Skyrmion Clustering and Dynamics in Strong Pinning or with Strong Magnus Force Dispersion

We show that skyrmions can exhibit density phase separation when driven over strong pinning. This occurs due to the Magnus force interaction, which creates regions of differing effective skyrmion Hall angle, combined with a density dependence of the skyrmion velocity, which is similar to the density dependent velocity observed at clustering transitions in active matter systems. We also investigate the effect of introducing a dispersion in the Magnus force experienced by individual skyrmions, which could arise if different skyrmion species or sizes coexist in a single sample. In this case, we find that the motion of the different species is locked at low drives, but as the drive increases, a disordering transition occurs when the species begin to slide past each other. At higher drives there is a transition to a species phase separated state in which skyrmions with the same Magnus force cluster together. These transitions are associated with signatures in the skyrmion structure, transport response, and velocity fluctuation spectra.