Light-scattering spectra of the glasses polystyrene (PS), polycarbonate (PC), and ${\mathrm{Ca}}_{0.4}{\mathrm{K}}_{0.6}({\mathrm{NO}}_3{)}_{1.4}$ (CKN) are measured in the frequency interval $3-10000\hspace{0.5em}\mathrm{GHz}$ covering a broad temperature range. The low frequency wing of the fast relaxation spectrum is found to show a power-law behavior with an exponent $\alpha =0.2\mathrm{}-\mathrm{0.6.}$ The exponent depends on system and temperature. No indication of a crossover to a white noise spectrum, as previously reported and discussed within mode-coupling analyses is found. It is shown that the Gilroy-Phillips model of thermally activated transitions in asymmetric double-well potentials well describes the power-law part of fast relaxation spectra in PS and CKN but fails in the case of PC. The distribution of barrier heights is extracted from the spectra.

• Received 16 July 1998

DOI:https://doi.org/10.1103/PhysRevB.58.14888