Majorana edge
modes by diagonalizing our proposed Hamiltonian on a finite-sized system. These
results establish twisted bilayer graphene as a promising platform to realize
topological superconductivity.
