Light scattering spectroscopy of the liquid-glass transition : comparison with idealized and extended mode coupling theory
Light scattering experiments were performed on several fragile glass-forming materials at temperatures ranging from above the melting temperature T, to below the calorimetric glass transition temperature T, in the frequency range 0.2 GHz to 4000 GHz. The experimental susceptibility spectra x”(o) exhibit both the form and the scaling properties predicted by the idealized mode coupling theory. In particular, the scaling frequency o, was found to obey the power law w, a 1 T T,_jzO for three different materials, confirming the predicted “critical slowing down” as T+ Tc from either above or below. At low frequencies and temperatures the spectra deviate from the predictions due to the neglect of ergodicity-restoring activated transport (hopping) processes. We have therefore reanalyzed the data using the extended version of the theory, including activated transport.
Light scattering spectroscopy of the liquid-glass transition
Supercooled liquids approaching the liquid-glass transition exhibit unusual properties associated with structural relaxation. The mode-coupling theory (MCT) of supercooled liquid dynamics shows that non-linear interactions among density fluctuation modes can produce effects resembling these observations. MCT produces detailed predictions for the dynamics of density fluctuations in supercooled liquids, including a crossover from liquid to glassy dynamics at a critical temperature Tc. Recent light scattering studies of supercooled CaKNO3 are described that have provided detailed tests of MCT both above and below Tc, including a cusp in the scaling time at Tc=105+or-5 degrees C. These experiments are reviewed, and comparisons with both the idealized and extended versions of MCT are described.
Light-scattering spectroscopy of the liquid-glass transition in CaKNO3 and in the molecular glass Salol: Extended-mode-coupling-theory analysis.
Recently reported light-scattering studies of Ca K NO 3 and Salol are reanalyzed, using the extended version of the mode-coupling theory of the liquid-glass transition including activated transport or hopping effects. Problems found in the original fits due to the neglect of hopping terms are largely corrected, and quantitative predictions for the susceptibility minimum below the crossover temperature T c are found. The results are also shown to accurately explain the CaKNO 3 neutron spin-echo data reported by Mezei [J. Non-Cryst. Solids 131-133, 317 (1991)]