  We present a simple model of an overdamped particle moving on a two
dimensional symmetric periodic substrate with a dc drive in the longitudinal
direction and additional ac drives in both the longitudinal and transverse
directions. For certain regimes we find that a finite longitudinal dc force
produces a net dc response only in the transverse direction, which we term
absolute transverse mobility. Additionally we find regimes exhibiting a ratchet
effect in the absence of an applied dc drive.
