Biased Motion in a Symmetric Periodic Potential by BreakingTemporal Symmetry

Unidirectional transport of a particle in a spatially periodic and symmetricpotential under a periodic force with broken temporal symmetry is studied.With a collaboration of the potential field and the asymmetric ac force, adc current can be observed. Resonant current steps are found for a finiteperiod of the ac force. A phase diagram of these resonant steps is given.Stochastic-resonance-like directional transport induced by thermal noises isrevealed.