Charge-transfer insulation in twisted bilayer graphene

Understand

We studied the real space structure of states in twisted bilayer graphene at the `magic angle' $\theta = 1.08^\circ$.

• The flat bands close to charge neutrality are composed of a mix of `ring' and `center' orbitals around the AA stacking region.
• An effective model with localized orbitals is constructed, which necessarily includes more than just the four flat bands.
• Long-range Coulomb interaction causes a charge-transfer at half-filling of the flat bands from the `center' to the `ring' orbitals.