Friction between atomically flat surfaces

We investigate the mechanism of atomic friction between two infinitely extended atomically flat surfaces within a two-dimensional Frenkel-Kontorova-Tomlinson model. The surfaces are identical but are rotated with respect to each other through an arbitrary misfit angle. The misfit gives rise to the formation of domains where every potential valley contains exactly one particle. They are separated by two sets of shift lines, crossing each other in topological defects. During quasistatic sliding, the whole domain pattern moves perpendicular to the driving force. Dissipation and friction hysteresis in the quasistatic limit are caused by irreversible jumps of these topological defects, which are the two-dimensional analogues of discommensurations occurring in the one-dimensional case of friction between chains with different lattice constants. Critical spring constants for the occurrence of instabilities can be derived. Friction shows strong dependence on both the misfit angle and the pulling direction. The relevance of the theoretical study for friction experiments on the nanoscale is discussed.