Vacuum Phonon Tunneling

Abstract

Field-induced phonon tunneling, a previously unknown mechanism of interfacial thermal transport, has been revealed by ultrahigh vacuum inelastic scanning tunneling microscopy (STM). Using thermally broadened Fermi-Dirac distribution in the STM tip as in situ atomic-scale thermometer we found that thermal vibrations of the last tip atom are effectively transmitted to sample surface despite few angstroms wide vacuum gap. We show that phonon tunneling is driven by interfacial electric field and thermally vibrating image charges, and its rate is enhanced by surface electron-phonon interaction.

Phonon transport across a vacuum gap

“…Although vacuum phonon transport is not typically considered in AFM and STM studies, we have shown that atomic interactions between leads separated by a vacuum-gap can result in energy transport at a rate that is four orders of magnitude greater than predictions of near-field radiation theory. Altfeder et al 1 experimentally observed a heat flux that was six orders of magnitude larger than predictions of near-field radiation theory. One explanation for this discrepency may be that the electric-field/lattice-deformation coupling mechanism proposed by Prunnila and Meltaus 2 is present in the STM tip-sample metal-metal junction studied by Altfeder et al 1 but is not considered here.…”

“…Altfeder et al 1 experimentally observed a heat flux that was six orders of magnitude larger than predictions of near-field radiation theory. One explanation for this discrepency may be that the electric-field/lattice-deformation coupling mechanism proposed by Prunnila and Meltaus 2 is present in the STM tip-sample metal-metal junction studied by Altfeder et al 1 but is not considered here. Because free electrons couple strongly to electric fields, the electron tunneling present in this STM experiment could have enhanced the atomic interactions between the leads.…”

“…Studying phonon transport across small gaps is also relevant to predicting the thermal resistance of rough interfaces 4,5 and in accounting for thermal resistance due to the presence of nanovoids. 6 Using ultrahigh-vacuum inelastic STM, Altfeder et al 1 studied the thermal coupling between a Pt/Ir STM-tip and a Au(111) film separated by 3Å of vacuum. They found that the local electric field of the STM tip couples the thermal vibrations of the tip and the film.…”

Scanning Tunneling Microscopy of Atomic Scale Phonon Standing Waves in Quasi-freestanding WSe2 Monolayers

“…The unusual behavior of phonons in nanomaterials manifests in such phenomena as spontaneous ripples on graphene [1], inelastic Friedel oscillations [2], phonon tunneling in subnanometer gaps [3], and phonon interference in quantum wells [4]. Experimental observation of atomic scale phonon standing waves that will be described here opens new page in this rapidly developing research field.…”

Local determination of the amount of integration of an atom into a crystal surface

“…Our method was recently used to study the electron-phonon interaction for Pb/Si(111) at the atomic scale 14 . Furthermore, phonons were utilized to identify the tip apex structure in a STM 15 and even its temperature 16 . High spatial resolution about the influence of point defects on surface phonon spectra was so far only provided by theory, for example, for phonons on pure metal surfaces and on surfaces with adatoms 17,18 .…”