Charge inversion and topological phase transition at a twist angle   induced van Hove singularity of bilayer graphene
  Van Hove singularities (VHS's) in the density of states play an outstanding and diverse role for the electronic and thermodynamic properties of crystalline solids. At the critical point the Fermi surface connectivity changes and topological properties undergo a transition. Opportunities to systematically pass a VHS at the turn of a voltage knob and study its diverse impact are however rare. With the advent of van der Waals heterostructures, control over the atomic registry of neigbouring graphene layers offers an unprecedented tool to generate a low energy VHS easily accessible with conventional gating. Here we have addressed magnetotransport when the chemical potential crosses the twist angle induced VHS in twisted bilayer graphene. A topological phase transition is experimentally disclosed in the abrupt conversion of electrons to holes or vice versa, a loss of a non-zero Berry phase and distinct sequences of integer quantum Hall states above and below the singularity. 
Youngwook Kim, Patrick Herlinger, Pilkyung Moon, Mikito Koshino,   Takashi Taniguchi, Kenji Watanabe and Jurgen.H Smet
cond-mat.mes-hall cond-mat.mtrl-sci cond-mat.str-el