Molecular-dynamics study of binary soft-sphere glasses: Quench-rate effects and aging effects

Molecular-dynamics simulations (MD) have been carried out on a soft-sphere model for binary alloys quenched into glassy states at different quench rates. The main purpose of the present work is to investigate slow relaxation phenomena of the quenched glassy states close to the glass transition by calculating both the static and dynamical structural changes upon aging (annealing) of them. For this purpose the MD simulations were performed for a time interval as long as an order of 2000\ensuremath{\tau}, where \ensuremath{\tau} is a microscopic time scale, equivalent to an order of the Einstein period, which is the characteristic period of oscillation of atoms. It is shown that the static properties exhibit neither significant changes during annealing of samples, nor changes for different samples. On the other hand, the dynamic properties show remarkable aging effects as well as sample-dependent behavior, meaning that the quenched glassy state cannot attain to an equilibrium state for the time scale of our simulations, due to dynamical slowing-down phenomena. The quenched sample is, however, shown to tend to a seemingly quasiequilibrium state after a sufficient annealing. Detailed discussions are made on both aging effects and sample-dependent behaviors of quenched glassy states, by paying particular attention to the behavior of the single-atom motion, mean-square displacement, and self-diffusion constant of the quenched glassy state. A non-Gaussian parameter is also calculated, which we propose as a good candidate to represent an order parameter of the glass transition.