Acoustic properties of borate glasses at low temperatures

Ultrasonic measurements were performed in Ag2O-B2O3 glasses at low temperatures (1.5–30 K) in order to study the influence of the stoichiometry on the low energy excitations, generally represented in terms of two-level systems (TLS). The analysis of the acoustic attenuation, which is regulated by the mechanisms of the TLS-phonon assisted relaxation, permits to obtain the spectral density of these excitations. The behaviour of with Ag2O content is in contrast with the prediction of the free-volume theory. Moreover by considering the structural modifications of the glassy network, due to Ag2O, the non-bridging oxygens, which are formed in relevant number when an high content of metallic oxide is added to boron oxide are suggested as main microscopic origin of TLS's the non-bridging oxygens, which are formed in relevant number when a high content of metallic oxide is added to boron oxide.