论文标题
扭曲的双双层石墨烯中的自发时间反转对称性破裂
Spontaneous time-reversal symmetry breaking in twisted double bilayer graphene
论文作者
论文摘要
扭曲的双重双层石墨烯（TDBG）包括两个Bernal堆叠的双层石墨烯片，它们之间有扭曲。施加在TDBG设备的顶部和后门上的栅极电压调整了电子带的平坦度和拓扑，从而实现了不寻常的实验控制水平。在具有扭曲角$ \ sim $ 1.2-1.3 $^\ circ $的TDBG设备中，已经观察到断裂的自旋/山谷对称金属状态，但是这些状态的拓扑和订单参数尚不清楚。我们报告了在TDBG相关金属状态下观察到异常的霍尔效应，磁滞回路跨越了平面磁场（$ b _ {\ perp} $），表现出自发损坏的时间逆向对称性。 $ b _ {\ perp} $滞后在平面内部持续存在，最多几个特斯拉，暗示山谷（轨道）铁磁性。同时，即使是平面磁场的MT尺度值，电阻率也会强烈影响，指向自旋 - 瓦利耦合或平面内场和山谷自由度之间的直接轨道耦合。
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. Broken spin/valley symmetry metallic states have been observed in tDBG devices with twist angles $\sim $ 1.2-1.3$^\circ$, 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 100s of mT in out-of-plane magnetic field ($B_{\perp}$) that demonstrate spontaneously broken time-reversal symmetry. The $B_{\perp}$ 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.
