Aging to Equilibrium Dynamics of SiO2

TL;DR

This study uses molecular dynamics simulations to analyze the aging and equilibrium dynamics of SiO2, revealing three distinct aging regimes and the conditions under which the system reaches equilibrium.

Contribution

It provides a detailed characterization of the transition from out-of-equilibrium to equilibrium dynamics in SiO2, including the development of a plateau and time superposition principles.

Findings

Identification of three aging regimes in SiO2 dynamics.

Development of a plateau in correlation functions at intermediate waiting times.

Confirmation of time superposition and scaling behaviors in aging SiO2.

Abstract

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 =5000K/3760K the system is quenched to lower temperatures T_f=2500K, 2750K, 3000K, 3250K and observed after a waiting time t_w. Since the simulation runs are long enough to reach equilibrium at T_f, 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 msd(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 msd(t_w,t_w+t) increases without forming a plateau. (II) With increasing t_w a plateau develops in C_q(t_w, t_w+t) and msd(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) where t_r=t_r(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 msd(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).