Thermal conductivity in glasses with a phononic crystal like structure

Abstract

We investigated the thermal conductivity of two glass samples with a phononic crystal like structure at temperatures below 100 mK. The samples consist of glass with holes on a triangular lattice. The diameters of the holes are 5µm and 25µm and the porosities are 62% and 45% respectively. The thermal transport in glasses at low temperatures is generally believed to be carried by phonons. The holes in the sample limit the mean free path for the thermal phonons and therefore reduce the heat transport. By measuring samples with restricted geometries it might be possible to observe heat transport channels which are not based on propagating thermal phonons. Such an additional heat transport channel should exist in glasses because of the existence of tunneling systems that interact with each other and therefore allow for an energy diffusion via resonant flip-flop processes. We present experimental data down to 20mK where the wavelength of the phonons becomes comparable to the lattice constant of the phononic crystal structure.