Experimental ratchet effect in superconducting films with periodic   arrays of asymmetric potentials
  A vortex lattice ratchet effect has been investigated in Nb films grown on arrays of nanometric Ni triangles, which induce periodic asymmetric pinning potentials. The vortex lattice motion yields a net dc-voltage when an ac driving current is applied to the sample and the vortex lattice moves through the field of asymmetric potentials. This ratchet effect is studied taking into account the array geometry, the temperature, the number of vortices per unit cell of the array and the applied ac currents. 
J. E. Villegas, E. M. Gonzalez and M. P. Gonzalez, J. V. Anguita, J.   L. Vicent