Electron-phonon heat exchange in layered nano-systems

Abstract

We analyze the heat power $P$ between electrons and phonons in thin metallic films deposited on free-standing dielectric membranes in a temperature range in which the phonon gas has a quasi two-dimensional distribution. The quantization of the electrons wavenumbers in the direction perpendicular to the film surfaces lead to the formation of quasi two-dimensional electronic sub-bands. The electron-phonon coupling is treated in the deformation potential model and, if we denote by $T_e$ the electrons temperature and by $T_{ph}$ the phonons temperature, we find that $P\equiv P^{(0)}(T_{e})-P^{(1)}(T_{e},T_{ph})$; $P^{(0)}$ is the power "emitted" by the electron system to the phonons and $P^{(1)}$ is the power "absorbed" by the electrons from the phonons. Due to the quantization of the electronic states, $P$ vs $(d,T_e)$ and $P$ vs $(d,T_{ph})$ show very strong oscillations with $d$, forming sharp crests almost parallel to the temperature axes. In the valleys between the crests, $P \propto T_e^{3.5} - T_{ph}^{3.5}$. From valley to crest, $P$ increases by more than one order of magnitude and on the crests $P$ does not have a simple power law dependence on temperature. The strong modulation of $P$ with the thickness of the film may provide a way to control the electron-phonon heat power and the power dissipation in thin metallic films. Eventually the same mechanism may be used to detect small variations of $d$ or surface contamination. On the other hand, the surface imperfections of the metallic films may make it difficult to observe the oscillations of $P$ with $d$ and eventually due to averaging the effects the heat flow would have a more smooth dependence on the thickness in real experiments.