Physics of Nanosystems, Department of Physics, Ludwig-Maximilians-Universität München, Munich, Germany.
Department of Physics, University of Texas at Dallas, Richardson, TX, USA.
Nature. 2021 Oct;598(7879):53-58. doi: 10.1038/s41586-021-03849-w. Epub 2021 Oct 6.
The quantum anomalous Hall (QAH) effect-a macroscopic manifestation of chiral band topology at zero magnetic field-has been experimentally realized only by the magnetic doping of topological insulators and the delicate design of moiré heterostructures. However, the seemingly simple bilayer graphene without magnetic doping or moiré engineering has long been predicted to host competing ordered states with QAH effects. Here we explore states in bilayer graphene with a conductance of 2 e h (where e is the electronic charge and h is Planck's constant) that not only survive down to anomalously small magnetic fields and up to temperatures of five kelvin but also exhibit magnetic hysteresis. Together, the experimental signatures provide compelling evidence for orbital-magnetism-driven QAH behaviour that is tunable via electric and magnetic fields as well as carrier sign. The observed octet of QAH phases is distinct from previous observations owing to its peculiar ferrimagnetic and ferrielectric order that is characterized by quantized anomalous charge, spin, valley and spin-valley Hall behaviour.
量子反常霍尔（QAH）效应——零磁场下手性能带拓扑的宏观表现——仅通过拓扑绝缘体的磁场掺杂和莫尔超晶格结构的精细设计得以实现。然而，看似简单的双层石墨烯，既没有磁场掺杂，也没有莫尔工程，长期以来一直被预测会出现具有 QAH 效应的竞争有序状态。在这里，我们研究了具有 2e/h 电导（其中 e 是电子电荷，h 是普朗克常数）的双层石墨烯中的状态，这些状态不仅在异常小的磁场下和五开尔文的温度下幸存下来，而且还表现出磁滞现象。实验特征共同提供了令人信服的证据，证明了轨道磁性驱动的 QAH 行为可以通过电场、磁场和载流子符号进行调节。所观察到的八分量子霍尔相不同于以往的观察结果，这是由于其独特的亚铁磁和铁电序，其特点是量子反常电荷、自旋、谷和自旋谷霍尔行为。
