Isotope effect of diffusion in a simple liquid

The diffusional isotope effect of a monatomic Lennard-Jones liquid is calculated by molecular-dynamics simulation. Mass differences of 10-40 % were used. At equilibrium density, with decreasing temperature a strong reduction of the isotope effect is found that indicates a marked increase of the collectivity of motion. Changing the density at constant temperature, the same effect is seen that shows that the density change is the main driving force behind the reduction of the isotope effect.

Dynamic heterogeneity of relaxations in glasses and liquids

We report an investigation of the heterogeneity in supercooled liquids and glasses using the non-Gaussianity parameter. We simulate selenium and a binary Lennard-Jones system by molecular dynamics. In the non-Gaussianity three time domains can be distinguished: an increase on the ps scale due to the vibrational (ballistic) motion of the atoms, followed by a growth, due to local relaxations ( beta relaxation) at not too high temperatures, and finally a slow drop at long times. The non-Gaussianity follows in the intermediate time domain a sqrt[t] law. This is explained by collective hopping and dynamic heterogeneity. We support this finding by a model calculation.