Diffusion in the Lennard-Jones glass model studied by equilibrium and nonequilibrium molecular dynamics

We present a constant-temperature nonequilibrium molecular-dynamics study of the mass transport in an amorphous Lennard-Jones system prepared by rapidly cooling the liquid at constant atmospheric pressure. The phase diagram of the liquid-supercooled liquid glass has been determined and the dependence of the glass stability on the quenching rate investigated. We calculated mobility averages when an external force is applied to the particles, thus bypassing the space-time limitations of equilibrium molecular dynamics which prevent the investigation of mass transport in glasses. The response of the system to the external perturbation is highly nonlinear: at each temperature, T, a linear fit of the mobility, μ, versus the external force reveals the existence of a threshold perturbation above which the mobility becomes liquidlike (D=μ${\mathit{k}}_{\mathit{B}}$T≳${10}^{\mathrm{-}6}$ ${\mathrm{cm}}^2$ ${\mathrm{s}}^{\mathrm{-}1}$) the system still being amorphous. Below this threshold, the mobility is vanishingly small. Our results show that for large perturbations the mobility is temperature independent whereas the data are consistent with an Arrhenius behavior in the range of smaller perturbations. A qualitative comparison of our results with recent experimental data on diffusion and viscosity of amorphous materials under irradiation is proposed.