A Unified Phonon Interpretation for the Non-Fourier Heat Conduction by Non-equilibrium Molecular Dynamics Simulations

Nanoconfinement induces many intriguing non-Fourier heat conduction phenomena that have been extensively studied in recent years, such as the nonlinear temperature profile inside the devices, the temperature jumps near the contacts, and the finite-size effects. The understanding of these phenomena, however, has been a matter of debate over the past two decades. In this work, we demonstrate a unified phonon interpretation of non-Fourier heat conduction which can help to understand these phenomena by a mode-to-mode correspondence between the non-equilibrium molecular dynamics (NEMD) simulations and the mode-resolved phonon Boltzmann transport equation (BTE). It is found that the nanoscale phonon transport characteristics including temperature profile, the heat flux value and the modal temperature depend on the applied thermal reservoirs on the two contacts. Our NEMD simulations demonstrate that Langevin thermostat behaves like an infinitely large thermal reservoir and provides thermally equilibrium mode-resolved phonon outlets, while biased reservoirs, e.g., Nose-Hoover chain thermostat and velocity rescaling method behave like non-equilibrium phonon outlets. Our interpretation clearly demonstrates that the non-Fourier heat transport phenomena are originated from a combination of non-diffusive phonon transport and phonon thermal nonequilibrium. This work provides a clear understanding of nanoscale heat transport and may guide the measurement and control of thermal transport in various applications.