Tip-jump statistics of stick-slip friction

Friction maps with atomic unit cell periodicity on highly oriented graphite in ultra-high vacuum are measured with the friction force microscope. The well-known stick-slip phenomenon is observed where the tip jumps between individual equilibrium positions. We perform a statistical analysis of the frictional forces that are necessary to induce the tip jumps. The corresponding histograms give a direct fingerprint of the distribution function of the tip-jump events. The histograms depend strongly on the atomic structure of the surface, which is related to the two-dimensionality of the surface potential. We compare the experimental to the theoretical distribution function, which is based on the thermally activated Tomlinson model for atomic friction, and quantitative values for the effective energy barriers are extracted.