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Topological and correlated states in twisted graphene systems

Date: 2021-05-14
Time: 10:00
Venue: D210
Speaker: 刘健鹏



We discuss the topological properties and correlated states in twisted bilayer graphene (TBG) and twisted multilayer graphene systems. We first show that the leading-order effect of the moire potential in TBG is to generate opposite pseudo magnetic fields coupled with two flavors of Dirac fermions, which gives rise to pseudo Landau levels, and the flat bands in magic-angle TBG are derived from the two zeroth pseudo Landau levels. The experimentally observed quantum anomalous Hall effects and the origin of the magic angles can be naturally explained using such a pseudo-Landau-level picture. We also show that the topologically nontrivial flat bands also generically exist in twisted multilayer graphene systems with tunable valley Chern numbers. We further study the nature of the correlated insulator states observed in TBG, and predict that the ground states at even integer fillings in TBG are valley polarized states associated with intriguing compensating current-loop patterns in real space. In the end we discuss the experimentally observed correlated insulator states in twisted multilayer graphene systems,and show that these insulator states at ½ filling are spin polarized nematic states, which may undergo topological phase transitions induced by weak vertical magnetic fields.


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