Chern insulators, van Hove singularities and topological flat bands in magic-angle twisted bilayer graphene
Chern insulators, van Hove singularities and topological flat bands in magic-angle twisted bilayer graphene
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DOI：
10.1038/s41563-020-00911-2
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发表时间：
2021-02-15
期刊：
影响因子：
41.2
通讯作者：
Andrei, Eva Y.
中科院分区：
文献类型：
--
作者：
Wu, Shuang;Zhang, Zhenyuan;Andrei, Eva Y.
魔角扭曲的双层石墨烯在其平坦的能带被部分填充时，表现出由增强的电子-电子相互作用触发的有趣的量子相变。然而，位相本身以及它们与假定的平坦带的非平凡拓扑之间的联系在很大程度上还没有被探索。在这里，我们报告的输运测量揭示了一系列掺杂诱导的Lifshitz相变，这些相变伴随着van Hove奇点，这有助于出现关联诱导的能隙和拓扑上的非平凡子带。在磁场存在的情况下，每个莫尔盒填充1、2、3载流子时的量子化霍尔平台揭示了子带的拓扑结构，并预示着陈数为C=3、2、1的陈氏绝缘体的出现。令人惊讶的是，对于超过5T的磁场，我们观察到填充度为3.5的范霍夫奇点，这表明可能存在分数的陈氏绝缘体。这种范霍夫奇点伴随着从低温金属到高温绝缘行为的交叉，这是熵驱动的波美兰丘克类转变的特征。
Magic-angle twisted bilayer graphene exhibits intriguing quantum phase transitions triggered by enhanced electron-electron interactions when its flat bands are partially filled. However, the phases themselves and their connection to the putative non-trivial topology of the flat bands are largely unexplored. Here we report transport measurements revealing a succession of doping-induced Lifshitz transitions that are accompanied by van Hove singularities, which facilitate the emergence of correlation-induced gaps and topologically non-trivial subbands. In the presence of a magnetic field, well-quantized Hall plateaus at a filling of 1,2,3 carriers per moire cell reveal the subband topology and signal the emergence of Chern insulators with Chern numbers, C = 3,2,1, respectively. Surprisingly, for magnetic fields exceeding 5 T we observe a van Hove singularity at a filling of 3.5, suggesting the possibility of a fractional Chern insulator. This van Hove singularity is accompanied by a crossover from low-temperature metallic, to high-temperature insulating behaviour, characteristic of entropically driven Pomeranchuk-like transitions.