Frequency and Temperature Dependence of the Sound Velocity in Amorphous Materials at Low Temperatures

Measurements of the sound velocity in vitreous silica and in amorphous metallic Pd-Si between 0.4 and 20 K and in the frequency range 10-450 MHz are reported. Above 4 K, the sound velocity varies linearly versus temperature and logarithmically versus frequency. From the analysis of the experimental results it is shown that the linear temperature dependence of the sound velocity cannot be explained with relaxation processes.