Dynamical ordering and directional locking for particles moving over quasicrystalline substrates

We use molecular dynamics simulations to study the driven phases of particles such as vortices or colloids moving over a decagonal quasiperiodic substrate. In the regime where the pinned states have quasicrystalline ordering, the driven phases can order into moving square or smectic states, or into states with aligned rows of both square and triangular tiling which we term dynamically induced Archimedean-like tiling. We show that when the angle of the drive is varied with respect to the substrate, directional locking effects occur where the particle motion locks to certain angles. It is at these locking angles that the dynamically induced Archimedean tiling appears. We also demonstrate that the different dynamical orderings and locking phases show pronounced changes as a function of filling fraction.     

Rectification of swimming bacteria and self-driven particle systems by arrays of asymmetric barriers

We show that the recent experimental observation of the rectification of swimming bacteria in a system with an array of asymmetric barriers occurs due to the ballistic component of the bacteria trajectories introduced by the bacterial "motor." Each bacterium selects a random direction for motion and then moves in this direction for a fixed period of time before randomly changing its orientation and moving in a new direction. In the limit where the bacteria undergo only Brownian motion on the size scale of the barriers, rectification does not occur. We examine the effects of steric interactions between the bacteria and observe a clogging effect upon increasing the bacteria density.     

Spontaneous transverse response and amplified switching in superconductors with honeycomb pinning arrays

Using numerical simulations, we show that a novel spontaneous transverse response can appear when a longitudinal drive is applied to type-II superconductors with honeycomb pinning arrays in a magnetic field near certain filling fractions. This response is generated by dynamical symmetry breaking that occurs at fields away from commensurability. We find a coherent strongly amplified transverse switching effect when an additional transverse ac drive is applied. The transverse ac drive can also be used to control switching in the longitudinal velocity response. We discuss how these effects could be used to create new types of devices such as current effect transistors.     

Jamming and diode effects for vortices in nanostructured superconductors

We examine jamming and ratchet effects for vortex matter in superconductors with asymmetric funnel geometries. We show that the vortex–vortex interactions can induce a clogging or jamming effect where it becomes increasingly difficult for the vortices to move through the system. We also find that commensurability effects can arise when certain vortex configurations form highly symmetrical structures in the funnel plaquettes. Due to the asymmetry, the critical currents are different for driving in different directions, leading to a diode effect. We also discuss other possible geometries and approaches that could be used to explore jamming in vortex matter, such as an analog to a granular hopper and a single driven vortex probe moving through an array of other vortices.     

Driving an individual vortex in the presence of a periodic pinning array

Recently it has been demonstrated experimentally that it is possible to manipulate an individual vortex in a type-II superconductor using a magnetic force microscope tip. Using numerical simulations, we investigate the dynamics of a single driven vortex in the presence of a periodic pinning array and other vortices. Remarkably, we find that the effective drag on the driven vortex is reduced at the matching fields, which is opposite from the behavior of the critical current when all the vortices are driven. We discuss this effect in the context of the type of dynamics that occur at matching and nonmatching fields.     

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) approximately q, the velocity-force relation is nonlinear and the velocity fluctuations of the driven particle are highly intermittent with a 1/f characteristic. When g(d) >q , the average velocity drops, 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.     

Depinning by fracture in a glassy background

We force a single particle through a two-dimensional simulated glass of smaller particles. We find that the particle velocity obeys a robust power law that persists to drive wells above threshold. As the single driven particle moves, it induces cooperative distortions in the surrounding medium. We show theoretically that a fracture model for these distortions produces power-law behavior and discuss implications for experimental probes of soft matter systems.     

Ratchet cellular automata

In this work we propose a ratchet effect which provides a general means of performing clocked logic operations on discrete particles, such as single electrons or vortices. The states are propagated through the device by the use of an applied ac drive. We numerically demonstrate that a complete logic architecture is realizable using this ratchet. We consider specific nanostructured superconducting geometries using superconducting materials under an applied magnetic field, with the positions of the individual vortices in samples acting as the logic states. These devices can be used as the building blocks for an alternative microelectronic architecture.