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Nanomachines: methods to induce a directed motion at nanoscale.

The motion of bodies in a periodic potential with weak damping is discussed. A spontaneous directed motion of particles is shown to be possible in the presence of external periodic forces at a velocity unambiguously defined by the frequency of the periodic action and the spatial period of the potential. The principle of inducing the directed motion at a precisely controlled velocity can be used to develop (i) means of handling individual molecules or molecular clusters on crystalline surfaces; (ii) "nanomachines"-objects capable of spontaneous motion not only in the absence of an external force but also under the action of a force opposite to the direction of motion (and thereby capable of carrying other particles); (iii) drives providing precisely controlled velocity of motion; (iv) controllable tribological systems constructed by profiling friction surfaces in a specified manner and applying an ultrasonic excitation. The dependence of the average system velocity on the average applied force is shown to have plateaus of constant velocity at zero velocity and a set of equidistant discrete velocities in the presence of periodic external perturbations. The problem of developing fully controlled nanomachines can be formulated as the problem of controlling the width and position of the plateaus.