Violation of Boltzmann Equipartition Theorem in Angular Phonon Phase Space Slows down Nanoscale Heat Transfer in Ultrathin Heterofilms

Abstract

Heat transfer through heterointerfaces is intrinsically hampered by a thermal boundary resistance originating from the discontinuity of the elastic properties. Here, we show that with shrinking dimensions the heat flow from an ultrathin epitaxial film through atomically flat interfaces into a single crystalline substrate is significantly reduced due to violation of Boltzmann equipartition theorem in the angular phonon phase space. For films thinner than the phonons mean free path, we find phonons trapped in the film by total internal reflection, thus suppressing heat transfer. Repopulation of those phonon states, which can escape the film through the interface by transmission and refraction, becomes the bottleneck for cooling. The resulting nonequipartition in the angular phonon phase space slows down the cooling by more than a factor of 2 compared to films governed by phonons diffuse scattering. These allow tailoring of the thermal interface conductance via manipulation of the interface.

Non-equilibrium phonon distribution caused by an electrical current

“…With novel, sophisticated experimental techniques it became feasible to measure deviations from thermal distributions in great detail, even with temporal resolution. Recent evidence for a non-equilibrium phonon distribution was reported by Hanisch-Blicharski et al [10]. This paper also provides an example for the impact on macroscopic processes (in this case the cooling of a thin metal film), which can be explained microscopically by the non-equilibrium phonon distribution.…”

“…In order to understand what actually causes this proliferation, we take a look at how the phonon distribution is affected by the electrons, i.e. ġ e−ph ( q ) given in equation (10) and shown in figure 6. We see that ġ e−ph ( q ) is highest at the north pole, which means that these phonons are emitted primarily.…”

“…We see that ġ e−ph ( q ) is highest at the north pole, which means that these phonons are emitted primarily. According to (10), the transfer function F + must be responsible for the generic structure of ġ e−ph ( q ), as the electron-phonon matrix element was ruled out. This is confirmed by considering F + in q -space for f( k ) = f FD ( k − k S , T e ) and g( q ) = g BE ( q , T ph = T e ).…”

Nanoscale Energy Transport Dynamics across Nonbonded Solid–Molecule Interfaces and in Molecular Thin Films

“…In contrast, time-resolved diffraction-based methods have the advantage of direct structure probing and suitable spatiotemporal resolution for laser-induced lattice and phonon dynamics, in addition to thermal transport information. To date, studies concerning out-of-plane photothermally induced dynamics have been conducted on substrate-supported elemental (semi)metal thin films, − semiconductor heterostructures, − two-dimensional (2D) materials, , and water and methanol ices − as well as a graphene–polymer interface . However, few reports looked into the energy transport across thin-film interfaces when composing units of two materials have significant size differences and are separated by weak (vdW) interactions.…”