Ratchet Effect in Surface Electromigration: Smoothing Surfaces by an ac   Field
  We demonstrate that for surfaces that have a nonzero Schwoebel barrier the application of an ac field parallel to the surface induces a net electro- migration current that points in the descending step direction. The magnitude of the current is calculated analytically and compared with Monte Carlo simulations. Since a downhill current smoothes the surface, our results imply that the application of ac fields can aid the smoothing process during annealing and can slow or eliminate the Schwoebel-barrier-induced mound formation during growth. 
Imre Derenyi, Choongseop Lee, Albert-Laszlo Barabasi