Diffusion mechanisms in metallic supercooled liquids and glasses

The mechanisms of atomic transport in supercooled liquids and the nature of the glass transition are long-standing problems(1-4) Collective atomic mot ion is thought to play an important role(4-6) in both phenomena. A metallic supercooled liquid represents an ideal system for studying intrinsic colle ctive motions because of its structural similarity to the "dense random pac king of spheres" model(7), which is conceptually simple. Unlike polymeric a nd network glasses, metallic supercooled liquids have only recently become experimentally accessible, following the discovery of bulk metallic glasses (8-12). Here we report a Be-9 nuclear magnetic resonance study of Zr-based bulk metallic glasses(8,9) in which we investigate microscopic transport in supercooled liquids around the glass transition regime. Combining our resu lts with diffusion measurements, we demonstrate that two distinct processes contribute to long-range transport in the supercooled liquid state: single -atom hopping and collective motion, the latter being the dominant process. The effect of the glass transition is clearly visible in the observed diff usion behaviour of the Be atoms.