Local Kekul\'e distortion turns twisted bilayer graphene into topological Mott insulators and superconductors

Abstract

Magic-angle twisted bilayer graphene displays at different fillings of the four flat bands lying around the charge neutrality point a wealth of notable phases that include magnetic Chern insulators, whose magnetization is mostly of orbital nature, and contiguous superconducting domes. Such rich phase diagram is here explained through the positive interplay of Coulomb repulsion and the electron coupling to a twofold optical mode that corresponds to Kekul\`e distortions localized into the small AA stacked regions of the moir\'e supercells. A static distortion stabilizes, at any integer filling of the flat bands, valence-bond insulators that carry finite Chern number away from charge neutrality. Similarly, a dynamic distortion that resonates between the two lattice vibrations leads to resonating-valence-bond topological insulators with built-in chiral d-wave pairs that have finite Chern number equal to the angular momentum, and thus are prone to turn superconducting upon doping away from integer filling.