Crossover from bulk to surface diffusion in the fluctuations of step edges on Pt(111)

Using low-energy electron microscopy we explore the fluctuations of step edges on Pt(111) in the temperature range 1190 K < T < 1520 K. Below 1400 K the relaxation times vary with wave vector q as q³, and above 1400 K as q², while the observed activation energy changes from 1.25 to 2.75 eV. A discussion shows that surface diffusion is responsible for the rates at low temperatures, and that bulk vacancy diffusion becomes dominant above 1400 K. We infer that a similar crossover must occur for steps on most metal surfaces. The step stiffness is determined as 175±20 meV nm^-1 at 1500 K.