Localization of lattice dynamics in low-angle twisted bilayer graphene
Localization of lattice dynamics in low-angle twisted bilayer graphene
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DOI：
10.1038/s41586-021-03252-5
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发表时间：
2021-02-18
期刊：
影响因子：
64.8
通讯作者：
Jorio, Ado
中科院分区：
文献类型：
--
作者：
Gadelha, Andreij C.;Ohlberg, Douglas A. A.;Jorio, Ado
扭曲的双层石墨烯是通过使双层石墨烯中的两个晶体网络相对于彼此轻微旋转而产生的。对于小的扭转角，材料经历自组织晶格重构，导致形成周期性重复的畴（1-3）。由此产生的超晶格调制材料内的振动（3，4）和电子（5，6）结构，导致电子-声子耦合（7，8）行为的变化，并观察到强相关性和超导性（9）。然而，访问这些调制和理解相关的影响是具有挑战性的，因为调制太小的实验技术，以准确地解决相关的能量水平和太大的理论模型，以正确地描述本地化的效果。在这里，我们报告了由纳米拉曼光谱仪（10）生成的重构（低角度）扭曲双层石墨烯中晶体超晶格的高光谱光学图像。通过纳米拉曼技术，可以用可见光观察晶体结构，该技术揭示了晶格动力学的局部化，存在应变孤子和拓扑点（1），导致可检测的光谱变化。结果通过原子模型得到合理化，该模型可以评估超晶格电子和振动态的局部密度。该评估突出了孤子和拓扑点的结构的振动和电子特性的相关性，特别是对于小的扭转角。我们的研究结果是理解原子和纳米尺度下声子相关效应的重要一步，例如Jahn-Teller效应（11）和电子库珀配对（12-14），并可能有助于在快速发展的双电子学领域（16）的背景下改善器件特性（15）。纳米拉曼光谱揭示了重建扭曲双层石墨烯中一些振动模式的局部化，并提供了电子-声子耦合如何影响材料的振动和电子性质的定性见解。
Twisted bilayer graphene is created by slightly rotating the two crystal networks in bilayer graphene with respect to each other. For small twist angles, the material undergoes a self-organized lattice reconstruction, leading to the formation of a periodically repeated domain(1-3). The resulting superlattice modulates the vibrational(3,4) and electronic(5,6) structures within the material, leading to changes in the behaviour of electron-phonon coupling(7,8) and to the observation of strong correlations and superconductivity(9). However, accessing these modulations and understanding the related effects are challenging, because the modulations are too small for experimental techniques to accurately resolve the relevant energy levels and too large for theoretical models to properly describe the localized effects. Here we report hyperspectral optical images, generated by a nano-Raman spectroscope(10), of the crystal superlattice in reconstructed (low-angle) twisted bilayer graphene. Observations of the crystallographic structure with visible light are made possible by the nano-Raman technique, which reveals the localization of lattice dynamics, with the presence of strain solitons and topological points(1) causing detectable spectral variations. The results are rationalized by an atomistic model that enables evaluation of the local density of the electronic and vibrational states of the superlattice. This evaluation highlights the relevance of solitons and topological points for the vibrational and electronic properties of the structures, particularly for small twist angles. Our results are an important step towards understanding phonon-related effects at atomic and nanometric scales, such as Jahn-Teller effects(11) and electronic Cooper pairing(12-14), and may help to improve device characterization(15) in the context of the rapidly developing field of twistronics(16). Nano-Raman spectroscopy reveals localization of some vibrational modes in reconstructed twisted bilayer graphene and provides qualitative insights into how electron-phonon coupling affects the vibrational and electronic properties of the material.