Observation of freezing in a structural glass over an exceptionally broad range of frequencies

Cooperative polarization fluctuations in mixed Rb1-x(NH4)xH2PO4 single crystals have been investigated with optical and dielectric measurements. Raman scattering reveals a soft mode at temperatures above those at which the glass anomalies first occur (T≳110 K). This mode continues slowing down on further cooling, but leaves behind a trail of hard modes. These evolve into a broad distribution of relaxations which have now been followed with Brillouin scattering, with audio-frequency dielectric measurements and with long-time sample-charging experiments. This freezing dynamics could be measured over approximately 17 orders of magnitude in frequency. One finds that the temperature dependence of the lowest frequency (or "cut-off" frequency) characterizing the distribution of relaxations is extremely well represented by the phenomenological Vogel-Fulcher law. It cannot be accuunted for as well by other laws, such as those proposed for spin glasses. © 1986 Springer-Verlag.