Theories for the correlated insulating states and quantum anomalous Hall effect phenomena in twisted bilayer graphene

The experimentally observed correlated insulating states and quantum anomalous Hall (QAH) effect in twisted bilayer graphene (TBG) have drawn significant attention. However, up to date, the specific mechanisms of these intriguing phenomena are still open questions. Using an all-band Hartree-Fock variational method, we have explained the correlated insulating states and QAH effects at some integer fillings of the flat bands in TBG. Our results indicate that states breaking flavor (valley and spin) symmetries are energetically favored at all integer fillings. In particular, the correlated insulating states at +/- 1/2 filling and at the charge neutrality point are all valley polarized states which break C-2z and time-reversal (T) symmetries but preserve C2zT symmetry. Such valley polarized states exhibit moire orbital antiferromagnetic ordering on an emergent honeycomb lattice with compensating circulating current pattern in the moire supercell. Within the same theoretical framework, our calculations indicate that the C = (-/+)1 QAH states at +/- 3/4 fillings of the magic-angle TBG are spin and orbital ferromagnetic states, which emerge when a staggered sublattice potential is present. We find that the nonlocalness of the exchange interactions tends to enhance the bandwidth of the low-energy bands due to the exchange-hole effect, which reduces the gaps of the correlated insulator phases. The nonlocal exchange interactions also dramatically enhance the spin polarization of the system, which significantly stabilizes the orbital and spin ferromagnetic QAH state at 3/4 filling of TBG aligned with hexagonal boron nitride (hBN). We also predict that, by virtue of the orbital ferromagnetic nature, the QAH effects at electron and hole fillings of hBN-aligned TBG would exhibit hysteresis loops with opposite chiralities.

Automated summary

Using an all-band Hartree-Fock variational method, the mechanisms behind correlated insulating states and quantum anomalous Hall effects in twisted bilayer graphene at integer fillings of flat bands have been explained. Correlated insulating states exhibit valley polarization and moire orbital antiferromagnetic ordering, while the quantum anomalous Hall states show spin and orbital ferromagnetic characteristics.