  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.
