Thermal properties in low dimensional structures below 1 K

Abstract

In this thesis thermal properties of low dimensional structures were experimentally studied at low temperatures with the help of tunnel junction thermometry and the Joule heating technique. The main objects of study were electron-phonon coupling in disordered thin metal films and phonon transport in suspended silicon nitride membranes. Our aim has been to clarify the effect of the phonon dimensionality, i.e. the effect of boundaries to the phonon modes and the transition from 3D to 2D phonons. The dimensionality cross over had not been observed before this work even though it is fabricationally a standard procedure to create the low dimensional environments for nanoscale applications and devices. The main conclusion here is that the nature of thermal phenomena can differ drastically in lower dimensional structures compared to bulk behaviour, and sometimes surprisingly advantageous ways.