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).