  For a vortex lattice moving in a periodic array we show analytically and
numerically that a new type of phase locking occurs in the presence of a
longitudinal dc driving force and a transverse ac driving force. This phase
locking is distinct from the Shapiro step phase locking found with longitudinal
ac drives. We show that an increase in critical current and a fundamental phase
locked step width scale with the square of the driving ac amplitude. Our
results should carry over to other systems such as vortex motion in
Josephson-junction arrays.
