Chapter 5 - Atomic-scale investigations of ultralow friction on crystal surfaces in ultrahigh vacuum

Controlling friction on the nanometer scale is one of nowaday challenges for scientists and engineers. Since the first observation of atomic friction reported by Mate et al. for a tungsten tip sliding on graphite, a lot of progress has been made in the understanding of this phenomenon on the atomic scale. An accurate description of the motion of a sharp tip elastically driven on a crystal surface by a microcantilever was first given by Tomanek et al., who based their interpretation on the Prandt-Tomlinson model. The lateral (friction) force acting on the tip can be estimated by measuring the angle of torsion of the cantilever. The tip sticks to a given equilibrium position on the surface lattice until the driving force becomes high enough to cause a slip into the closest equilibrium position along the pull direction. The resulting stick-slip motion corresponds to a sawtooth-shaped time evolution of the lateral force with the atomic periodicity of the surface lattice. However, this scenario is observed only if a precise condition is fulfilled. The lateral stiffness of the driving spring must be lower than the curvature of the tip-surface interaction potential. If this is not the case, the tip slides on the surface without abrupt jumps, and a superlubricity� scenario is observed.