Acoustic properties of a-${\mathrm{B}}_2$${\mathrm{O}}_3$ and a-${\mathrm{GeO}}_2$ are investigated at low frequencies in the temperature range 10 mK<T<200 K, using a vibrating-reed technique; the results confirm earlier measurements on other amorphous dielectrics. Whereas the low-temperature data are well described by Jäckle’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.

• Received 26 May 1995

DOI:https://doi.org/10.1103/PhysRevB.52.7179