Novel Colloidal Crystalline States on Two Dimensional Periodic Substrates

We show using numerical simulations that a rich variety of novel colloidal crystalline states are realized on square and triangular two dimensional periodic substrates which can be experimentally created using crossed laser arrays. When there are more colloids than potential substrate minima, multiple colloids are trapped at each substrate minima and act as a single particle with a rotational degree of freedom, giving rise to a new type of orientational order. We call these states colloidal molecular crystals. A two-step melting can also occur in which individual colloidal molecules initially rotate, destroying the overall orientational order, followed by the onset of inter-well colloidal hopping.Comment: 4 pages, 4 postscript figure

Random Organization and Plastic Depinning

We provide evidence that plastic depinning falls into the same class of phenomena as the random organization which was recently studied in periodically driven particle systems [L. Corte et al., Nature Phys. 4, 420 (2008)]. In the plastic flow system, the pinned regime corresponds to the quiescent state and the moving state corresponds to the fluctuating state. When an external force is suddenly applied, the system eventually organizes into one of these two states with a time scale that diverges as a power law at a nonequilibrium transition. We propose a simple experiment to test for this transition in colloidal systems and superconducting vortex systems with random disorder.Comment: 4 pages, 4 postscript figures; version to appear in Phys. Rev. Let

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.

Active Matter Ratchets with an External Drift

When active matter particles such as swimming bacteria are placed in an asymmetric array of funnels, it has been shown that a ratchet effect can occur even in the absence of an external drive. Here we examine active ratchets for two dimensional arrays of funnels or L-shapes where there is also an externally applied dc drive or drift. We show that for certain conditions, the ratchet effect can be strongly enhanced, and that it is possible to have conditions under which run-and-tumble particles with one run length move in the opposite direction from particles with a different run length. For the arrays of L-shapes, we find that the application of a drift force can enhance a transverse rectification in the direction perpendicular to the drift. When particle-particle steric interactions are included, we find that the ratchet effects can be either enhanced or suppressed depending on barrier geometry, particle run length, and particle density.Comment: 9 pages, 12 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