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 T-i is an element of {5000 K, 3760 K}, the system is quenched to lower temperatures T-f is an element of {2500 K, 2750 K, 3000 K, 3250 K} and observed after a waiting time t(w). 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 C-q(t(w), t(w) + t), and for the mean-square displacement Delta r(2)(t(w), t(w) + t). We conclude that there are three different t(w) regions: (I) At very short waiting times, C-q(t(w), t(w) + t) decays very fast without forming a plateau. Similarly Delta r(2)(t(w), t(w) + t) increases without forming a plateau. (II) With increasing tw a plateau develops in C-q(t(w), t(w) + t) and Delta r(2)(t(w), t(w) + t). For intermediate waiting times the plateau height is independent of t(w) and T-i. Time superposition applies, i.e., C-q=C-q(t/t(r)(Cq)) where t(r)(Cq)=t(r)(Cq)(t(w)) is a waiting time-dependent decay time. Furthermore C-q=C(q, t(w), t(w) + t) scales as C-q=C(q, z(t(w), t) where z is a function of t(w) and t only, i.e., independent of q. (III) At large t(w) the system reaches equilibrium, i.e., C-q(t(w), t(w) + t) and Delta r(2)(t(w), t(w) + t) are independent of t(w) and T-i. For C-q(t(w), t(w) + t) we find that the time superposition of intermediate waiting times (II) includes the equilibrium curve (III)