  We study by molecular dynamics computer simulation a binary soft-sphere
mixture that shows a pronounced decoupling of the species' long-time dynamics.
Anomalous, power-law-like diffusion of small particles arises, that can be
understood as a precursor of a double-transition scenario, combining a glass
transition and a separate small-particle localization transition. Switching off
small-particle excluded-volume constraints slows down, rather than enhances,
small-particle transport. The data are contrasted with results from the
mode-coupling theory of the glass transition.
