Van Hove singularities and Lifshitz transitions in magic-angle twisted bilayer graphene

The moiré superlattice in magic angle twisted bilayer graphene gives rise to strong electron correlations when the Fermi level is aligned with its flat bands. We carried out magneto-transport and Hall density measurements which provide access to the Fermi surface topology and its evolution as a function of doping. As the Fermi level sweeps through the flat bands, we observe that the Fermi surface undergoes a sequence of Lifshitz transitions at integer moiré fillings where discontinuous changes in the Fermi surface topology are accompanied by van Hove singularities (VHS). We find that these VHS facilitate the emergence of correlation-induced gaps and topologically non-trivial sub-bands at fillings of 1, 2, 3 carriers per moiré cell. The non-trivial topology of these sub-bands is revealed in the presence of a magnetic field by the appearance of precisely quantized Hall plateaus with Chern numbers, C=3,2,1 respectively.