First-principles calculations of phonon transport across a vacuum gap
Takuro Tokunaga, M. Arai, Kazuaki Kobayashi, Wataru Hayami, Shigeru Suehara, Takuma Shiga, Keunhan Park, Mathieu Francoeur
Abstract
Phonon transport across a vacuum gap separating intrinsic silicon crystals is predicted via the three-dimensional 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 $\ensuremath{\sim}1$ nm, and follows a ${d}^{\ensuremath{-}11.9\ifmmode\pm\else\textpm\fi{}1.2}$ 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.