LOW-ENERGY EXCITATIONS AND HYPERSONIC PROPERTIES AT LOW-TEMPERATURES IN AMORPHOUS MEDIA

Brillouin scattering measurements of velocity and inverse mean free path of phonons are presented for three silicate glasses and two samples of PMMA, in the temperature range from 1. 7 to 15 K. The velocity maximum due to resonant absorption of phonons is observed in the silicate glasses for longitudinal phonons of frequency 35 GHz. These experimental results, as well as the measurements of the inverse mean free path, can be described satisfactorily by a model assuming an interaction between phonons and two level systems. In PMMA, the velocity of 18 GHz phonons tends to be constant below 5 K, while the inverse mean free path is proportional to temperature in the whole temperature range. These results can be considered as an evidence of a relaxation process, even if a quantitative description is not possible on the basis of these data only.