Peak Effect, Melting, and Transport in Skyrmion Crystals

We numerically examine the transport of skyrmions driven over weak random quenched disorder using a modified Thiele approach that includes the thermal softening of skyrmion pairwise interactions introduced by Wang et al., Phys. Rev. Appl. 18, 044024 (2022). The depinning transition is elastic at low temperatures but becomes plastic with a reduced threshold at higher temperatures due to the competition between thermal creep and thermal softening, indicating a temperature-induced order to disorder transition into a glass state. The resulting non-monotonic critical depinning forces and crossing of the velocity-force curves are similar to what is observed in the peak effect for type-II superconducting vortex lattices, where the softening of vortex-vortex interactions with temperature leads to an order-disorder transition. For low skyrmion densities the peak effect is absent since the system is always in a disordered state. We map the dynamical phase transition as a function of temperature, density, drive, and materials parameters, and show that the signatures are similar to those of the superconducting peak effect. Our results are consistent with recent experiments.