  We examine the motion of an individual skyrmion driven through an assembly of
other skyrmions by a constant or increasing force in the absence of quenched
disorder. The skyrmion behavior is determined by the ratio of the damping and
Magnus terms, as expressed in terms of the intrinsic skyrmion Hall angle. For a
fixed driving force in the damping dominated regime, the effective viscosity
decreases monotonically with increasing skyrmion density, similar to what is
observed in overdamped systems where it becomes difficult for the driven
particle to traverse the surrounding medium at high densities. In contrast, in
the Magnus dominated regime the velocity dependence on the density is
nonmonotonic, and there is a regime in which the skyrmion moves faster with
increasing density, as well as a pronounced speed-up effect in which a skyrmion
traveling through a dense medium moves more rapidly than it would at low
densities or in the single particle limit. At higher densities, the effective
damping increases and the velocity decreases. The velocity-force curves in the
Magnus-dominated regime show marked differences from those in the
damping-dominated regimes. Under an increasing drive we find that there is a
threshold force for skyrmion motion which increases with density. Additionally,
the skyrmion Hall angle is drive dependent, starting near zero at the threshold
for motion and increasing with increasing drive before reaching a saturation
value, similar to the behavior found for skyrmions driven over quenched
disorder. We map dynamic phase diagrams showing the threshold for motion,
nonlinear flow, speed-up, and saturation regimes. We also find that in some
cases, increasing the density can reduce the skyrmion Hall angle while
producing a velocity boost, which could be valuable for applications.
