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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

(上海科技大学)

Abstract:

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.

References: 

Jianpeng Liu,  Junwei Liu, Xi Dai,  Phys. Rev. B,  99 (15), 155415 (2019)

Jianpeng Liu, Zhen Ma, Jinhua Gao, Xi Dai, Phys. Rev. X, 9 (3), 031021 (2019)

Jianpeng Liu,  Xi Dai, Phys. Rev. B, 103 (3), 035427 (2021)

Shihao Zhang, Jianpeng Liu, arXiv:2101.04711v2

报告人简介:

刘健鹏,上海科技大学博导、研究员以及终身制序列助理教授。2010年本科毕业于南开大学物理学院,随即前往美国罗格斯大学攻读博士学位,并于2015年取得博士学位。随后前往UCSB、香港科技大学先后担任博士后以及研究助理教授等职位。从2020年5月至今,刘健鹏在上海科技大学任博导、研究员以及终身制序列助理教授。研究方向为理论和计算凝聚态物理学,具体包括:拓扑材料与拓扑物态、磁性材料、转角二维体系以及摩尔二维体系电子结构和物性的理论研究,以及基于密度泛函理论的计算方法研究等。

联系人:杨威特聘研究员(82648050)