Breakdown of Optical Phonons’ Splitting in Two-Dimensional Materials,Nano Letters

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Breakdown of Optical Phonons’ Splitting in Two-Dimensional Materials
Nano Letters ( IF 9.6 ) Pub Date : 2017-05-18 00:00:00 , DOI: 10.1021/acs.nanolett.7b01090
Thibault Sohier ₁ , Marco Gibertini ₁ , Matteo Calandra ₂ , Francesco Mauri _{3,

4} , Nicola Marzari ₁

Affiliation

We investigate the long-wavelength dispersion of longitudinal and transverse optical phonon modes in polar two-dimensional materials, multilayers, and their heterostructures. Using analytical models and density-functional perturbation theory in a two-dimensional framework, we show that at variance with the three-dimensional case these modes are degenerate at the zone center but the macroscopic electric field associated with the longitudinal-optical modes gives rise to a finite slope at the zone center in their corresponding phonon dispersions. This slope increases linearly with the number of layers and it is determined solely by the Born effective charges of the material and the dielectric properties of the surrounding media. Screening from the environment can greatly reduce the slope splitting between the longitudinal and transverse optical modes and can be seen in the experimentally relevant case of boron nitride–graphene heterostructures. As the phonon momentum increases, the intrinsic screening properties of the two-dimensional material dictate the transition to a momentum-independent splitting similar to that of three-dimensional materials. These considerations are essential to understand electrical transport and optical coupling in two-dimensional systems.

中文翻译：

二维材料中光子的分裂

我们研究纵向和横向光学声子模式在极性二维材料，多层及其异质结构中的长波色散。在二维框架中使用分析模型和密度泛函微扰理论，我们表明，在三维情况下，这些模式在区域中心退化，但与纵向光学模式相关的宏观电场产生了相应声子色散在区域中心的有限斜率。该斜率随层数线性增加，并且仅由材料的Born有效电荷和周围介质的介电特性决定。从环境中进行筛选可以大大减少纵向和横向光学模式之间的斜率分裂，并且可以在实验相关的氮化硼-石墨烯异质结构案例中看到。随着声子动量的增加，二维材料的固有屏蔽特性决定了与动量无关的分裂的过渡，类似于三维材料。这些考虑对于理解二维系统中的电传输和光耦合至关重要。

更新日期：2017-05-24

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