Chern mosaic and Berry-curvature magnetism in magic-angle graphene

Berry连接和曲率 物理 凝聚态物理 Chern类 双层石墨烯 磁性 拓扑绝缘体 量子霍尔效应 曲率 几何相位 电子 石墨烯 量子力学 拓扑(电路) 几何学 组合数学 数学
作者
Sameer Grover,Matan Bocarsly,Aviram Uri,Petr Stepanov,Giorgio Di Battista,Indranil Roy,Jiewen Xiao,Alexander Y. Meltzer,Y. Myasoedov,Keshav Pareek,Kenji Watanabe,Takashi Taniguchi,Binghai Yan,Ady Stern,Erez Berg,Dmitri K. Efetov,E. Zeldov
出处
期刊:Nature Physics [Nature Portfolio]
卷期号:18 (8): 885-892 被引量:79
标识
DOI:10.1038/s41567-022-01635-7
摘要

Charge carriers in magic-angle graphene come in eight flavours described by a combination of their spin, valley and sublattice polarizations. When inversion and time-reversal symmetries are broken, this ‘flavour’ degeneracy can be lifted, and their corresponding bands can be sequentially filled. Due to their non-trivial band topology and Berry curvature, each band is classified by a topological Chern number C, leading to quantum anomalous Hall and Chern insulator states. Using a scanning superconducting quantum interference device on a tip, we image the nanoscale equilibrium orbital magnetism induced by the Berry curvature, the polarity of which is governed by C, and detect its two constituent components associated with the drift and self-rotation of the electronic wavepackets. At integer filling v = 1, we observe a zero-field Chern insulator, which—rather than being described by a global topologically invariant C—forms a mosaic of microscopic patches of C = −1, 0 or 1. On further filling, we find a first-order phase transition due to the recondensation of electrons from valley K to K′, leading to irreversible flips of the local Chern number and magnetization, as well as to the formation of valley domain walls, giving rise to hysteretic anomalous Hall resistance. Topological states characterized by Chern numbers are usually considered to be the global properties of a material. Now a spatial patchwork of different Chern insulator states is imaged in twisted bilayer graphene.
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