  We show numerically that in a binary system of Yukawa particles, a dispersity
driven disordering transition occurs. In the presence of quenched disorder this
disordering transition coincides with a marked increase in the depinning
threshold, known as a peak effect. We find that the addition of poorly pinned
particles can increase the overall pinning in the sample by increasing the
amount of topological disorder present. If the quenched disorder is strong
enough to create a significant amount of topological disorder in the
monodisperse system, addition of a poorly pinned species generates further
disorder but does not produce a peak in the depinning force. Our results
indicate that for binary mixtures, optimal pinning occurs for topological
defect fraction densities of 0.2 to 0.25. For defect densities below this
range, the system retains orientational order. We determine the effect of the
pinning density, strength, and radius on the depinning peak and find that the
peak effect is more pronounced in weakly pinning systems.
