  Ratchet effect, observed in many systems starting from living organism to
artificially designed device, is a manifestation of motion in asymmetric
potential. Here we report results of a conductivity study of Polypyrrole
nanowires, which have been prepared by a simple method to generate a variation
of doping concentration along the length. This variation gives rise to an
asymmetric potential profile that hinders the symmetry of the hopping process
of charges and hence the value of measured resistance of these nanowires become
sensitive to the direction of current flow. The asymmetry in resistance was
found to increase with decreasing nanowire diameter and increasing temperature.
The observed phenomena could be explained with the assumption that the spatial
extension of localized state involved in hopping process reduces as the doping
concentration reduces along the length of the nanowires.
