Chaotic transport of particles in two-dimensional periodic potentials driven by ac forces.

The diffusive and directed transport of particles in a two-dimensional periodic potential subjected to frictional and time-periodic forces is analyzed in detail. The model represents diffusion of atoms adsorbed on metal surfaces under an applied ac electric field (surface electromigration) in the low-temperature limit. The second dimension and the potential energy coupling are shown to play an important role on both diffusion and net currents, depending on the direction of the drive. A properly chosen biharmonic field is able to control the directed ratchetlike dynamics of atoms on symmetric surfaces, since current reversals take place by different stabilization of attractors. Reversals identified with hysteresis loops between periodic running attractors are robust against an increase of the second harmonic amplitude, and against temperature effects inside the experimental range for measurements of surface diffusion.