Cynthia J. Olson Reichhardt

Casimir effect in active matter systems.

We numerically examine run-and-tumble active matter particles in Casimir geometries composed of two finite parallel walls. We find that there is an attractive force between the two walls of a magnitude that increases with increasing run length. The attraction exhibits an unusual exponential dependence on the wall separation, and it arises due to a depletion of swimmers in the region between the walls by a combination of the motion of the particles along the walls and a geometric shadowing effect. This attraction is robust as long as the wall length is comparable to or smaller than the swimmer run length, and is only slightly reduced by the inclusion of steric interactions between swimmers. We also examine other geometries and find regimes in which there is a crossover from attraction to repulsion between the walls as a function of wall separation and wall length.

Random organization and plastic depinning.

We provide evidence that the general phenomenon of plastic depinning can be described as an absorbing phase transition, and shows the same features as the random organization which was recently studied in periodically driven particle systems [L. Corte, Nature Phys. 4, 420 (2008)]. In the plastic flow system, the pinned regime corresponds to the absorbing 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 systems with random disorder.

Local melting and drag for a particle driven through a colloidal crystal.

We numerically investigate a colloidal particle driven through a colloidal crystal as a function of temperature. When the charge of the driven particle is larger or comparable to that of the colloids comprising the crystal, a local melting can occur, characterized by defect generation in the lattice surrounding the driven particle. The generation of the defects is accompanied by an increase in the drag force on the driven particle, as well as large noise fluctuations. We discuss the similarities of these results to the peak effect phenomena observed for vortices in superconductors.

Hysteresis and Return Point Memory in Artificial Spin Ice Systems

Using computer simulations, we investigate hysteresis loops and return-point memory for artificial square and kagome spin ice systems by cycling an applied bias force and comparing microscopic effective spin configurations throughout the hysteresis cycle. Return-point memory loss is caused by motion of individual defects in kagome ice or of grain boundaries in square ice. In successive cycles, return-point memory is recovered rapidly in kagome ice. Memory is recovered more gradually in square ice due to the extended nature of the grain boundaries. Increasing the amount of quenched disorder increases the defect density but also enhances the return-point memory since the defects become trapped more easily.

Nonequilibrium phases for driven particle systems with effective orientational degrees of freedom.

We study the driving of colloidal molecular crystals over periodic substrates such as those created with optical traps. The n-merization that occurs in the colloidal molecular crystal states produces a remarkably rich variety of distinct dynamical behaviors, including polarization effects within the pinned phase and the formation of both ordered and disordered sliding phases. Using computer simulations, we map the dynamic phase diagrams as a function of substrate strength for dimers and trimers on a triangular substrate, and correlate features on the phase diagram with transport signatures.

Disordering transitions in vortex matter: peak effect and phase diagram

Using numerical simulations of magnetically interacting vortices in disordered layered superconductors we obtain the static vortex phase diagram as a function of magnetic field and temperature. For increasing field or temperature, we find a transition from ordered straight vortices to disordered decoupled vortices. This transition is associated with a peak effect in the critical current. For samples with increasing disorder strength the field at which the decoupling occurs decreases. Long range, nonlinear interactions in the c-axis are required to observe the effect.

Dynamical freezing of active matter

When collections of interacting particles are externally driven, they typically form strongly fluctuating states with short-lived structures or patterns that are continually created and destroyed, such as those found in turbulent fluids (1) or fluctuating granular matter (2). In some cases, however, such driven systems do not remain in a strongly fluctuating state but instead organize into a peculiar dynamical state in which the fluctuations are strongly reduced or even completely lost. Such states typically involve the formation of some type of pattern, and they are considered to be dynamically “frozen” because the motion that does occur appears in a repeatable fashion or is confined only to specific localized regions. When the system has reached one of these states, the lack of significant fluctuations means that it becomes dynamically trapped or absorbed; hence, entry into such a state is termed an absorbing transition (3, 4). A broader question is whether this type of nonequilibrium phase transition is limited to a select few types of system or whether it is much more common and arises under generic nonequilibrium conditions such as those relevant to biology. In PNAS (5), Schaller et al. show that active matter systems with simple ingredients can indeed undergo a transition into an absorbing state.

Metastability and transient effects in vortex matter near a decoupling transition

We examine metastable and transient effects both above and below the first-order decoupling line in a three-dimensional (3D) simulation of magnetically interacting pancake vortices. We observe pronounced transient and history effects as well as supercooling and superheating between the 3D coupled, ordered and 2D decoupled, disordered phases. In the disordered supercooled state as a function of dc driving, reordering occurs through the formation of growing moving channels of the ordered phase. No channels form in the superheated region; instead the ordered state is homogeneously destroyed. When a sequence of current pulses is applied we observe memory effects. We find a ramp rate dependence of the

Noise at the crossover from Wigner liquid to Wigner glass.

V(I)

Fluctuations and noise signatures of driven magnetic skyrmions

curves on both sides of the decoupling transition. The critical current that we obtain depends on how the system is prepared.