  A variety of new and interesting correlated states have been predicted in
graphene monolayer doped to Van Hove singularities (VHSs) of its
density-of-state (DOS). However, tuning the Fermi energy to reach a VHS of
graphene by either gating or chemical doping is prohibitively difficult, owning
to their large energy distance (3 eV). Therefore, these correlated states,
which arise from effects of strong electron-electron interactions at the VHSs,
have remained experimentally elusive. Here, we report experimental evidences of
electron-electron interactions around the VHSs of a twisted bilayer graphene
(TBG) through scanning tunneling microscopy measurements. By introducing a
small twisted angle between two adjacent graphene sheets, we are able to
generate low-energy VHSs arbitrarily approaching the Fermi energy. The split of
the VHSs are observed and the symmetry breaking of electronic states around the
VHSs are directly visualized. These results experimentally demonstrate the
important effects of electron-electron interactions on electronic properties
around the VHSs of the TBG, therefore providing motivation for further
theoretical and experimental studies in graphene systems with considering
many-body interactions.
