  Current theoretical approaches to the analysis of radiative heat exchange at
the nanoscale are based on Rytov's stochastic electrodynamics. However, this
approach falls short in the description of microscale energy transfer since it
overlooks non-resonant contributions arising from the coupling between
different modes of relaxation in the material. We show that the phonon density
of states given through a log-normal distribution accounts for such
mode-coupling and leads to a general expression for the heat transfer
coefficient which includes non-resonant contributions. This expression fits the
existing experimental results with remarkable accuracy. Thus, our theory goes
beyond stochastic electrodynamics and offers an overall explanation of energy
transfer through micrometric gaps regardless of geometrical configurations.
