  We examine skyrmions under a dc drive interacting with a square array of
obstacles for varied obstacle size and damping. When the drive is applied in a
fixed direction, we find that the skyrmions are initially guided in the drive
direction but also move transverse to the drive due to the Magnus force. The
skyrmion Hall angle, which indicates the difference between the skyrmion
direction of motion and the drive direction, increases with drive in a series
of quantized steps as a result of the locking of the skyrmion motion to
specific symmetry directions of the obstacle array. On these steps, the
skyrmions collide with an integer number of obstacles to create a periodic
motion. The transitions between the different locking steps are associated with
jumps or dips in the velocity-force curves. In some regimes, the skyrmion Hall
angle is actually higher than the intrinsic skyrmion Hall angle that would
appear in the absence of obstacles. In the limit of zero damping, the skyrmion
Hall angle is 90$^\circ$, and we find that it decreases as the damping
increases. For multiple interacting skyrmion species in the collective regime,
we find jammed behavior at low drives where the different skyrmion species are
strongly coupled and move in the same direction. As the drive increases, the
species decouple and each can lock to a different symmetry direction of the
obstacle lattice, making it possible to perform topological sorting in analogy
to the particle sorting methods used to fractionate different species of
colloidal particles moving over two-dimensional obstacle arrays.
