Speaker
Description
Magic-angle twisted bilayer graphene exhibits diverse, fascinating phases when the four flat bands around the charge neutrality point are partially filled. These phases include magnetic Chern insulators with predominantly orbital magnetization and adjacent superconducting regions. Recent scanning tunneling microscopy (STM) measurements on low strain samples have provided evidence of a local Kekulé pattern at the graphene scale in the half-filling insulating state.
The emergence of these intriguing phases can be attributed to the cooperative effects of Coulomb repulsion and electron coupling to a twofold optical mode associated with Kekulé distortions and localized in the small AA-stacked regions of the moiré supercells. At any integer filling of the flat bands, a static distortion stabilizes valence-bond insulators with non-zero Chern numbers away from charge neutrality. Furthermore, a dynamic distortion arising from resonating lattice vibrations gives rise to resonating-valence-bond topological insulators, featuring chiral d-wave pairs with Chern numbers equal to the angular momentum. This picture naturally leads to superconductivity upon doping away from integer fillings.
The compelling agreement between this phonon-mediated local Kekulé distortion scenario and experimental observations underscores the pivotal role of phonons in driving the ground state properties of twisted bilayer graphene.
