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In magic-angle twisted bilayer graphene (MA-TBG) system, as the Fermi level is swept through the flat bands, their quenched kinetic energy facilitates interaction-induced instabilities leading to strong correlated behaviors. Beyond many-body interactions, the existence of non-trivial band-topology plays a crucial role in shaping these correlated phases in this system. However, the phases themselves and their connection to the putative non-trivial topology of the flat bands are largely unexplored. Here we report on magneto-transport and Hall density measurements that reveal a succession of doping-induced Lifshitz transitions of van Hove singularity (VHS) which 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 VHS at a filling of 3.5 electrons per moiré cell, suggesting the possibility of a topological sub-band at a fractional moiré filling.
Reference
S. Wu, et al, arXiv:2007.03735 (2020) (Nature Materials, in press (2021))
J. Liu, X. Dai, Phys. Rev. B 99, 155415 (2019)
A. Maharaj, et al., Phys. Rev. B 96, 045132 (2017)
Dr. Shuang Wu received his Ph.D. in Physics from Institute of Physics, Chinese Academy of Sciences in 2016. Then he started the postdoc research at UC Berkeley (2016-2018) and later at Rutgers University (2018 - present). His current research is focusing on strong correlations and topology in graphene Moiré system. He is the author of 20+ papers in peer-reviewed journals, including Nature research journals and Physics Review journals.
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