Heat vortex in hydrodynamic phonon transport of two-dimensional materials.

We study hydrodynamic phonon heat transport in two-dimensional (2D) materials. Starting from the Peierls-Boltzmann equation with the Callaway model approximation, we derive a 2D Guyer-Krumhansl-like equation describing hydrodynamic phonon transport, taking into account the quadratic dispersion of flexural phonons. In addition to Poiseuille flow, second sound propagation, the equation predicts heat current vortices and negative non-local thermal conductance in 2D materials, which are common in classical fluids but have not yet been considered in phonon transport. Our results also illustrate the universal transport behaviors of hydrodynamics, independent of the type of quasi-particles and their microscopic interactions.

Intrinsically low thermal conductivity of bismuth oxychalcogenides originating from interlayer coupling.

The anharmonicity of phonons in a solid is ultimately rooted in the chemical bonding. However, the direct connection between phonon anharmoncity and chemical bonding is difficult to make experimentally or theoretically, mainly due to their complicated lattice structures. Here, with the help of first-principles calculations, we show that the intrinsically low lattice thermal conductivity (κ) of Bi2O2X (X = S, Se, Te) shows a strong connection to the electrostatic inter-layer coupling. We explain our results by the strong anharmonic chemical bonding between Bi and chalcogen atoms. Additionally, due to the strong anharmonicity, a large portion of phonon modes has a mean free path shorter than the average atomic distance. We employ a recently proposed two-channel model to take into account their contribution to κ.