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

TL;DR

This paper uses a Hartree-Fock approach to explain the correlated insulating states and quantum anomalous Hall effects in twisted bilayer graphene, revealing flavor symmetry breaking, moiré orbital antiferromagnetism, and the influence of nonlocal exchange interactions.

Contribution

It provides a comprehensive theoretical framework explaining various correlated states and QAH phenomena in TBG, including the effects of nonlocal exchange interactions and alignment with hBN.

Findings

Flavor symmetry breaking favors insulating states at all integer fillings.

Valley polarized states exhibit moiré orbital antiferromagnetic order.

QAH states at certain fillings are stabilized by spin and orbital ferromagnetism.

Abstract

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 a fully unrestricted Hartree-Fock variational method, we have explained the correlated insulating states and QAH effects at various 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 $\pm 1/2$ filling and at the charge neutrality point are all valley polarized sates which break $C_{2z}$ and time-reversal ($\mathcal{T}$) symmetries, but preserves $C_{2z}\mathcal{T}$ symmetry. Such valley polarized states exhibit "moir\'e orbital antiferromagnetic ordering" on an emergent honeycomb lattice with compensating circulating current pattern in the moir\'e supercell. Within the same theoretical framework, our calculations indicate that the $C\!=\!\mp 1$ QAH states at $\pm 3/4$ 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 tend 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 stabilize 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.