Symmetry breaking and skyrmionic transport in twisted bilayer graphene

Understand

Motivated by recent low-temperature magnetoresistance measurements in twisted bilayer graphene aligned with hexagonal Boron Nitride substrate, we perform a systematic study of possible symmetry breaking orders in this device at a filling of two electrons per Moir\'e unit cell.

• We find that the surprising non-monotonic dependence of the resistance on an out-of-plane magnetic field is difficult to reconcile with particle-hole charge carriers from the low-energy bands in symmetry broken phases.
• We invoke the non-zero Chern numbers of the twisted bilayer graphene flat bands to argue that skyrmion textures provide an alternative for the dominant charge carriers.
• Via an effective field-theory for the spin degrees of freedom, we show that the effect of spin Zeeman splitting on the skyrmion excitations provides a possible explanation for the non-monotonic magnetoresistance.