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

Sliding friction between atomically clean surfaces with and without impurities is studied numerically and theoretically. We show that the maximum static frictional force shows complicated behavior as a function of the elastic constant of the material, which results from various discommensurate structures. We also show that impurities can diminish the maximum static frictional force. Velocity dependences of the kinetic frictional force in the absence and presence of impurities are calculated. Transverse pinning, which yields 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.