Cascade of electronic transitions in magic-angle twisted bilayer graphene
Cascade of electronic transitions in magic-angle twisted bilayer graphene
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DOI：
10.1038/s41586-020-2339-0
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发表时间：
2020-06-01
期刊：
影响因子：
64.8
通讯作者：
Yazdani, Ali
中科院分区：
文献类型：
--
作者：
Wong, Dillon;Nuckolls, Kevin P.;Yazdani, Ali
魔角扭曲双层石墨烯表现出多种电子态，包括相关绝缘体(1-3)、超导体(2-4)和拓扑相(3,5,6)。了解导致这些相的微观机制需要确定电子-电子相互作用和量子简并性之间的相互作用（后者是由于自旋和谷自由度）。在最近的光谱测量中，在平坦电子带的部分填充处观察到了强电子-电子相关性的特征(7-10)，并且输运实验表明，填充处朗道能级简并性的变化对应于每个莫尔晶胞的整数个电子(2-4)。然而，相互作用效应与系统简并性之间的相互作用目前尚不清楚。在这里，我们报告了魔角扭曲双层石墨烯光谱特性的级联转变作为电子填充的函数，这是使用高分辨率扫描隧道显微镜确定的。我们发现在莫尔平带的每个整数填充处，化学势发生了明显的变化，并且低能激发发生了重新排列。这些光谱特征是库仑相互作用的直接结果，库仑相互作用将简并平带分裂成哈伯德子带。我们发现这些相互作用（我们可以通过实验提取其强度）对垂直磁场的存在非常敏感，这会强烈改变光谱跃迁。我们在这里报道的级联跃迁表征了相关的高温母相（11,12），在魔角扭曲双层石墨烯中，各种绝缘和超导基态相在低温下出现。魔角扭曲双层石墨烯中的电子-电子相互作用可以分裂通常简并的电子带，从而产生光谱揭示的级联电子跃迁。
Magic-angle twisted bilayer graphene exhibits a variety of electronic states, including correlated insulators(1-3), superconductors(2-4)and topological phases(3,5,6). Understanding the microscopic mechanisms responsible for these phases requires determination of the interplay between electron-electron interactions and quantum degeneracy (the latter is due to spin and valley degrees of freedom). Signatures of strong electron-electron correlations have been observed at partial fillings of the flat electronic bands in recent spectroscopic measurements(7-10), and transport experiments have shown changes in the Landau level degeneracy at fillings corresponding to an integer number of electrons per moire unit cell(2-4). However, the interplay between interaction effects and the degeneracy of the system is currently unclear. Here we report a cascade of transitions in the spectroscopic properties of magic-angle twisted bilayer graphene as a function of electron filling, determined using high-resolution scanning tunnelling microscopy. We find distinct changes in the chemical potential and a rearrangement of the low-energy excitations at each integer filling of the moire flat bands. These spectroscopic features are a direct consequence of Coulomb interactions, which split the degenerate flat bands into Hubbard sub-bands. We find these interactions, the strength of which we can extract experimentally, to be surprisingly sensitive to the presence of a perpendicular magnetic field, which strongly modifies the spectroscopic transitions. The cascade of transitions that we report here characterizes the correlated high-temperature parent phase(11,12)from which various insulating and superconducting ground-state phases emerge at low temperatures in magic-angle twisted bilayer graphene.Electron-electron interactions in magic-angle twisted bilayer graphene can split usually degenerate electronic bands, giving rise to a cascade of electronic transitions revealed by spectroscopy.