Low-frequency vibrational states in Spectrosil-WF

The heat capacity of Spectrosil-WF (vitreous silica containing 20 ppm of OH) in the temperature range from 2 to 16 K as well as the Raman measurements are presented. The heat capacity is similar to that of Spectrosil-B (vitreous silica containing 1200 ppm of OH) but is about 20% higher than the Heralux (vitreous silica containing 130--180 ppm of OH). The density of low-frequency vibrational states, g({ν}), has been determined. It has been found that the form of g({ν}) is nonquadratic, which can be explained on the basis of a soft-potential model. About 110 atoms participate in a soft mode. The Raman and infrared coupling constants have similar dependence on frequency. These are explained on the basis of a single-coupling-constant bond model. It is found that the form of the Raman coupling constant cannot be explained on the basis of Martin-Brenig theory.