Intermediate range order dynamics near the glass transition

Correlations in the atomic structure and dynamics extend in glasses and (supercooled) liquids to an intermediate range, which is a few times longer than the interatomic spacing. Motion on this length scale can be expected to be particularly revealing of collective phenomena. However, due to neutron scattering experimental difficulties, namely to all-pervasive spurious multiple scattering effects and to the weakness of the signal at wave numbers corresponding to this length scale, very little is experimentally known so far about the dynamics in this range. We have developed a new experimental strategy to overcome these difficulties, and applied this novel approach to the study of the archetypal fragile glass former CKN. Our results provide the first direct experimental evidence for the existence of a first, fast structural relaxation step in the supercooled liquid phase, which does not scale with the viscosity and which cannot be attributed to extra vibrational modes, either propagating or local. The enhanced slowing down with decreasing temperature of the slow relaxation process in the wave number domain of the intermediate range order also allowed us to investigate the temperature dependence of the amplitude of this relaxation step in a model independent manner. In contrast to some previous findings, our direct results show no sign of any critical singularity above the glass transition temperature.