Correlated insulator behaviour at half-filling in magic-angle graphene superlattices
Correlated insulator behaviour at half-filling in magic-angle graphene superlattices
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DOI：
10.1038/nature26154
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发表时间：
2018-04-05
期刊：
影响因子：
64.8
通讯作者：
Jarillo-Herrero, Pablo
中科院分区：
文献类型：
--
作者：
Cao, Yuan;Fatemi, Valla;Jarillo-Herrero, Pablo
范德华异质结构是一种由垂直堆叠的二维结构块组成的超材料，由层间的范德华力结合在一起。这种设计意味着范德华异质结构的特性可以被精确地设计，甚至比二维材料的特性更精确(1)。其中一个性质是异质结构中不同层之间的“扭转”角。这个角度在范德华异质结构的电子特性中起着至关重要的作用，但在其他类型的异质结构中没有直接的类似物，例如使用分子束外延生长的半导体。对于小的扭转角，二维层之间的晶格错位产生的云纹图案会产生长程的堆叠顺序调制。到目前为止，关于扭曲角对范德华异质结构影响的研究主要集中在单层石墨烯在六方氮化硼表面的异质结构上，由于六方氮化硼的带隙较大，其层间相互作用相对较弱（2-5）。本文研究了一种由双层石墨烯组成的异质结构，其中两层石墨烯相互以一定角度扭曲。我们通过实验证明，正如理论预测的那样(6)，当这个角度接近“魔”角时，由于强层间耦合，接近零费米能量的电子能带结构变得平坦。这些平坦带在半填充时表现出绝缘状态，这在电子之间缺乏相关性的情况下是不被期望的。我们发现，在半填充状态下，这些相关状态与Mott-like绝缘体状态一致，这种绝缘体状态可能是由云纹图案诱导的超晶格中的电子局域化引起的。魔角扭曲双层石墨烯异质结构的这些特性表明，这些材料可以用于在没有磁场的二维条件下研究其他奇异的多体量子相。通过电可调性和通过扭转角的带宽可调性来获得平带，可以为更奇特的相关系统铺平道路，例如非常规超导体和量子自旋液体。
A van der Waals heterostructure is a type of metamaterial that consists of vertically stacked two-dimensional building blocks held together by the van der Waals forces between the layers. This design means that the properties of van der Waals heterostructures can be engineered precisely, even more so than those of two-dimensional materials(1). One such property is the 'twist' angle between different layers in the heterostructure. This angle has a crucial role in the electronic properties of van der Waals heterostructures, but does not have a direct analogue in other types of heterostructure, such as semiconductors grown using molecular beam epitaxy. For small twist angles, the moire pattern that is produced by the lattice misorientation between the two-dimensional layers creates long-range modulation of the stacking order. So far, studies of the effects of the twist angle in van der Waals heterostructures have concentrated mostly on heterostructures consisting of monolayer graphene on top of hexagonal boron nitride, which exhibit relatively weak interlayer interaction owing to the large bandgap in hexagonal boron nitride(2-5). Here we study a heterostructure consisting of bilayer graphene, in which the two graphene layers are twisted relative to each other by a certain angle. We show experimentally that, as predicted theoretically(6), when this angle is close to the 'magic' angle the electronic band structure near zero Fermi energy becomes flat, owing to strong interlayer coupling. These flat bands exhibit insulating states at half-filling, which are not expected in the absence of correlations between electrons. We show that these correlated states at half-filling are consistent with Mott-like insulator states, which can arise from electrons being localized in the superlattice that is induced by the moire pattern. These properties of magic-angle-twisted bilayer graphene heterostructures suggest that these materials could be used to study other exotic many-body quantum phases in two dimensions in the absence of a magnetic field. The accessibility of the flat bands through electrical tunability and the bandwidth tunability through the twist angle could pave the way towards more exotic correlated systems, such as unconventional superconductors and quantum spin liquids.