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.