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

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

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

Vortex Guidance and Transport in Channeled Pinning Arrays

We numerically examine vortices interacting with pinning arrays where a portion of the pinning sites have been removed in order to create coexisting regions of strong and weak pinning. The region without pinning sites acts as an easy-flow channel. For driving in different directions with respect to the channel, we observe distinct types of vortex flow. When the drive is parallel to the channel, the flow first occurs in the pin free region followed by a secondary depinning transition in the pinned region. At high vortex densities there is also an intermediate plastic flow phase due to the coupling between the weak and strong pinning regions. For driving applied perpendicular to the channel, we observe a jammed phase in which vortices accumulate on the boundary of the pinned region due to the vortex-vortex repulsion, while at higher drives the vortices begin to flow through the pinning array. For driving at an angle to the channel, depending on the filling we observe a drive-induced reentrant pinning effect as well as negative differential mobility which occurs when vortices move from the unpinned to the pinned portion of the sample.Comment: 8 pages, 12 postscript figure