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二维材料及其异质结的声子物理研究

The phonon physics of two-dimensional materials and related heterostructures

  • 摘要: 声子是固体最重要的元激发之一,是理解材料摩尔热容、德拜温度以及热膨胀系数等热力学性质的基础,同时电声子相互作用也决定了固体的电导和超导等特性。拉曼光谱是表征固体声子物理的重要实验手段,不仅能表征材料的结构和质量,还能提供材料声子性质、电子能带结构、电声耦合等信息。文章将拉曼光谱应用于二维材料及其范德瓦尔斯异质结的声子物理研究。先简单介绍二维材料的层间振动声子模式和层内振动声子模式,其中层间振动声子模式的频率可用线性链模型来计算,而强度则可用层间键极化率模型来解释;同类层内振动声子模式的Davydov劈裂峰之间的频率差异可用范德瓦尔斯模型拟合。随后,将这些模型推广到二维范德瓦尔斯异质结中,以转角多层石墨烯、MoS2/石墨烯和hBN/WS2为例介绍了范德瓦尔斯异质结的声子谱,阐述如何应用线性链模型和经典键极化率模型计算层间振动模的频率和强度,并由此给出二维范德瓦尔斯异质结的界面耦合强度和各层间呼吸模的电声耦合强度等重要参数。

     

    Abstract: Phonon is one of the most important elementary excitations, and is fundamental for understanding thermodynamic properties, such as heat capacity, Debye temperature, and the coefficient of thermal expansion. Furthermore, electron-phonon coupling can determine the electric conductivity and superconductivity of materials. Raman spectroscopy is the most important tool to study phonon physics, and can not only be utilized to explore the lattice structure and quality of materials but also their phonon properties, electronic band structure and electron-phonon coupling. Here, we investigate the phonon physics of two-dimensional (2D) materials and the related van der Waals heterostructures by Raman spectroscopy. First, we will introduce interlayer and intralayer phonon modes. The frequency of the interlayer phonon modes can be well reproduced by the linear chain model while their intensity can be calculated by the interlayer bond polarizability model; in addition, the splitting frequency between Davydov components in multilayer 2D materials originating from the same intralayer mode in monolayer counterpart can be well fitted by the van der Waals model. Secondly, we extend these models to 2D van der Waals heterostructures. By taking twisted multilayer graphene, MoS2/graphene and hBN/WS2 heterostructures as examples, we demonstrate how to calculate the frequency and Raman intensity of the interlayer modes by the linear chain and interlayer polarizability models, respectively, which can further give the strength of the interlayer coupling and electron-phonon coupling for layer-breathing modes in van der Waals heterostructures.

     

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