Apparent nonergodic behavior of supercooled liquids above the glass transition temperature - art. no. 031503

A speckle pattern can be observed in the polarized component of light scatt ered from glass forming liquids far above their glass transition temperatur e. This speckle pattern fluctuates with characteristic time that correspond s to the relaxation time of the additional ultraslow component in the corre lation function and is about seven orders of magnitude longer than the rela xation time of the a-process. This slow process is out of the experimental time window when the a-process is measured by means of the photon correlati on spectroscopy and results in an apparent nonergodicity which can be seen as a baseline offset in the ensemble-averaged correlation function. In cont rast. the time-averaged field correlation functions which have been measure d in practically all light scattering studies always decay to zero. The slo w process contributes a q-dependent excess intensity to the polarized compo nent of scattered light. The values of the nonergodicity parameters obtaine d from the static and dynamic light scattering experiments are equal. Both the slow component and the excess intensity result from denser regions of f ractal character which develop in glass-forming liquids on approaching the glass transition.