How universal are the low temperature acoustic properties of glasses?

Abstract

We evaluated already published data on ultrasound absorption and velocity changes below 1 K for a set of 18 different glasses. By uniformly analysing these data according to the tunnelling model we have calculated the standard spectral density $$\bar P$$ of two-level-systems (TLS) and their coupling energyγ 1 andγ 1 to longitudinal and transversal phonons. Contrary to literature, we have not observed any correlation between $$\bar P$$ and the glass transition temperatureT g for our set of 18 glasses. As far asT g is concerned we only found a relation τ min 0.1K withn≃2.5 and where τmin is the shortest TLS-phonon relaxation time at 0.1 K. Since the values for $$\bar P$$ andγ 1,t scatter within half an order of magnitude, we conclude that the TLS parameters of a certain glass are only weakly dependent on bulk properties. We find that the ratioγ t 2 /C 44 (whereC 44 is the shear elastic constant) is a more sensitive parameter for tests on the variation of the TLS spectral density of glasses.