Femtosecond optically excited coherent acoustic phonon modes (CAPs) are investigated in a free-standing van der Waals heterostructure composed of a 20-nm transparent hexagonal boron nitride (hBN) and a 42-nm opaque graphite layer. Employing ultrafast electron diffraction, which allows for the independent evaluation of strain dynamics in the constituent material layers, three different CAP modes are identified within the bilayer stack after the optical excitation of the graphite layer. An analytical model is used to discuss the creation of individual CAP modes. Furthermore, their excitation mechanisms in the heterostructure are inferred from the relative phases of these modes by comparison with a numerical linear-chain model. The results support an ultrafast heat transfer mechanism from graphite to the hBN lattice system, which is important to consider when using this material combination in devices.

中文翻译：

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耦合六方氮化硼-石墨异质结构中的相干声子

在由 20 nm 透明六方氮化硼 (hBN) 和 42 nm 不透明石墨层组成的独立范德华异质结构中研究飞秒光激发相干声声子模式 (CAP)。采用超快电子衍射，可以独立评估组成材料层中的应变动力学，在石墨层光学激发后，在双层堆叠内识别出三种不同的 CAP 模式。分析模型用于讨论各个 CAP 模式的创建。此外，通过与数值线性链模型进行比较，可以从这些模式的相对相位推断出它们在异质结构中的激发机制。结果支持从石墨到六方氮化硼晶格系统的超快传热机制，这一点在设备中使用这种材料组合时需要考虑。