Superdiffusive motion of the Pb wetting layer on the Si(111) surface.

Mass transport in the Pb wetting layer on the Si(111) surface is investigated by observing nonequilibrium coverage profile evolution with low energy electron microscopy and microlow energy electron diffraction. Equilibration of an initial coverage step profile occurs by the exchange of mass between oppositely directed steep coverage gradients that each move with unperturbed shape. The bifurcation of the initial profile, the shape of the profile between the two moving edges, and the time dependence of equilibration are all at odds with expectations for classical diffusion behavior. These observations signal a very unusual coverage dependence of diffusion or may even reveal an exceptional collective superdiffusive mechanism.