  Despite recent experiments exhibiting an impressive enhancement in radiative
heat flux between parallel planar silica surfaces with gap sizes of about 10
nm, the exploration of sub-nanometric gap distances remains unexplored. In this
work, by employing non-equilibrium molecular dynamics (NEMD) simulations, we
study the heat transfer between two SiO2 plates in both their amorphous and
crystalline forms. When the gap size is 2 nm, we find that the heat transfer
coefficient experiences a substantial ~30-fold increase compared to the
experimental value at the gap size of 10 nm confirming the dependence on the
distance inversely quadratic as predicted by the fluctuational electrodynamics
(FE) theory. Comparative analysis between NEMD and FE reveals a generally good
agreement, particularly for amorphous silica. Spectral heat transfer analysis
demonstrates the profound influence of gap size on heat transfer, with peaks
corresponding to the resonances of dielectric function. Deviations from
fluctuational electrodynamics theory at smaller gap sizes are interpreted in
the context of acoustic phonon tunneling and the effects of a gradient of
permittivity close to the surfaces.
