  Using a simple numerical model of skyrmions in a two-dimensional system
interacting with a quasi-one dimensional periodic substrate under combined dc
and ac drives where the dc drive is applied perpendicular to the substrate
periodicity, we show that a rich variety of novel phase locking dynamics can
occur due to the influence of the Magnus term on the skyrmion dynamics. Instead
of Shapiro steps, the velocity response in the direction of the dc drive
exhibits a series of spikes, including extended dc drive intervals over which
the skyrmions move in the direction opposite to the dc drive, producing
negative mobility. There are also specific dc drive values at which the
skyrmions move exactly perpendicular to the dc drive direction, giving a
condition of absolute transverse mobility.
