  Twisted bilayer graphene is an excellent example of highly correlated system
demonstrating a nearly flat electron band, the Mott transition and probably a
spin liquid state. Besides the one-electron picture, analysis of Dirac points
is performed in terms of spinon Fermi surface in the limit of strong
correlations. Application of gauge field theory to describe deconfined spin
liquid phase is treated. Topological quantum transitions, including those from
small to large Fermi surface in the presence of van Hove singularities, are
discussed.
