ACOUSTIC PROPERTIES OF OXIDE GLASSES AT LOW-TEMPERATURES

Acoustic properties of alpha-B2O3 and alpha-GeO2 are investigated at l ow frequencies in the temperature range 10 mK < T < 200 K, using a vib rating-reed technique; the results confirm earlier measurements on oth er amorphous dielectrics. Whereas the low-temperature data are well de scribed by Jackle's perturbation theory in the framework of the tunnel ing model, the strong relaxation above 5 K requires a different approa ch. Up to 20 K incoherent tunneling of two-level systems seems to gove rn the relaxational dynamics; at still higher temperature the two-leve l description breaks down, and thermally activated barrier crossing le ads to a Arrhenius-like relaxation rate.