Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA.
National Institute for Materials Science, Tsukuba, Japan.
Nature. 2020 Jul;583(7815):215-220. doi: 10.1038/s41586-020-2260-6. Epub 2020 May 6.
The recent discovery of correlated insulator states and superconductivity in magic-angle twisted bilayer graphene has enabled the experimental investigation of electronic correlations in tunable flat-band systems realized in twisted van der Waals heterostructures. This novel twist angle degree of freedom and control should be generalizable to other two-dimensional systems, which may exhibit similar correlated physics behaviour, and could enable techniques to tune and control the strength of electron-electron interactions. Here we report a highly tunable correlated system based on small-angle twisted bilayer-bilayer graphene (TBBG), consisting of two rotated sheets of Bernal-stacked bilayer graphene. We find that TBBG exhibits a rich phase diagram, with tunable correlated insulator states that are highly sensitive to both the twist angle and the application of an electric displacement field, the latter reflecting the inherent polarizability of Bernal-stacked bilayer graphene. The correlated insulator states can be switched on and off by the displacement field at all integer electron fillings of the moiré unit cell. The response of these correlated states to magnetic fields suggests evidence of spin-polarized ground states, in stark contrast to magic-angle twisted bilayer graphene. Furthermore, in the regime of lower twist angles, TBBG shows multiple sets of flat bands near charge neutrality, resulting in numerous correlated states corresponding to half-filling of each of these flat bands, all of which are tunable by the displacement field as well. Our results could enable the exploration of twist-angle- and electric-field-controlled correlated phases of matter in multi-flat-band twisted superlattices.
最近在魔角扭曲双层石墨烯中发现了关联绝缘体态和超导性，这使得人们能够在扭曲的范德华异质结构中实现的可调平带系统中实验研究电子关联。这种新的扭曲角自由度和控制应该可以推广到其他二维系统，这些系统可能表现出类似的关联物理行为，并可以实现调节和控制电子-电子相互作用强度的技术。在这里，我们报告了一个基于小角度扭曲双层双层石墨烯（TBBG）的高度可调谐关联系统，它由两个旋转的双层石墨烯的 Bernal 堆叠片组成。我们发现 TBBG 表现出丰富的相图，具有可调谐的关联绝缘体态，对扭曲角和施加的电位移场都非常敏感，后者反映了 Bernal 堆叠双层石墨烯的固有极化率。在莫尔单元的所有整数电子填充下，位移场可以打开和关闭关联绝缘体态。这些关联态对磁场的响应表明存在自旋极化的基态证据，这与魔角扭曲双层石墨烯形成鲜明对比。此外，在较低扭曲角的情况下，TBBG 在电荷中性附近显示出多组平带，导致与这些平带的每个半填充相对应的多个关联态，所有这些态都可以通过位移场进行调节。我们的结果可以使人们在多平带扭曲超晶格中探索转角和电场控制的关联物质相。
