  The emergence of flat bands in twisted bilayer graphene at the magic angle
can be understood in terms of a vanishing Fermi velocity of the Dirac cone.
This is associated with van Hove singularities approaching the Fermi energy and
becoming higher-order. In the density of states this is reflected by flanking
logarithmic van Hove divergences pinching off the central Dirac cone in energy
space. The low-energy pseudogap of the Dirac cone away from the magic angle is
replaced by a power-law divergence due to the higher-order van Hove singularity
at the magic angle.
  This plays an important role in the exotic phenomena observed in this
material, such as superconductivity and magnetism, by amplifying electronic
correlation effects. Here we investigate one such correlation effect -- the
Kondo effect due to a magnetic impurity embedded in twisted bilayer graphene.
We use the Bistritzer-MacDonald model to extract the low-energy density of
states of the material as a function of twist angle, and study the resulting
quantum impurity physics using perturbative and numerical renormalization group
methods. Although at zero temperature the impurity is only Kondo screened
precisely at the magic angle, we find highly nontrivial behavior at finite
temperatures relevant to experiment, due to the complex interplay between
Dirac, van Hove, and Kondo physics.
