[论文解读] Electrically tunable correlated and topological states in twisted monolayer-bilayer graphene

Shao-Wen Chen, Minhao He+9 more|arXiv (Cornell University)|Apr 23, 2020

Graphene research and applications被引用 18

一句话总结

本研究证明，扭曲的单层-双层石墨烯（tMBG）可实现电场可调的关联与拓扑态，包括绝缘相和金属相，其铁磁性与大反常霍尔效应可通过栅压进行调控。该体系的低对称性使得通过反转位移场可在tBLG型与tDBG型行为之间切换，从而在零磁场下实现对磁性与能带拓扑的完全电控。

ABSTRACT

Twisted van der Waals heterostructures with flat electronic bands have recently emerged as a platform for realizing correlated and topological states with an extraordinary degree of control and tunability. In graphene-based moiré heterostructures, the correlated phase diagram and band topology depend strongly on the number of graphene layers, their relative stacking arrangement, and details of the external environment from the encapsulating crystals. Here, we report that the system of twisted monolayer-bilayer graphene (tMBG) hosts a variety of correlated metallic and insulating states, as well as topological magnetic states. Because of its low symmetry, the phase diagram of tMBG approximates that of twisted bilayer graphene when an applied perpendicular electric field points from the bilayer towards the monolayer graphene, or twisted double bilayer graphene when the field is reversed. In the former case, we observe correlated states which undergo an orbitally driven insulating transition above a critical perpendicular magnetic field. In the latter case, we observe the emergence of electrically tunable ferromagnetism at one-quarter filling of the conduction band, with a large associated anomalous Hall effect. Uniquely, the magnetization direction can be switched purely with electrostatic doping at zero magnetic field. Our results establish tMBG as a highly tunable platform for investigating a wide array of tunable correlated and topological states.

研究动机与目标

• 探索具有低晶体对称性的扭曲单层-双层石墨烯（tMBG）中的关联与拓扑态。
• 确定扭转角、栅极掺杂与位移场之间的相互作用如何调控电子态。
• 研究在无外加磁场条件下，tMBG中出现铁磁性与拓扑序的机制。
• 确立tMBG作为实现并调控关联与拓扑量子态的高可调平台。

提出的方法

• 采用‘切割与堆叠’方法制备tMBG异质结构，扭转角在0.89°至1.55°之间。
• 采用hBN封装的双栅器件，实现对载流子密度（n）与位移场（D）的精确控制。
• 在4端子几何结构下，于0.3 K温度下使用锁相技术（13.3 Hz或17.7 Hz）测量电输运特性。
• 通过上下栅压调节位移场D，定义D > 0为场强从单层石墨烯指向双层石墨烯。
• 通过磁场中的量子振荡确定扭转角，使用关系式ns = 8θ² / (√3 a²)，其中a = 0.246 nm。
• 采用500 pA激励测量磁滞回线，并使用直流电流偏置控制磁态。

实验结果

研究问题

• RQ1 位移场方向如何调控tMBG的相图，在tBLG型与tDBG型行为之间切换？
• RQ2 能否在零磁场下于tMBG中实现电场可调的铁磁性与大反常霍尔效应？
• RQ3 tMBG的低对称性在实现对拓扑与关联态的电控中起到何种作用？
• RQ4 在不同填充因子下，关联绝缘态与金属态如何在tMBG中出现？
• RQ5 能否仅通过电致掺杂在无外加磁场条件下切换tMBG中的磁化方向？

主要发现

• 当D > 0（场强从双层石墨烯指向单层石墨烯）时，tMBG在所有整数填充因子下均表现出关联绝缘态，类似于扭曲双层石墨烯（tBLG），且在临界垂直磁场以上出现轨道驱动的绝缘相变。
• 当D < 0（场强从单层石墨烯指向双层石墨烯）时，在导带四分之一填充处出现大反常霍尔效应，表明存在电场可调的铁磁性。
• 在铁磁态下，磁化方向可仅通过电致掺杂在零磁场下实现切换，证明了对磁序的完全电控。
• 由于tMBG中C2与My对称性的破缺，该体系的相图在D > 0时近似为tBLG型，在D < 0时近似为tDBG型。
• 所观测到的拓扑态与非平庸能带拓扑一致，表明在强关联下可能实现分数陈绝缘体态。
• 结果确立了tMBG作为仅通过栅压与位移场即可探索广泛关联与拓扑量子相的高可调平台。