Thermal transport at the nanoscale : from fourier diffusion to phonon hydrodynamics

Abstract

From the global pursuit of clean and efficient sources of energy to the challenges presented by the high power densities in the semiconductor industry to the problem of decoherence in quantum systems, thermal processes are ubiquitous across all scales of space and time. Work done in the last decade has led to a number of experimental and theoretical developments that have enabled scientists and engineers to construct an accurate picture of thermal transport at small length and time scales. In this work, we employ and contribute to this modern toolset by testing and pushing the limits of our understanding. First, we experimentally examine the effects of domain walls and crystal structure in ferroelectric thin films on thermal transport. We move on to study the effect of crystal structure and defects in oxide thin films, in which we demonstrate a reversible process that can tune thermal conductivity across one order of magnitude. Secondly, we experimentally and theoretically examine deviations from the diffusive regime of thermal transport in SiGe alloys, thereby extending current theory and experiment to the study of size effects in thermal transport to opaque materials. Finally, we go beyond the single mode approximation to the Boltzmann transport equation and develop a formalism to study size effects and hydrodynamic phenomena by solving the full scattering matrix version of the linearized Boltzmann transport equation. Using this formalism as a guide, we report the experimental observation of second sound in graphite.