Measuring the Nonergodicity in Glasses by Ensemble-Averaged Photon Correlation Spectroscopy

Although dynamic light scattering is used to monitor the dynamics of glass-forming liquids above the glass transition temperature, in the glass phase the absence of ergodicity results in a partial arrest of these dynamics and traditional time-averaged measures fail to monitor the remaining dynamics. Instead, scattering data must be processed in an ensemble-averaged manner by integrating the scattering from multiple regions by slowly translating the sample. We report studies of glass-forming 2Ca(NO3)2:3KNO3 (CKN) obtained below the glass transition temperature using a motorized translation system. Our findings will be used to assess the temperature dependence of the so-called nonergodic level that is predicted by certain mode-coupling theories to exhibit ``cusp'' near the mode coupling critical temperature.