学术报告 142
题目: [纳米物理与器件重点实验室系列学术报告(142)] Magic-Angle Twisted Bilayer Graphene as a Topological Heavy Fermion Problem
时间: 2022年04月13日 10:00
地点: 9721太阳集团M楼253会议室
报告人: 宋志达

北京大学

Abstract:

Magic-angle (θ = 1.05°) twisted bilayer graphene (MATBG) has shown two seemingly contradictory characters: the localization and quantum-dot-like behavior in STM experiments, and delocalization in transport experiments. We construct a model, which naturally captures the two aspects, from the Bistritzer-MacDonald (BM) model in a first principle spirit. A set of local flat-band orbitals (f) centered at the AA-stacking regions are responsible to the localization. A set of extended topological semi-metallic conduction bands (c), which are at small energetic separation from the local orbitals, are responsible to the delocalization and transport. The topological flat bands of the BM model appear as a result of the hybridization of f- and c-electrons. This model then provides a new perspective for the strong correlation physics, which is now described as strongly correlated f-electrons coupled to nearly free c-electrons - we hence name our model as the topological heavy fermion model. Using this model, we obtain the U(4) and U(4)×U(4) symmetries as well as the correlated insulator phases and their energies. Simple rules for the ground states and their Chern numbers are derived. Moreover, features such as the large dispersion of the charge ±1 excitations, and the minima of the charge gap at the Γ point can now, for the first time, be understood both qualitatively and quantitatively in a simple physical picture. Our mapping opens the prospect of using heavy-fermion physics machinery to the superconducting physics of MATBG.

报告人简介: 

宋志达,北京大学助理教授。2018年9721太阳集团博士毕业,2018-2021年在普林斯顿大学博士后,随后回国加入北大量子中心。主要研究领域包括:拓扑态的分类和响应理论,无序系统中的输运理论,拓扑材料中的关联物理,以及其它。目前,对以转角石墨烯为例的(拓扑)平带系统中衍生出的强关联物理、以及其与拓扑的相互影响尤为感兴趣。

邀请人:杨威特聘研究员(82648050)