  Classical theories of crystal nucleation and growth from the liquid assume
activated processes that are interface limited, with the atoms individually
joining the growing interface by jumps that occur at a rate that is determined
by the diffusion coefficient in the liquid phase. These assumptions are in
contradiction with the results of molecular dynamics studies that are presented
here for supercooled Ni and Al20Ni60Zr20. Instead of diffusion-based attachment
across the interface, atoms join the interface by making small changes so as to
match the orientational order parameter of the nucleating crystal. Further,
instead of joining individually multiple atoms join cooperatively, with the
number of cooperative atoms increasing with decreasing temperature.
