Kondo effect in Twisted Bilayer Graphene

Graphene has a lot of interesting properties. However, it's low density of states near the charge neutrality point means that it can not show a Kondo effect under generic circumstances. Depending on the hybridization, magnetic impurities embedded on its surface remain unscreened even at the lowest temperatures. We show that this is no longer the case in magic angle twisted bilayer graphene. By studying the effective Bistritzer-MacDonald model, we capture features of the density of states at low energies as a Dirac cone flanked by van Hove singularities. As the two layers of graphene are twisted toward the magic angle, these logarithmic van Hove singularities approach each other in energy, and pinch off the pseudogap associated with the cone, forming a higher order order power law singularity at the magic angle. The enhanced density of states at the Fermi energy plays a critical role in the re-entrance of strongly correlated phenomena like the Kondo effect due to a magnetic impurity. By studying the resulting quantum impurity physics using perturbative and numerical renormalization group methods, we find that at zero temperature the impurity is only Kondo screened precisely at the magic angle. We also offer predictions of highly nontrivial behavior at finite temperatures relevant to experiments, due to the complex interplay between Dirac, van Hove, and Kondo physics.