Heat vortices of ballistic and hydrodynamic phonon transport in two-dimensional materials

Heat conduction in solid materials may behave like fluid dynamics when normal (N) scattering dominates phonon transport. In this hydrodynamic regime, the heat vortices have been predicted with frequency-independent relaxation time. So can this phenomena appear in other regimes? And what are the differences? In order to answer these questions, in this work, the heat vortices in two-dimensional materials are investigated based on the frequency-dependent phonon Boltzmann transport equation (BTE) under the Callaway model. We find that apart from the hydrodynamic regime, the heat vortices can appear in the ballistic and other transition regimes, which is a wider window of heat vortices compared to previous study (Fig. 1). The differences of heat vortices in the ballistic and hydrodynamic regime are investigated. In ribbon structure, the vortices sizes increase with system length in the ballistic regime but are confined by the system width in the hydrodynamic regime. In porous structures, the critical values of tau(R) (or tau(N)) about when the heat vortices disappear (or when the vortices size starts to become smaller) are found. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

Heat conduction in solid materials can exhibit fluid dynamics behavior when normal scattering dominates phonon transport, leading to the prediction of heat vortices with frequency-independent relaxation time in the hydrodynamic regime. This phenomenon can also appear in other regimes such as the ballistic regime, with differences in vortex sizes and critical values observed. The study provides insights into the wider window of heat vortices in different regimes and their characteristics in different material structures.