Anomalous reduction in black phosphorus phonon velocities driven by lattice anisotropy

Phonon dynamics and transport determines how heat is utilized or dissipated in materials. In anisotropic 2D materials for miniaturized optoelectronics or thermoelectrics, the impact of material geometry and nanoscale material structure on phonon propagation is central to controlling thermal conduction. Here, we directly observe {in-plane} phonon propagation and dynamics in intrinsically anisotropic black phosphorus (BP) using ultrafast electron microscopy. We directly measure coherent acoustic phonon group velocities on the nanoscale along each of the Bravais lattice vectors (zigzag and armchair) and in an intermediate lattice direction that is a linear combination of the Bravais lattice vectors. We identify an anomalously low group velocity, deviating strongly from theoretical calculations, for coherent acoustic phonons travelling in the intermediate lattice direction. A machine learning-based model, calculating the phonon properties of an >8000 atom supercell, reveals the low group phonon velocity results from a mixing of the transverse and longitudinal acoustic phonons in BP that is independent of material morphology. This work demonstrates how transport of coherent phonons may be controlled through material geometry, while also highlighting how new avenues for directional control of nanoscale heat transport remain undiscovered even within well-known corrugated layered materials like BP.