TUNNELING STATES IN VITREOUS GEO2

Ultrasonic measurements of the attenuation and the velocity variation have been carried out in amorphous GeO2 at low temperature (0.3-10 K) and high frequencies (80-210 MHz). From numerical fits to the tunnelin g model, the typical parameters of the tunneling states (TS) were dete rmined and compared to those found for vitreous SiO2. Tne study reveal s that in a-GeO2, which is considered as a close structural analog to a-SiO2, although the density of states is found to be very similar in both materials, the coupling between the TS and the phonons is signifi cantly smaller. In the model of coupled tetrahedra as the origin of th e TS, this difference can be understood in view of the fact that numer ical calculations about the vibrational characteristics of network amo rphous solids indicate that the tetrahedra are more decoupled in vitre ous GeO2 than in vitreous silica.