Numerical and theoretical study of friction between atomically clean surfaces and effects of impurities

Sliding friction between atomically clean surfaces with and without impurit ies is studied numerically and theoretically. We show that the maximum stat ic frictional force shows complicated behavior as a function of the elastic constant of the material, which results from various discommensurate struc tures. We also show that impurities can diminish the maximum static frictio nal force. Velocity dependences of the kinetic frictional force in the abse nce and presence of impurities are calculated. Transverse pinning, which yi elds static frictional force along the transverse direction to the sliding direction, shows peculiar behavior. It is also shown that the direction of the sliding motion can deviate from that of the external force.