Accurate tight-binding model for twisted bilayer graphene describes topological flat bands without geometric relaxation

S Pathak and T Rakib and R Hou and A Nevidomskyy and E Ertekin and HT Johnson and LK Wagner, PHYSICAL REVIEW B, 105, 115141 (2022).

DOI: 10.1103/PhysRevB.105.115141

A major hurdle in understanding the phase diagram of twisted bilayer graphene is the roles of lattice relaxation and electronic structure on isolated band flattening near magic twist angles. In this work, the authors develop an accurate local environment tight-binding model fit to tight-binding parameters computed from ab initio densityfunctional theory calculations across many atomic configurations. With the accurate parametrization, it is found that the magic angle shifts to slightly lower angles than often quoted, from around 1.05 degrees. to around 0.99 degrees, and that isolated flat bands appear for rigidly rotated graphene layers, with enhancement of the flat bands when the layers are allowed to distort. Study of the orbital localization supports the emergence of fragile topology in the isolated flat bands without the need for lattice relaxation.

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