Flat band topology of magic angle graphene on a transition metal dichalcogenide

TL;DR

This paper explores how proximity-induced spin-orbit coupling in twisted bilayer graphene on transition metal dichalcogenides creates flat, topologically nontrivial bands, enabling potential realization of topological insulators.

Contribution

It demonstrates the impact of substrate-induced spin-orbit coupling on flat band topology and identifies conditions for topological phases in twisted bilayer graphene.

Findings

Presence of extremely flat bands across the mini-Brillouin zone.

Identification of parameter regimes for valley Chern and topological insulators.

Possibility of realizing a time-reversal protected topological insulator through doping.

Abstract

We consider twisted bilayer graphene on a transition metal dichalcogenide substrate, where proximity-induced spin-orbit coupling significantly alters the eight flat bands which occur near the magic angle. The resulting band structure features a pair of extremely flat bands across most of the mini-Brillouin zone. Further details depend sensitively on the symmetries of the heterostructure; we find semiconducting band structures when all two-fold rotations around in-plane axis are broken, and semi-metallic band structures otherwise. We calculate the Chern numbers of the different isolated bands, and identify the parameter regimes and filling factors where valley Chern insulators and topological insulators are realized. Interestingly, we find that for realistic values of the proximity-induced terms, it is possible to realize a topological insulator protected by time-reversal symmetry by doping two holes or two electrons per superlattice unit cell into the system.