Nonlocal phonon thermal transport in graphene in hydrodynamic regime

The hydrodynamic behavior of phonons is of particular interest and importance owing to the strong demand for highly thermal conductive materials. Thermal transport in hydrodynamic regime becomes essentially nonlocal, which can give rise to a number of new and counterintuitive phenomena. In this work, we present a direct numerical study of nonlocal phonon thermal transport in graphene ribbon with vicinity geometry based on the phonon Boltzmann transport equation with first-principles inputs. We demonstrate the viscosity-dominated hydrodynamic transport behaviors with two abnormal thermal transport phenomena: heat current whirlpools and negative nonlocal effect, which originate from phonon viscosity. Phonon viscosity produces the vorticity of shear flows, leading to the backflow of the heat current and the generation of negative nonlocal vicinity response. The system average temperature and the ribbon width as well as the relative positions of the heat sources play a pivotal role in the occurrence of heat current whirlpools and negative nonlocal temperature response. The present work provides solid evidence for phonon hydrodynamic transport in graphene and a potential avenue for experimental detection in the future.