Kuiri Manabendra, Coleman Christopher, Gao Zhenxiang, Vishnuradhan Aswin, Watanabe Kenji, Taniguchi Takashi, Zhu Jihang, MacDonald Allan H, Folk Joshua
Department of Physics and Astronomy & Stewart Blusson Quantum Matter Institute, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada.
Research Center for Functional Materials, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki, 305-0044, Japan.
Nat Commun. 2022 Oct 29;13(1):6468. doi: 10.1038/s41467-022-34192-x.
Twisted double bilayer graphene (tDBG) comprises two Bernal-stacked bilayer graphene sheets with a twist between them. Gate voltages applied to top and back gates of a tDBG device tune both the flatness and topology of the electronic bands, enabling an unusual level of experimental control. Metallic states with broken spin and valley symmetries have been observed in tDBG devices with twist angles in the range 1.2-1.3°, but the topologies and order parameters of these states have remained unclear. We report the observation of an anomalous Hall effect in the correlated metal state of tDBG, with hysteresis loops spanning hundreds of mT in out-of-plane magnetic field (B) that demonstrate spontaneously broken time-reversal symmetry. The B hysteresis persists for in-plane fields up to several Tesla, suggesting valley (orbital) ferromagnetism. At the same time, the resistivity is strongly affected by even mT-scale values of in-plane magnetic field, pointing to spin-valley coupling or to a direct orbital coupling between in-plane field and the valley degree of freedom.
扭曲双层双层石墨烯（tDBG）由两个伯纳尔堆叠的双层石墨烯片组成，它们之间存在扭曲。施加到tDBG器件顶部和背面栅极的栅极电压可调节电子能带的平坦度和拓扑结构，从而实现不同寻常的实验控制水平。在扭曲角为1.2-1.3°的tDBG器件中观察到了具有破缺自旋和谷对称性的金属态，但这些态的拓扑结构和序参量仍不清楚。我们报告了在tDBG的关联金属态中观察到反常霍尔效应，在面外磁场（B）中，磁滞回线跨越数百毫特斯拉，这表明时间反演对称性自发破缺。对于高达几特斯拉的面内磁场，B磁滞仍然存在，这表明存在谷（轨道）铁磁性。同时，即使是毫特斯拉量级的面内磁场值也会强烈影响电阻率，这表明存在自旋-谷耦合或面内磁场与谷自由度之间的直接轨道耦合。
