Acoustic properties of oxide glasses at low temperatures

Acoustic properties of a-B₂{O}₃$ and a-${GeO}₂$ are investigated at low frequencies in the temperature range 10 mKT200 K, using a vibrating-reed technique; the results confirm earlier measurements on other amorphous dielectrics. Whereas the low-temperature data are well described by J\"ackle's perturbation theory in the framework of the tunneling model, the strong relaxation above 5 K requires a different approach. Up to 20 K incoherent tunneling of two-level systems seems to govern the relaxational dynamics; at still higher temperature the two-level description breaks down, and thermally activated barrier crossing leads to a Arrhenius-like relaxation rate.