We present a macroscopic, phenomenological theory for the heat flow between two material half-spaces of differing temperatures whose surfaces are separated by a gap of width l. Our calculation parallels Liftshitz’s calculation of the van der Waals force between two dielectric slabs. For l sufficiently small, the heat flow is enhanced by a contribution from evanescent waves, and in the limit of a very small gap varies as ${\mathit{l}}^{\mathrm{-}2}$.

• Received 25 August 1994

DOI:https://doi.org/10.1103/PhysRevB.50.18517