Low Temperature Behavior of Potassium and Sodium - Glasses.

The low-temperature properties of a system of well characterized potassium and sodium silicate glasses have been measured in an effort to better understand the nature of the low-energy excitations which exist in all glasses and to further investigate relationships between the low-temperature behavior of glasses and their behavior at the glass transition temperature, T(,g). In addition to the low-temperature measurements which included specific heat, thermal expansion, thermal conductivity, and dielectric dispersion, each sample also had known glass transition temperature, mass density, phonon velocity, and concentration and density fluctuations. The results of this study have provided further support for an empirical relationship between the low-temperature phonon mean free path and T(,g). This seems to indicate P (PROPORTIONAL) 1/T(,g) where P is the portion of the localized two-level-state (TLS) excitations which couple strongly to phonons. This is in tangential agreement with the free-volume model which predicts that the total density of states of TLS should vary as T(,g)(' -1). The tunneling-states model of TLS could be fitted to the data, but only at the expense of a more complex array of adjustable parameters than originally envisioned. No support has been found for other models developed to explain the low-temperature specific heat and/or thermal conductivity. In particular, no evidence could be found for phonon scattering from microscopic fluctuations.