First-principles calculations of phonon transport across a vacuum gap

TL;DR

This study predicts phonon-mediated heat transfer across a silicon vacuum gap using first-principles calculations, revealing that phonons dominate heat transfer at sub-nanometer gaps, which is crucial for understanding heat transfer in extreme near-field conditions.

Contribution

The paper introduces a first-principles atomistic Green's function approach to accurately quantify phonon transport across vacuum gaps, highlighting the role of electron wave function overlap.

Findings

Phonon transport exceeds near-field radiation for gaps smaller than ~1 nm.

Acoustic phonon modes primarily contribute to heat transfer.

Weak covalent interactions enable phonon pathways across the vacuum gap.

Abstract

Phonon transport across a vacuum gap separating intrinsic silicon crystals is predicted via the atomistic Green's function method combined with first-principles calculations of all interatomic force constants. The overlap of electron wave functions in the vacuum gap generates weak covalent interaction between the silicon surfaces, thus creating a pathway for phonons. Phonon transport, dominated by acoustic modes, exceeds near-field radiation for vacuum gaps smaller than ~ 1 nm. The first-principles-based approach proposed in this work is critical to accurately quantify the contribution of phonon transport to heat transfer in the extreme near field.