Lattice Anisotropy-Driven Reduction of Phonon Velocities in Black Phosphorus

Phonon dynamics and transport determine how heat is utilized and dissipated in materials. In 2D systems for optoelectronics and thermoelectrics, the impact of nanoscale material structure on phonon propagation is central to controlling thermal conduction. Here, we directly observe in-plane coherent acoustic phonon propagation in black phosphorus (BP) using ultrafast electron microscopy. We identify a significant reduction of the group velocities in directions intermediate to the armchair and zigzag lattice directions. Using a machine learning-based model with an >8000 atom supercell, we find that this slowing results from the mixing of in-plane transverse and longitudinal acoustic phonons and is independent of broken symmetries of edge reconstructions. This work demonstrates how coherent phonon transport is sensitive to propagation direction in the lattice plane.声子动力学与输运特性决定了热量在材料中的利用与耗散方式。在光电子学与热电学领域的二维体系中，纳米尺度材料结构对声子传播的影响是调控热传导的核心所在。本研究借助超快电子显微术，直接观测到黑磷（black phosphorus, BP）面内相干声学声子的传播行为，并发现扶手椅型与锯齿型晶格方向之间的中间取向处，群速度出现显著降低。本研究基于包含8000余个原子的超胞构建机器学习模型，证实该速度减慢现象源于面内横向声学声子与纵向声学声子的耦合混合，且与边缘重构引发的对称性破缺无关。本研究表明，相干声子输运对晶格平面内的传播方向具有显著敏感性。