Symmetry breaking in the double moir\'{e} superlattices of relaxed twisted bilayer graphene on hexagonal boron nitride

We study the atomic and electronic structures of the commensurate double moir'{e} superlattices in fully relaxed twisted bilayer graphene (TBG) nearly aligned with the hexagonal boron nitride (BN). The single-particle effective Hamiltonian (${\widehat{H}}^0$) taking into account the relaxation effect and the full moir'{e} Hamiltonian introduced by BN has been built for TBG/BN. The mean-field (MF) band structures of the self-consistent Hartree-Fock (SCHF) ground states at different number ($\nu$) of filled flat bands relative to the charge neutrality point (CNP) are obtained based on ${\widehat{H}}^0$ in the plane-wave-like basis. The single-particle flat bands in TBG/BN become separated by the opened gap at CNP due to the symmetry breaking in ${\widehat{H}}^0$. We find that the broken $C_2$ symmetry in ${\widehat{H}}^0$ mainly originates from the intralayer inversion-asymmetric structural deformation in the graphene layer adjacent to BN, which introduces spatially non-uniform modifications of the intralayer Hamiltonian. The gapped flat bands have finite Chern numbers. For TBG/BN with the magic twist angle, the SCHF ground states with $\mathrm{\mid }\nu \mathrm{\mid }$ = 1-3 are all insulating with narrow MF gaps. When the flat conduction bands are filled, the gap at $\nu$ = 1 is smaller than that at $\nu$ = 3, suggesting that the nontrivial topological properties associated with the flat Chern bands are more likely to be observed at $\nu =3$. This is similar for negative $\nu$ with empty valence bands. The dependence of the electronic structure of TBG/BN on positive $\nu$ is roughly consistent with recent experimental observations.