Crossover from Intermittent to Continuum Dynamics for Locally Driven Colloids

We simulate a colloid with charge q_d driven through a disordered assembly of interacting colloids with charge q and show that, for q_d \approx q, the velocity-force relation is nonlinear and the velocity fluctuations of the driven particle are highly intermittent with a 1/f characteristic. When q_d >> q, the average velocity ddrops, the velocity force relation becomes linear, and the velocity fluctuations are Gaussian. We discuss the results in terms of a crossover from strongly intermittent heterogeneous dynamics to continuum dynamics. We also make several predictions for the transient response in the different regimes.Comment: 4 pages, 5 postscript figures. Version to appear in Physical Review Letter

Shear Banding and Spatiotemporal Oscillations in Vortex Matter in Nanostructured Superconductors

We propose a simple nanostructured pinning array geometry where a rich variety of complex vortex shear banding phenomena can be realized. A single row of pinning sites is removed from a square pinning array. Shear banding effects arise when vortex motion in the pin-free channel nucleates motion of vortices in the surrounding pinned regions, creating discrete steps in the vortex velocity profile away from the channel. Near the global depinning transition, the width of the band of moving vortices undergoes oscillations or fluctuations that can span the entire system. We use simulations to show that these effects should be observable in the transport properties of the system. Similar large oscillations and shear banding effects are known to occur for sheared complex fluids in which different dynamical phases coexist.Comment: 4 pages, 4 postscript figure

Noise at the Wigner Glass Transition

Using a simple model for interacting electrons in two dimensions with random disorder, we show that a crossover from a Wigner liquid to a Wigner glass occurs as a function of charge density. The noise power increases strongly at the transition and the characteristics of the 1/f^alpha noise change. When the temperature is increased, the noise power decreases. We compare these results with recent noise measurements in systems with two-dimensional metal-insulator transitions.Comment: 4 pages, 5 postscript figure

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