NATURE OF THE CONSTANT-LOSS DIELECTRIC RESPONSE OF VARIOUS CRYSTALS AND GLASSES

For a wide range of crystalline materials and glasses, it has been shown that, at low temperatures, the ac conductivity sigma(omega) is proportional to frequency omega, corresponding to a loss per cycle that is independent of frequency. This paper explores the features of this constant-loss behavior, showing that it is a truly bulk phenomenon, that it is only slowly dependent on temperature (i.e., not activated) and, in the case of glasses, that it does not give rise to a mixed alkali effect. The ''pair approximation'' model with hopping over an asymmetric barrier is fitted to some of the results, but doubts remain as to whether this model is appropriate for systems with low defect concentrations.