18.104.22.168.1 Roughening transition of stepped metal surfaces
This chapter describes phase transitions. Phase transitions have a long tradition for 3D materials. There are, however, a few phase transitions inherent to the clean surface. At low temperatures, the surface is well ordered with the surface atoms in a nearly bulk arrangement (1 x 1 structure) or in a superstructure. The high temperature phase shows a reduced order in a way characteristic for the phase transition given. If the displacement occurs into another lattice site on top of the originally complete top layer the transition is called roughening transition. The roughening may proceed by increasing the number of partially occupied levels with a series of transitions, until at the roughening transition the number of levels and the roughness (or asperity height) diverges. If the displacements allow any lateral position with respect to the lattice sites of the substrate, the transition is called melting. The basic model is the so called solid-on-solid (SOS) model, where atoms can only reside on top of atoms. The decrease of the peak intensity due to the roughening transition (experimental points together with model predictions) is shown. A qualitative illustration of the phase transition is given. A profile analysis by fitting experimental points with power law and Lorentzian shape is shown and the results are summarized. Roughening parameters for Ni and Cu are tabulated. By measuring both the roughness of the step edge and the probability of kink formation and edge diffusion at various temperatures, activation energies for the roughening transition are determined directly.