Chern Insulators and Topological Flat-bands in Magic-angle Twisted Bilayer Graphene

Magic-angle twisted bilayer graphene (MATBG) hosts low energy flat bands whose quenched kinetic energy facilitates interaction-induced instabilities when the Fermi level is swept through. Beyond many-body interactions, the existence of non-trivial band-topology in MATBG is expected to play a crucial role in the emergence of correlated phases. However, questions concerning the nature of these phases and their connection to the putative non-trivial band-topology are largely unanswered. Here we report on magneto-transport and Hall density measurements that reveal a succession of doping-induced Lifshitz transitions where discontinuous changes in the Fermi surface topology are accompanied by van Hove singularities (VHS) that facilitate the emergence of correlation-induced gaps and topologically non-trivial sub-bands. In the presence of a magnetic field the topology of the sub-bands at filling of 1, 2, 3 carriers per moiré cell, is revealed through well quantized Hall plateaus signaling the appearance of Chern insulators with Chern numbers, C=3, 2, 1, respectively. Surprisingly, for magnetic fields exceeding 5T we observe a new VHS at a filling of 3.5 electrons per moiré cell, suggesting the possibility of a topological sub-band at a fractional moiré filling.
1. S. Wu, Z. Zhang, K. Watanabe, T. Taniguchi, and Eva Y Andrei, "Chern Insulators and Topological Flat-bands in Magic-angle Twisted Bilayer Graphene", arXiv:2007.03735