Step fluctuation studies of surface diffusion and step stiffness for the Ni(111) surface

Step edge fluctuations on clean Ni(111) are investigated using low-energy electron microscopy. When interpreted as capillary waves the fluctuations yield values of the surface mass diffusion coefficient Ds and the step edge stiffness β in the temperature range 1050-1340 K. β (θ,T) is of magnitude ∼300 meV nm at 1200 K, almost independent of step orientation θ, and decreases with increasing temperature T. At the lower temperatures, the decay of capillary modes depends on wave vector q as q3, as expected for surface diffusion over terraces next to the step. Also, the deduced surface diffusion coefficient Ds = 10-4±0.5 exp (-0.65 0.1 eV kB T) cm2 s is consistent with that on similar surfaces when scaled to homologous temperatures by the melting temperature Tm, in keeping with a recently suggested universality. A component of step relaxation driven by bulk diffusion above 0.65 Tm is reasonably consistent with bulk diffusion results obtained much earlier using radio tracer methods. This result is contrasted with earlier discussions that postulate a regime of high-temperature surface diffusion with a large activation energy and very large prefactor. Sublimation detected here by step edge flow near 0.75 Tm is consistent with the known cohesive energy.