Moire Materials Group

Thursday Nov 5, 2020, 9:30 AM → 11:00 AM America/New_York

Speaker: Eslam Khalaf (Harvard)

Hidden symmetry, correlated insulators and superconductivity in magic angle twisted bilayer graphene
 
When two sheets of graphene are twisted relative to each other by an angle around 1 degree, a host of interaction-driven phases including correlated insulators and superconductors are observed. I will discuss the nature of these correlated insulators, their low energy excitations and how they give rise to superconductivity upon doping. Starting by projecting Coulomb interaction onto the flat bands, I will show that the problem is characterized by a hidden approximate U(4)xU(4) symmetry. This approximate symmetry allows us to map the problem to that of a pair of tunnel-coupled multilayer Chern insulators with opposite Chern numbers whose ground state at some integer fillings can be obtained exactly. In addition to identifying the insulating ground states at integer fillings, I will discuss the properties of their low energy bosonic excitations including their count, degeneracies, and symmetry quantum numbers. This data constitutes a fingerprint of the symmetry breaking insulating state which enables its experimental identification. At the end, I will show that doping such insulators leads to superconductivity via  pairing topological skyrmion textures. Remarkably, this new mechanism of superconductivity, which is distinct from weak coupling phonon-mediated pairing and unconventional pairing mechanisms in cuprates, arises solely from repulsive interactions. I will discuss how these insights not only clarify why certain correlated moire materials do not become superconducting, but they also point to promising new platforms where robust superconductivity is anticipated.