Vortex ratchet
  We present a new class of thermal ratchets operating under the action of a symmetry breaking non-Hermitian perturbation which rectifies thermal fluctuations, and driven by a unbiased periodic force. The peculiar non-Hermitian dynamics which follows causes energy transduction from the force to the system in such a way that an average `uphill' particle current is induced. We discuss physical realizations in assemblies of orientable particles, in itinerant oscillator models, and in problems of diffusion in disordered media. 
A. Perez-Madrid, T. Alarcon, J.M. Rubi