Unconventional Superconductivity and Density Waves in Twisted Bilayer Graphene

Abstract

We study electronic ordering instabilities of twisted bilayer graphene around the filling of n=2 electrons per supercell, where correlated insulator state and superconductivity have been recently observed. Motivated by the Fermi surface nesting and the proximity to Van Hove singularity, we introduce a hot-spot model to study the effect of various electron interactions systematically. Using the renormalization group method, we find that d or p-wave superconductivity and charge or spin density wave emerge as the two types of leading instabilities driven by Coulomb repulsion. The density-wave state has a gapped energy spectrum around n=2 and yields a single doubly degenerate pocket upon doping to n>2. The intertwinement of density wave and superconductivity and the quasiparticle spectrum in the density-wave state are consistent with experimental observations.