Time: 10:00am, May 17, 2019

Venue: Rm. S401, Kavli ITS Meeting Room [View Map]

Speaker: Jian-Peng Liu (HKU)

Abstract

In this talk we discuss the electronic structures and topological properties of twisted bilayer and multilayer graphene systems. We propose that the two low-energy flat bands in twisted bilayer  graphene (TBG) are equivalent to two zeroth pseudo Landau levels with opposite sublattice polarizations and carry opposite Chern numbers +/-1. Such a pseudo Landau-level  representation of TBG naturally explains the origin of ``magic angles" in TBG, and have significant implications on the nature of the correlated insulating phases observed in experiments. The pseudo Landau-level representation can be further generalized to twisted multilayer graphene (TMG) systems, in which a universal valley Chern-number hierarchy can be derived for the two low-energy flat bands of the system. The nontrivial Chern numbers in the TMG systems are associated with large and valley contrasting orbital magnetizations, which generate circulating current loops and local magnetic field distributions on the moire scale, which can be experimentally detected. The nontrivial valley Chern numbers, the emergence of the flat bands, and the orbital magnetism make the TMG system a perfect platform to study the interplay between strong correlations and nontrivial band topology.