arXiv subjects

Takuro Tokunaga

Publications and source records attributed to Takuro Tokunaga.

3 recordsLinked to original sources

arXiv2022

Role of Acoustic Phonon Transport in Near- to Asperity-Contact Heat Transfer

Acoustic phonon transport is revealed as a potential radiation-to-conduction transition mechanism for single-digit nanometer vacuum gaps. To show this, we measure heat transfer from a feedback-controlled platinum nanoheater to a laterally oscillating silicon tip as the tip-nanoheater vacuum gap distance is precisely controlled from a single-digit nanometer down to bulk contact in a high-vacuum shear force microscope. The measured thermal conductance shows a gap dependence of $d^{-5.7\pm1.1}$ in the near-contact regime, which is in good agreement with acoustic phonon transport modeling based on the atomistic Green's function framework. The obtained experimental and theoretical results suggest that acoustic phonon transport across a nanoscale vacuum gap can be the dominant heat transfer mechanism in the near- and asperity-contact regimes and can potentially be controlled by an external force stimuli.

cond-mat.mes-hall↗

arXiv2021

First-principles calculations of phonon transport across a vacuum gap

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.

cond-mat.mes-hall↗

arXiv2021

Extreme Near-Field Heat Transfer Between Gold Surfaces

Extreme near-field heat transfer between metallic surfaces is a subject of debate as the state-of-the-art theory and experiments are in disagreement on the energy carriers driving heat transport. In an effort to elucidate the physics of extreme near-field heat transfer between metallic surfaces, this Letter presents a comprehensive model combining radiation, acoustic phonon and electron transport across sub-10-nm vacuum gaps. The results obtained for gold surfaces show that in the absence of bias voltage, acoustic phonon transport is dominant for vacuum gaps smaller than ~2 nm. The application of a bias voltage significantly affects the dominant energy carriers as it increases the phonon contribution mediated by the long-range Coulomb force and the electron contribution due to a lower potential barrier. For a bias voltage of 0.6 V, acoustic phonon transport becomes dominant at a vacuum gap of 5 nm, whereas electron tunneling dominates at sub-1-nm vacuum gaps. The comparison of the theory against experimental data from the literature suggests that well-controlled measurements between metallic surfaces are needed to quantify the contributions of acoustic phonon and electron as a function of the bias voltage.

cond-mat.mes-hall↗