Atomistic modeling of extreme near-field heat transport across nanogaps between two polar dielectric materials

The understanding of extreme near-field heat transport across vacuum nanogaps between polar dielectric materials remains an open question. In this work, we present a molecular dynamic simulation of heat transport across MgO-MgO nanogaps, together with a consistent comparison with the continuum fluctuational-electrodynamics theory using local dielectric properties. The dielectric function is computed by Green-Kubo molecular dynamics with the anharmonic damping properly included. As a result, the direct atomistic modeling shows significant deviation from the continuum theory even up to a gap size of few nanometers due to non-local dielectric response from acoustic and optical branches as well as phonon tunneling. The lattice anharmonicity is demonstrated to have a large impact on the energy transmission and thermal conductance, in contrast to its moderate effect reported for metallic vacuum nanogaps. The present work thus provides further insight into the physics of heat transport in the extreme near-field regime between polar materials, and put forward a methodology to account for anharmonic effects.