We present a numerical study of classical particles obeying a Langevin equation moving on a solid bcc (110) surface. The particles are subject to a two dimensional periodic and symmetric potential of rectangular symmetry and to an external dc field along one of the diagonals of the structure. One observes a bias current with a component orthogonal to the dc field. The drift velocity (magnitude and direction) and diffusion of the particle depend on the surface potential and external field

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... ers, the temperature, and the friction coefficient. We numerically explore these dependences. Because the potential perceived by a particle as well as its friction coefficient depend on the nature of the particle, so might the angle between the particle velocity and the dc field. This scenario may thus provide a useful particle sorting technique.