Impact of Mechanical Deformation on Phonon Hydrodynamics in Two-Dimensional Materials*

The route to room temperature phonon hydrodynamics remains an open challenge. One unexplored avenue is to engineer phonon properties via mechanical deformation. By leveraging first-principles calculations, this work elucidates the interplay between mechanical strain, compression, and phonon hydrodynamics in 2D (graphene, hexagonal boron nitride) and quasi-2D materials (bi-layer graphene and graphite). First, we investigate the effect of strain on phonon dispersion relations, noting considerable shifts in phonon frequency. Furthermore, by analyzing the phonon dispersion relations in graphite, we observe that a decrease in interlayer distance correspondingly elevates the frequency of the ZO' mode, attributed to the enhanced van der Waals interactions between the layers. Consequently, stronger interlayer interactions induce stiffer layer coupling, manifesting as higher vibrational frequencies. Secondly, we examine the impact of strain on 3^rd and 4^th order force constants and anharmonic scattering rates. We then examine the impact of tensile and compressive strain on the phonon hydrodynamics window. To do so, we employ Guyer's criteria and solutions to the full scattering matrix Boltzmann transport equation to obtain the window of phonon hydrodynamics in the temperature-length scale phase space as a function of mechanical deformation. We demonstrate that as strain and compression are applied, the hydrodynamic window may shift to higher temperatures.