Diffusion in crystalline solids is controlled by the energy barrier an atom must overcome to hop between sites. In glasses, the same picture was assumed: measure the local barrier, predict the diffusion rate. But the measured macroscopic diffusion barriers in glasses are much larger than the local rearrangement barriers. Something was missing.
This paper identifies the missing ingredient: correlations. When an atom hops forward in a glass, the reverse hop has a different barrier than the forward hop — the energy landscape is asymmetric. An atom that moves forward is likely to move backward again, wasting its effort. The back-and-forth motion, not the height of any single barrier, is what slows diffusion.
The insight resolves the discrepancy quantitatively. Molecular dynamics simulations of metallic glasses show that the correlation contribution to the activation energy dominates the local-barrier contribution. The effective diffusion barrier is not the height of the walls but the fraction of hops that are productive — that advance the atom's net position rather than shuttling it back and forth.
The mechanism extends to other disordered materials — SiO₂ glasses and Lennard-Jones glasses show the same physics. The effect comes from structural disorder itself, not from specific chemistry. Any amorphous material with an asymmetric energy landscape will exhibit it.
Surface diffusion is faster not because surface barriers are lower, but because surface correlations are weaker. Atoms at surfaces are less likely to reverse their hops because the asymmetry is reduced. The bottleneck moves from energy to geometry: in the bulk, the landscape sends atoms back; at the surface, it lets them go.