[论文解读] Gate-Tunable Mott Insulator in Trilayer Graphene-Boron Nitride Moir\'e Superlattice

Guorui Chen, Lili Jiang+8 more|arXiv (Cornell University)|Mar 6, 2018

Graphene research and applications被引用 11

一句话总结

本研究在ABC三重层石墨烯/六方氮化硼异质结的莫尔超晶格中实现了栅压可调的莫特绝缘体。由于自旋和谷简并，每个莫尔晶胞的窄能带可容纳四个电子，莫特绝缘态在1/4和1/2填充时出现。通过垂直栅压可电调制莫特能隙，从而实现对电子掺杂的原位调控，并实现不同莫特绝缘态之间的转变。

ABSTRACT

Mott insulator plays a central role in strongly correlated physics, where the repulsive Coulomb interaction dominates over the electron kinetic energy and leads to insulating states with one electron occupying each unit cell. Doped Mott insulator is often described by the Hubbard model, which can give rise to other correlated phenomena such as unusual magnetism and even high temperature superconductivity. A tunable Mott insulator, where the competition between the Coulomb interaction and the kinetic energy can be varied in situ, can provide an invaluable model system for the study of Mott physics. Here we report the realization of such a tunable Mott insulator in the ABC trilayer graphene (TLG) and hexagonal boron nitride (hBN) heterostructure with a Moir\'e superlattice. Unlike massless Dirac electrons in monolayer graphene, electrons in pristine ABC TLG are characterized by quartic energy dispersion and large effective mass that are conducive for strongly correlated phenomena. The Moir\'e superlattice in TLG/hBN heterostructures leads to narrow electronic minibands that are gate-tunable. Each filled miniband contains 4 electrons in one Moir\'e lattice site due to the spin and valley degeneracy of graphene. The Mott insulator states emerge at 1/4 and 1/2 fillings, corresponding to one electron and two electrons per site, respectively. Moreover, the Mott states in the ABC TLG/hBN heterostructure exhibit unprecedented tunablility: the Mott gap can be modulated in situ by a vertical electrical field, and at the mean time the electron doping can be gate-tuned to fill the band from one Mott insulating state to another. Our observation of a tunable Mott insulator opens up tremendous opportunities to explore novel strongly correlated phenomena in two-dimensional Moir\'e superlattice heterostructures.

研究动机与目标

• 在二维莫尔超晶格体系中实现可调的莫特绝缘体。
• 探索在栅压可调平台中电子关联与动能之间的相互作用。
• 研究在三重层石墨烯/hBN异质结中特定填充下莫特绝缘态的出现。
• 实现对莫特能隙和电子掺杂的原位调控，以探测强关联物理行为。

提出的方法

• 通过设计ABC三重层石墨烯/六方氮化硼异质结，形成具有窄电子亚带的莫尔超晶格。
• 采用双栅场效应器件结构施加垂直电场，以调节莫特能隙。
• 通过电输运测量识别亚带在1/4和1/2填充时的绝缘态。
• 利用ABC三重层石墨烯的大有效质量和四次方能带色散特性，增强电子关联效应。
• 利用自旋和谷简并，实现每个莫尔晶胞容纳四个电子。
• 通过栅压扫描监测绝缘能隙和能带填充的演化，观察可调的莫特相变。

实验结果

研究问题

• RQ1 是否可在三重层石墨烯/hBN莫尔超晶格中实现并电调莫特绝缘体？
• RQ2 在具有窄亚带的关联电子系统中，莫特能隙如何响应外部栅压？
• RQ3 电子填充（1/4和1/2）在该系统中稳定莫特绝缘态方面起什么作用？
• RQ4 是否可通过栅压控制的掺杂在不同莫特绝缘态之间进行调控？
• RQ5 自旋和谷自由度如何影响莫尔亚带中关联绝缘态的形成？

主要发现

• 在ABC三重层石墨烯/hBN异质结的窄莫尔亚带中，1/4和1/2填充时出现莫特绝缘态。
• 通过垂直栅压可电调制莫特能隙，实现了对绝缘态的原位调控。
• 每个填充的亚带包含四个电子/每个莫尔晶胞，归因于自旋和谷简并。
• 该系统支持栅压可调的电子掺杂，可实现不同莫特绝缘态之间的转变。
• ABC三重层石墨烯中大的有效质量和四次方色散特性增强了电子关联效应，有利于莫特绝缘体的形成。
• 莫特能隙和能带填充的可调性为研究二维莫尔系统中的强关联现象提供了多功能平台。