  We propose and study a novel way to produce a DC transport of vortices when
applying an AC electrical current to a sample. Specifically, we study
superconductors with a graduated random pinning density, which transports
interacting vortices as a ratchet system. We show that a ratchet effect appears
as a consequence of the long range interactions between the vortices. The
pinned vortices create an asymmetric periodic flux density profile, which
results in an asymmetric effective potential for the unpinned interstitial
vortices. The latter exhibit a net longitudinal rectification under an applied
transverse AC electric current.
