V. N. Novikov

Nuclear quantum effects in atomic motions are well known at low temperatures [equation]K, but analyses of structural relaxation in liquids and description of the glass transition...

By combining, and modestly extending, a variety of theoretical concepts for the dynamics of liquids in the supercooled regime, we formulate a simple analytic model for the temperature and wavevector dependent collective density fluctuation relaxation time that is measurable using coherent dynamic neutron scattering. Comparison with experiments on the ionic glass-forming liquid Ca–K–NO3 in the lightly supercooled regime suggests the model captures the key physics in both the local cage and mesoscopic regimes, including the unusual wavevector dependence of the collective structural relaxation time. The model is consistent with the idea that the decoupling between diffusion and viscosity is reflected in a different temperature dependence of the collective relaxation time at intermediate wavevectors and near the main (cage) peak of the static structure factor. More generally, our analysis provides support for the ideas that decoupling information and growing dynamic length scales can be at least qualitatively deduced by analyzing the collective relaxation time as a function of temperature and wavevector, and that there is a strong link between dynamic heterogeneity phenomena at the single and many particle level. Though very simple, the model can be applied to other systems, such as molecular liquids.

The correlation between the fragility of glass-forming systems and the ratio of instantaneous bulk to shear moduli, K _∞/G _∞, of the corresponding glasses is analyzed for bulk metallic and polymeric materials. We emphasize the importance of the free electron contribution to the bulk modulus of metals and a specific intramolecular contribution to fragility in the case of polymers for some long molecules. We argue that these two contributions are the main reasons for the failure of the correlation between fragility and K _∞/G _∞ observed for these materials.

Relaxation processes in polymethylmethacrylate were investigated using light scattering and mechanical relaxation techniques. The acoustic attenuation data cover the frequency range from 1 Hz to 17 GHz and the light scattering from 0.5 to 10 000 GHz. Analysis of the data in a broad temperature range (80–350 K) shows that the model of the asymmetric double-well potentials, traditionally used for description of the acoustic attenuation in glasses, cannot describe all the data consistently. It is shown that assuming an additional relaxation in GHz region with a constant loss spectrum provides a good agreement of the model with experimental data.