First-principles calculations of phonon transport across a vacuum gap

Phonon transport across a vacuum gap separating intrinsic silicon crystals is predicted via the threedimensional atomistic Green's function method combined with first-principles calculations, based on the density functional theory, of all interatomic force constants. Phonon transport, dominated by acoustic modes, exceeds near-field radiation for vacuum gaps d smaller than similar to 1 nm, and follows a d-11.9 +/- 1.2 power law. It is shown that overlapping electron wave functions in the vacuum gap generates weak covalent interaction between the silicon surfaces, thus creating a pathway for phonons. The first-principles-based approach proposed in this paper is critical to accurately quantify the contribution of phonon transport to heat transfer in the extreme near field.