Mode-locking in driven vortex lattices with transverse ac-drive and random pinning

We find mode-locking steps in simulated current-voltage characteristics of driven vortex lattices with {\it random} pinning when an applied ac-current is {\it perpendicular} to the dc-current. For low frequencies there is mode-locking only above a non-zero threshold ac force amplitude, while for large frequencies there is mode-locking for any small ac force. This is consistent with the nature of {\it transverse} temporal order in the different regimes in the absence of an applied ac-drive. For large frequencies the magnitude of the fundamental mode-locked step depends linearly with the ac force amplitude.