Abstract
Superlubric state was found in a molecular dynamic simulation study of incommensurate Frenkel-Kontorova model. This finding was supported by an experimental study with ultra-high vacuum scanning tunneling microscopy. Recently, superlubric state was experimentally found in friction of tungsten tip sliding on graphite in ambient laboratory air. Since there are likely to be some impurities at the frictional interface in this experiment, the experimental result indicates that the superlubric state is stable against contaminations of impurities. Here we examined this indication by performing a molecular dynamic simulation study of incommensurate Frenkel-Kontorova model with impurities. In case of attractive impurities, the impurities were placed at the peaks of the sinusoidal substrate potential; in case of repulsive ones, they were placed at the bottoms. The concentration of the impurities was set to be 1/1500. The interaction potential between the impurities and the sliding atoms was expressed by Gaussian function. The strength of the interaction was varied from 1 to 3 times of the amplitude of the sinusoidal substrate potential. For each value of the strength of the impurities, motions of the sliding atoms were observed for various initial velocities and amplitudes of the substrate potential. Observing the sliding motions, friction diagrams, which show where in the parameter space friction and superlubric states are, were constructed. If the velocity of the center of mass of the sliding crystal decreased, the sliding motion was defined to be in frictional state. If the center of mass velocity did not decrease, the sliding motion was defined to be in superlubric state. The obtained friction diagrams showed that if the interaction strength between the impurities and the sliding atoms was smaller than 3 times of the amplitude of the substrate potential, the friction diagram remained unchanged. Hence, the superlubric state was shown to be stable against the weak impurities.
