On the nature of low-energy excitations in amorphous materials

Comparing recent experimental results of the thermal and acoustic properties of glasses, it appears that the degree of purity and the method of sample preparation have a noticeable influence on the specific heat, but hardly any effect on the phonon transport properties such as the thermal conductivity, ultrasonic attenuation and the sound velocity. This leads to the hypothesis that only a fraction of the low-energy excitations seen in specific heat measurements have a very strong coupling to phonons, and that this fraction, which mainly determine the transport properties, are rather insensitive to chemical and structural details.