Disordering Transitions and Peak Effect in Polydisperse Particle Systems

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.Comment: 8 pages, 8 postscript figures. Version to appear in PR

Fluctuations, Jamming, and Yielding for a Driven Probe Particle in Disordered Disk Assemblies

Using numerical simulations we examine the velocity fluctuations of a probe particle driven with constant force through a two-dimensional disordered assembly of disks which has a well-defined jamming point J at a density of \phi_J=0.843. As \phi increases toward \phi_J, the average velocity of the probe particle decreases and the velocity fluctuations show an increasingly intermittent or avalanchelike behavior. When the system is within a few percent of the jamming density, the velocity distributions are exponential, while when the system is less than a percent away from jamming, the velocity distributions have a non-exponential or power law character. The velocity power spectra exhibit a crossover from a Lorentzian form to a 1/f shape near jamming. We extract a correlation exponent \nu which is in good agreement with recent shear simulations. For \phi > \phi_J, there is a critical threshold force F_c that must be applied for the probe particle to move through the sample which increases with increasing \phi. The onset of the probe motion above \phi_J occurs via a local yielding of the particles around the probe particle which we term a local shear banding effect.Comment: 11 pages, 20 postscript figure

Simulations of Noise in Disordered Systems

We use particle dynamics simulations to probe the correlations between noise and dynamics in a variety of disordered systems, including superconducting vortices, 2D electron liquid crystals, colloids, domain walls, and granular media. The noise measurements offer an experimentally accessible link to the microscopic dynamics, such as plastic versus elastic flow during transport, and can provide a signature of dynamical reordering transitions in the system. We consider broad and narrow band noise in transport systems, as well as the fluctuations of dislocation density in a system near the melting transition.Comment: 12 pages, 9 postscript figures, requires spie.cls. SPIE Conference on Fluctuations and Noise 2003, invited contributio

Commensurability Effects at Nonmatching Fields for Vortices in Diluted Periodic Pinning Arrays

Using numerical simulations, we demonstrate that superconductors containing periodic pinning arrays which have been diluted through random removal of a fraction of the pins have pronounced commensurability effects at the same magnetic field strength as undiluted pinning arrays. The commensuration can occur at fields significantly higher than the matching field, produces much greater critical current enhancement than a random pinning arrangement due to suppresion of vortex channeling, and persists for arrays with up to 90% dilution. These results suggest that diluted periodic pinning arrays may be a promising geometry to increase the critical current in superconductors over a wide magnetic field range.Comment: 6 pages, 5 postscript figures. Version to appear in Phys. Rev.