Correlation-driven topological phases in magic-angle twisted bilayer graphene
Correlation-driven topological phases in magic-angle twisted bilayer graphene
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DOI：
10.1038/s41586-020-03159-7
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发表时间：
2021-01-18
期刊：
影响因子：
64.8
通讯作者：
Nadj-Perge, Stevan
中科院分区：
文献类型：
--
作者：
Choi, Youngjoon;Kim, Hyunjin;Nadj-Perge, Stevan
魔角扭曲双层石墨烯（MATBG）表现出一系列源自强电子-电子相互作用的相关现象。当 +/- 1、+/- 2 和 +/- 3 电子占据每个莫尔晶胞时，这些相互作用使得费米表面极易重建，并导致各种相关相位的形成 (1-4)。尽管一些相已被证明具有非零陈数(5,6)，但许多其他相的局部微观性质和拓扑特征尚未确定。在这里，我们介绍一组使用扫描隧道显微镜来绘制有限磁场中 MATBG 中出现的拓扑相的技术。通过静电掺杂和磁场跟踪费米能级局部态密度的演化，我们创建了局部朗道扇形图，使我们能够将陈数直接分配给所有观察到的相。我们发现了六个拓扑相的存在，这些拓扑相是由有限域中的整数填充产生的，并且源于由相关性（7,8）驱动的一系列对称破缺跃迁。这些拓扑相只能在魔角周围的小范围扭转角内形成，这进一步将它们与在电荷中性附近观察到的朗道能级区分开来。此外，我们观察到，即使是在低场下获取的电荷中性朗道谱也会因相互作用而发生相当大的改变，表现出显着的电子空穴不对称性，并且在零朗道能级（约 3 至 5 毫电子伏）之间出现出人意料的大分裂。我们的结果表明，强烈的电子相互作用如何影响 MATBG 能带结构并导致相关拓扑相。
Magic-angle twisted bilayer graphene (MATBG) exhibits a range of correlated phenomena that originate from strong electron-electron interactions. These interactions make the Fermi surface highly susceptible to reconstruction when +/- 1, +/- 2 and +/- 3 electrons occupy each moire unit cell, and lead to the formation of various correlated phases(1-4). Although some phases have been shown to have a non-zero Chern number(5,6), the local microscopic properties and topological character of many other phases have not yet been determined. Here we introduce a set of techniques that use scanning tunnelling microscopy to map the topological phases that emerge in MATBG in a finite magnetic field. By following the evolution of the local density of states at the Fermi level with electrostatic doping and magnetic field, we create a local Landau fan diagram that enables us to assign Chern numbers directly to all observed phases. We uncover the existence of six topological phases that arise from integer fillings in finite fields and that originate from a cascade of symmetry-breaking transitions driven by correlations(7,8). These topological phases can form only for a small range of twist angles around the magic angle, which further differentiates them from the Landau levels observed near charge neutrality. Moreover, we observe that even the charge-neutrality Landau spectrum taken at low fields is considerably modified by interactions, exhibits prominent electron-hole asymmetry, and features an unexpectedly large splitting between zero Landau levels (about 3 to 5 millielectronvolts). Our results show how strong electronic interactions affect the MATBG band structure and lead to correlation-enabled topological phases.