How do the properties of a glass depend on the cooling rate? A computer simulation study of a Lennard-Jones system

Abstract

Using molecular dynamics computer simulations we investigate how the glass transition and the properties of the resulting glass depend on the cooling rate with which the sample has been quenched. This is done by studying a two component Lennard-Jones system which is coupled to a heat bath whose temperature is decreased from a high temperature, where the system is a liquid, to zero temperature, where the system is a glass. The temperature $T_b$ of the heat bath is decreased linearly in time, i.e. $T_b=T_0-γt$, where $γ$ is the cooling rate. In accordance with simple theoretical arguments and with experimental observations we find that the glass transition, as observed in the specific heat and the thermal expansion coefficient, becomes sharper when $γ$ is decreased. A decrease of the cooling rate also leads to a decrease of the glass transition temperature $T_g$ and we show that the dependence of $T_g$ on $γ$ can be rationalized by assuming that the temperature dependence of the relaxation times of the system is given by either a Vogel-Fulcher law or a power-law. By investigating the structural properties of the glass, such as the radial distribution functions, the coordination numbers and the angles between three neighbor-sharing particles, we show how the local order of the glass increases with decreasing cooling rate. The enthalpy and the density of the glass decrease and increase, respectively, with decreasing $γ$. By investigating the $γ$ dependence of clusters of nearest neighbors, we show how these observations can be understood from a microscopic point of view. We also show that the spectrum of the glass, as computed from the dynamical matrix, shows a shift towards higher frequencies when $γ$ is decreased. All these effects show that there is a significant