We present molecular dynamics simulations of a binary Lennard-Jones mixture at temperatures below the kinetic glass transition. The "mobility" of a particle is characterized by the amplitude of its fluctuation around its average position. The 5% particles with the largest/smallest mean amplitude are thus defined as the relatively most mobile/immobile particles. We investigate for these 5% particles their spatial distribution and find them to be distributed very heterogeneously in that mobile as well as immobile particles form clusters. The reason for this dynamic heterogeneity is traced back to the fact that mobile/immobile particles are surrounded by fewer/more neighbors which form an effectively wider/narrower cage. The dependence of our results on the length of the simulation run indicates that individual particles have a characteristic mobility time scale, which can be approximated via the non-Gaussian parameter.02.70. Ns, 61.20.Lc, 61.43.Fs,

The structure of a uniform simple liquid is related to that of a reference fluid with purely repulsive intermolecular forces in a self-consistently determined external reference field (ERF) phi_ R. The ERF can be separated into a harshly repulsive part phi_ R0 generated by the repulsive core of a reference particle fixed at the origin and a more slowly varying part phi_ R1 arising from a mean field treatment of the attractive forces. We use a generalized linear response method to calculate the reference fluid structure, first determining the response to the smoother part phi_ R1 of the ERF alone, followed by the response to the harshly repulsive part. Both steps can be carried out very accurately, as confirmed by MD simulations, and good agreement with the structure of the full LJ fluid is found.Comment: 11 pages, 7 figure

We study a binary Lennard-Jones system below the glass transition with molecular dynamics simulations. To investigate the dynamics we focus on events (jumps) where a particle escapes the cage formed by its neighbors. Using single particle trajectories we define a jump by comparing for each particle its fluctuations with its changes in average position. We find two kinds of jumps: "reversible jumps," where a particle jumps back and forth between two or more average positions, and "irreversible jumps," where a particle does not return to any of its former average positions, i.e., successfully escapes its cage. For all investigated temperatures both kinds of particles jump and both irreversible and reversible jumps occur. With increasing temperature, relaxation is enhanced by an increasing number of jumps and growing jump lengths in position and potential energy. However, the waiting time between two successive jumps is independent of temperature. This temperature independence might be due to aging, which is present in our system. We therefore also present a comparison of simulation data with three different histories. The ratio of irreversible to reversible jumps is also increasing with increasing temperature, which we interpret as a consequence of the increased likelihood of changes in the cages, i.e., a blocking of the "entrance" back into the previous cage. In accordance with this interpretation, the fluctuations both in position and energy are increasing with increasing temperature. A comparison of the fluctuations of jumping particles and nonjumping particles indicates that jumping particles are more mobile even when not jumping. The jumps in energy normalized by their fluctuations are decreasing with increasing temperature, which is consistent with relaxation being increasingly driven by thermal fluctuations. In accordance with subdiffusive behavior are the distributions of waiting times and jump lengths in position.

We investigate the aging dynamics of amorphous SiO(2) via molecular dynamics simulations of a quench from a high temperature T(i) to a lower temperature T(f). We obtain a microscopic picture of aging dynamics by analyzing single particle trajectories, identifying jump events when a particle escapes the cage formed by its neighbors, and determining how these jumps depend on the waiting time t(w), the time elapsed since the temperature quench to T(f). We find that the only t(w)-dependent microscopic quantity is the number of jumping particles per unit time, which decreases with age. Similar to previous studies for fragile glass formers, we show here for the strong glass former SiO(2) that neither the distribution of jump lengths nor the distribution of times spent in the cage are t(w) dependent. We conclude that the microscopic aging dynamics is surprisingly similar for fragile and strong glass formers.