Aging to equilibrium dynamics of SiO2.

Molecular dynamics computer simulations are used to study the aging dynamics of SiO2 (modeled by the BKS model). Starting from fully equilibrated configurations at high temperatures Ti∊{5000 K,3760 K}, the system is quenched to lower temperatures Tf∊{2500 K,2750 K,3000 K,3250 K} and observed after a waiting time tw. Since the simulation runs are long enough to reach equilibrium at Tf, we are able to study the transition from out-of-equilibrium to equilibrium dynamics. We present results for the partial structure factors, for the generalized incoherent intermediate scattering function Cq(tw,tw+t), and for the mean-square displacement Δr2(tw,tw+t). We conclude that there are three different tw regions: (I) At very short waiting times, Cq(tw,tw+t) decays very fast without forming a plateau. Similarly Δr2(tw,tw+t) increases without forming a plateau. (II) With increasing tw a plateau develops in Cq(tw,tw+t) and Δr2(tw,tw+t). For intermediate waiting times the plateau height is independent of tw and Ti. Time superposition applies, i.e., Cq=Cq(t/trCq) where trCq=trCq(tw) is a waiting time-dependent decay time. Furthermore Cq=C(q,tw,tw+t) scales as Cq=C(q,z(tw,t)) where z is a function of tw and t only, i.e., independent of q. (III) At large tw the system reaches equilibrium, i.e., Cq(tw,tw+t) and Δr2(tw,tw+t) are independent of tw and Ti. For Cq(tw,tw+t) we find that the time superposition of intermediate waiting times (II) includes the equilibrium curve (III).

Single particle jumps in a binary Lennard-Jones system below the glass transition.

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.