Soft-sphere model for the glass transition in binary alloys: Pair structure and self-diffusion

Molecular-dynamics simulations have been carried out on a ‘‘soft-sphere’’ model for binary alloys quenched into supercooled and amorphous states. The main emphasis of the work is on the static and dynamic characterization of the glass transition. A comparison between molecular-dynamics data and the results of a self-consistent integral equation shows that the equation of state bifurcates into ‘‘glass’’ and ‘‘fluid’’ branches below a glass transition temperature $T_g$. The static pair structures differ significantly along the two branches. The structurally relaxed ‘‘fluid’’ branch leads to a phase separation at very low temperatures. Close to the glass transition, the atomic mean-square displacements of the two species go over more and more slowly to the asymptotic diffusive regime, due to the emergence of an intermediate time scale linked to the slowing down of structural relaxation. The diffusion constants of the two species follow closely a scaling law, as predicted by mode-coupling theory, except in the immediate vicinity of the glass transition where activated processes lead to residual diffusion.