This work reports the real-time observation of the interlayer lattice vibrations in bilayer and few-layer PtSe2 by means of the coherent phonon method. The layer-breathing mode and standing wave mode of the interlayer vibrations are found to coexist in such a kind of group-10 transition metal dichalcogenides (TMDCs). The interlayer breathing force constant standing for perpendicular coupling (per effective atom) is derived as 7.5 N/m, 2.5 times larger than that of graphene. The interlayer shearing force constant is comparable to the interlayer breathing force constant, which indicates that PtSe2 has nearly isotropic interlayer coupling. The low-frequency Raman spectroscopy elucidates the polarization behavior of the layer-breathing mode that is assigned to have A1g symmetry. The standing wave mode shows redshift with the increasing number of layers, which successfully determines the out-of-plane sound velocity of PtSe2 experimentally. Our results manifest that the coherent phonon method is a good tool to uncover the interlayer lattice vibrations, beyond the conventional Raman spectroscopy limit. The strong interlayer interaction in group-10 TMDCs reveals their promising potential in high-frequency (∼terahertz) micro-mechanical resonators.

中文翻译：

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直接观察双层和少层 PtSe2 中的层间相干声学声子动力学

这项工作报告了通过相干声子方法实时观察双层和少层 PtSe2 中的层间晶格振动。发现层间振动的层呼吸模式和驻波模式共存于这种10族过渡金属二硫属化物（TMDC）中。垂直耦合的层间呼吸力常数（每个有效原子）推导出为 7.5 N/m，是石墨烯的 2.5 倍。层间剪切力常数与层间呼吸力常数相当，表明 PtSe2 具有近乎各向同性的层间耦合。低频拉曼光谱阐明了具有 A1g 对称性的层呼吸模式的极化行为。驻波模式随着层数的增加显示出红移，这成功地通过实验确定了 PtSe2 的面外声速。我们的结果表明，相干声子方法是揭示层间晶格振动的好工具，超出了传统的拉曼光谱极限。第 10 组 TMDC 中的强层间相互作用揭示了它们在高频（～太赫兹）微机械谐振器中的潜力。