Ferromagnetic Mott state in twisted graphene bilayers at the magic angle*

We address the effective tight-binding Hamiltonian that describes the insulating Mott state of twisted graphene bilayers at a magic angle [1]. In that configuration, twisted bilayers form a honeycomb superlattice of localized states, characterized by the appearance of flat bands with fourfold degeneracy. After calculating the maximally localized superlattice Wannier wave functions, we derive the effective spin model that describes the Mott state at quarter filling, which has an emergent SU(4) symmetry that is reminiscent of spin and valley quantum numbers. We suggest that the system is an exotic ferromagnetic Mott insulator, with well-defined experimental signatures.
[1] K. Seo, V. N. Kotov, B. Uchoa, PRL 122,246402 (2019).