arXiv subjects

C. J. Olson Reichhardt

Publications and source records attributed to C. J. Olson Reichhardt.

At least 73 records · Page 4Linked to original sources

arXiv2011

Dynamically Induced Locking and Unlocking Transitions in Driven Layered Systems with Quenched Disorder

Using numerical simulations, we examine a simple model of two or more coupled one-dimensional channels of driven particles with repulsive interactions in the presence of quenched disorder. We find that this model exhibits a remarkably rich variety of dynamical behavior as a function of the strength of the quenched disorder, coupling between channels, and external drive. For weaker disorder, the channels depin in a single step. For two channels we find dynamically induced decoupling transitions that result in coexisting pinned and moving phases as well as moving decoupled phases where particles in both channels move at different average velocities and slide past one another. Decoupling can also be induced by changing the relative strength of the disorder in neighboring channels. At higher drives, we observe a dynamical recoupling or locking transition into a state with no relative motion between the channels.This recoupling produces unusual velocity-force signatures, including negative differential conductivity. The depinning threshold shows distinct changes near the decoupling and coupling transitions and exhibits a peak effect phenomenon of the type that has been associated with transitions from elastic to plastic flow in other systems. We map several dynamic phase diagrams showing the coupling-decoupling transitions and the regions in which hysteresis occurs. We also examine the coexistence regime for channels with unequal amounts of quenched disorder. For multiple channels, multiple coupling and decoupling transitions can occur; however, many of the general features found for the two channel system are still present. Our results should be relevant to depinning in layered geometries in systems such as vortices in layered or nanostructured superconductors and Wigner or colloidal particles confined in nano-channels; they are also relevant to the general understating of plastic flow.

cond-mat.stat-mech↗

arXiv2011

Positive and Negative Drag, Dynamic Phases, and Commensurability in Coupled One-Dimensional Channels of Particles with Yukawa Interactions

We introduce a simple model consisting of two or three coupled one-dimensional channels of particles with Yukawa interactions. For the two channel system, when an external drive is applied only to the top or primary channel, we find a transition from locked flow where particles in both channels move together to decoupled flow where the particles in the secondary or undriven channel move at a slower velocity than the particles in the primary or driven channel. Pronounced commensurability effects in the decoupling transition occur when the ratio of the number of particles in the top and bottom channels is varied, and the coupling of the two channels is enhanced when this ratio is an integer or a rational fraction. Near the commensurate fillings, we find additional features in the velocity-force curves caused by the slipping of individual vacancies or incommensurations in the secondary channels. For three coupled channels, when only the top channel is driven we find a remarkably rich variety of distinct dynamic phases, including multiple decoupling and recoupling transitions. These transitions produce pronounced signatures in the velocity response of each channel. We also find regimes where a negative drag effect can be induced in one of the non-driven channels. The particles in this channel move in the opposite direction from the particles in the driven channel due to the mixing of the two different periodic frequencies produced by the discrete motion of the particles in the two other channels. In the two channel system, we also demonstrate a ratchet effect for the particles in the secondary channel when an asymmetric drive is applied to the primary channel. This ratchet effect is similar to that observed in superconducting vortex systems when there is a coupling between two different species of vortices.

cond-mat.soft↗

arXiv2011

Jamming in Granular Polymers

We examine the jamming transition in a two-dimensional granular polymer system using compressional simulations. The jamming density ϕ_c decreases with increasing length of the granular chain due to the formation of loop structures, in excellent agreement with recent experiments. The jamming density can be further reduced in mixtures of granular chains and granular rings, also as observed in experiment. We show that the nature of the jamming in granular polymer systems has pronounced differences from the jamming behavior observed for polydisperse two-dimensional disk systems at Point J. This result provides further evidence that there is more than one type of jamming transition.

cond-mat.soft↗

arXiv2011

Structural Transitions and Dynamical Regimes for Directional Locking of Particles Driven over Periodic Substrates

We numerically investigate collective ordering and disordering effects for vortices in type-II superconductors interacting with square and triangular substrate arrays under a dc drive that is slowly rotated with respect to the fixed substrate. A series of directional locking transitions occur when the particle motion locks to symmetry directions of the substrate, producing a series of steps in the velocity-force curves. The locking transitions coincide with structural transitions between triangular, square, smectic, or disordered particle arrangements, which can be identified using the structure factor. We show that the widths of the locking steps pass through local minima and maxima as a function of the ratio of the number of particles to the number of substrate minima. Unlike a static system, where matching effects occur for simple integer commensuration ratios, our system exhibits dynamical commensuration effects where an integer number of particle chains flow between one-dimensional lines of substrate minima. As the system enters and exits the locking steps, order-disorder transitions in the structure of the moving particle assembly occur. We identify two distinct symmetry locking regimes as a function of substrate strength which produce different locking step characteristics. For weak substrates, all the particles are in motion and a portion of the particles flow through the substrate minima, leading to structural transitions at certain driving angles. For strong substrates, some particles are permanently pinned while the remaining particles flow around them. At the crossover between these two regimes of substrate strength, the locking steps are destroyed due to the onset of chaotic plastic flow which produces pronounced changes in the transport characteristics. Similar effects occur for colloidal particles driven over square and triangular substrate arrays.

cond-mat.supr-con↗

arXiv2010

The Effect of Pinning on Drag in Coupled One-Dimensional Channels of Particles

We consider a simple model for examining the effects of quenched disorder on drag consisting of particles interacting via a Yukawa potential that are placed in two coupled one-dimensional channels. The particles in one channel are driven and experience a drag from the undriven particles in the second channel. In the absence of pinning, for a finite driving force there is no pinned phase; instead, there are two dynamical regimes of completely coupled or locked flow and partially coupled flow. When pinning is added to one or both channels, we find that a remarkably rich variety of dynamical phases and drag effects arise that can be clearly identified by features in the velocity force curves. The presence of quenched disorder in only the undriven channel can induce a pinned phase in both channels. Above the depinning transition, the drag on the driven particles decreases with increasing pinning strength, and for high enough pinning strength, the particles in the undriven channel reach a reentrant pinned phase which produces a complete decoupling of the channels. We map out the dynamic phase diagrams as a function of pinning strength and the density of pinning in each channel. Our results may be relevant for understanding drag coupling in 1D Wigner crystal phases, and the effects we observe could also be explored using colloids in coupled channels produced with optical arrays, vortices in nanostructured superconductors, or other layered systems where drag effects arise.

cond-mat.soft↗

arXiv2010

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.

cond-mat.soft↗

arXiv2010

Anisotropic Sliding Dynamics, Peak Effect, and Metastability in Stripe Systems

A variety of soft and hard condensed matter systems are known to form stripe patterns. Here we use numerical simulations to analyze how such stripe states depin and slide when interacting with a random substrate and with driving in different directions with respect to the orientation of the stripes. Depending on the strength and density of the substrate disorder, we find that there can be pronounced anisotropy in the transport produced by different dynamical flow phases. We also find a disorder-induced "peak effect" similar to that observed for superconducting vortex systems, which is marked by a transition from elastic depinning to a state where the stripe structure fragments or partially disorders at depinning. Under the sudden application of a driving force, we observe pronounced metastability effects similar to those found near the order-disorder transition associated with the peak effect regime for three-dimensional superconducting vortices. The characteristic transient time required for the system to reach a steady state diverges in the region where the flow changes from elastic to disordered. We also find that anisotropy of the flow persists in the presence of thermal disorder when thermally-induced particle hopping along the stripes dominates. The thermal effects can wash out the effects of the quenched disorder, leading to a thermally-induced stripe state. We map out the dynamical phase diagram for this system, and discuss how our results could be explored in electron liquid crystal systems, type-1.5 superconductors, and pattern-forming colloidal assemblies.

cond-mat.soft↗

arXiv2010

Commensurability, Jamming, and Dynamics for Vortices in Funnel Geometries

We use numerical simulations to examine vortex states and dynamics in periodic funnel geometries where a drive is applied in the easy flow direction. We show that this system exhibits a number of different commensurability effects when the vortex configurations match to both the periodicity of the array and the geometry of the funnels. The vortex configurations in this system are generally different from those observed for single isolated triangular superconducting samples due to the coupling of vortices in adjacent funnels. At certain matching fields, peaks in the critical current are absent due to the particular vortex configurations that occur at these fields. We find that the overall depinning force increases with increasing vortex density as a result of the enhanced vortex-vortex interactions caused by a crowding effect at the funnel tips. When a system becomes less mobile as a result of increased particle interactions, it is said to exhibit a jamming behavior. Under an applied drive we observe a series of elastic and plastic vortex flow phases which produce pronounced features such as jumps or dips in the transport curves. In all of the flow phases, only one vortex can pass through the funnel tip at a time due to the vortex-vortex repulsion forces. As a consequence of this constraint, we observe the remarkable result that the sum of the vortex velocities at a fixed drive remains nearly constant with increasing magnetic field B rather than increasing linearly. This result is similar to the behavior of sand in an hourglass. We also show how noise fluctuations can be used to distinguish the different flow phases. Our results should be readily generalizable to other systems of particles flowing in periodic funnel geometries, such as colloids or Wigner crystals.

cond-mat.supr-con↗

arXiv2010

Structural Transitions, Melting, and Intermediate Phases for Stripe and Clump Forming Systems

We numerically examine the properties of a two-dimensional system of particles which have competing long range repulsive and short range attractive interactions as a function of density and temperature. For increasing density, there are well defined transitions between a low density clump phase, an intermediate stripe phase, an anticlump phase, and a high density uniform phase. To characterize the transitions between these phases we propose several measures which take into account the different length scales in the system. For increasing temperature, we find an intermediate phase that is liquid-like on the short length scale of interparticle spacing but solid-like on the larger length scale of the clump, stripe, or anticlump pattern. This intermediate phase persists over the widest temperature range in the stripe state when the local particle lattice within an individual stripe melts well below the temperature at which the entire stripe structure breaks down, and is characterized by intra-stripe diffusion of particles without inter-stripe diffusion. This is followed at higher temperatures by the onset of inter-stripe diffusion in an anisotropic diffusion phase, and then by breakup of the stripe structure. We identify the transitions between these regimes through diffusion, specific heat, and energy fluctuation measurements, and find that within the intra-stripe liquid regime, the excess entropy goes into disordering the particle arrangements within the stripe rather than affecting the stripe structure itself. The clump and anticlump phases also show multiple temperature-induced diffusive regimes which are not as pronounced as those of the stripe phase.

cond-mat.soft↗

arXiv2009

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.

cond-mat.supr-con↗

arXiv2009

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.

cond-mat.stat-mech↗

arXiv2009

Fluctuation Theorem in Driven Nonthermal Systems with Quenched Disorder

We demonstrate that the fluctuation theorem of Gallavotti and Cohen can be used to characterize the class of dynamics that arises in nonthermal systems of collectively interacting particles driven over random quenched disorder. By observing the frequency of entropy-destroying trajectories, we show that there are specific dynamical regimes near depinning in which this theorem holds. Hence the fluctuation theorem can be used to characterize a significantly wider class of nonequilibrium systems than previously considered. We discuss how the fluctuation theorem could be tested in specific systems where noisy dynamics appear at the transition from a pinned to a moving phase such as in vortices in type-II superconductors, magnetic domain walls, and dislocation dynamics.

cond-mat.stat-mech↗

arXiv2009

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 ϕ_J=0.843. As ϕincreases toward ϕ_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 νwhich is in good agreement with recent shear simulations. For ϕ> ϕ_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 ϕ. The onset of the probe motion above ϕ_J occurs via a local yielding of the particles around the probe particle which we term a local shear banding effect.

cond-mat.soft↗

arXiv2009

Coherent and Incoherent Vortex Flow States in Crossed Channels

We examine vortex flow states in periodic square pinning arrays with one row and one column of pinning sites removed to create an easy flow crossed channel geometry. When a drive is simultaneously applied along both major symmetry axes of the pinning array such that vortices move in both channels, a series of coherent flow states develop in the channel intersection at rational ratios of the drive components in each symmetry direction when the vortices can cross the intersection without local collisions. The coherent flow states are correlated with a series of anomalies in the velocity force curves, and in some cases can produce negative differential conductivity. The same general behavior could also be realized in other systems including colloids, particle traffic in microfluidic devices, or Wigner crystals in crossed one-dimensional channels.

cond-mat.supr-con↗

arXiv2009

Switching and Jamming Transistor Effect for Vortex Matter in Honeycomb Pinning Arrays with ac Drives

We show that a remarkable variety of dynamical phenomena, including switching, polarization, symmetry locking, and dynamically induced pinning, can occur for vortices in type-II superconductors in the presence of a honeycomb pinning array and an ac or combined ac and dc drive. These effects occur at the second matching field where there are two vortices per pinning site, and arise due to the formation of vortex dimer states in the interstitial regions of the honeycomb array. The orientation of the pinned and moving vortex dimers can be controlled externally by the application of a drive. We term this a polarization effect and demonstrate that it can lock or unlock the vortex motion into different symmetry directions of the underlying pinning lattice. If the moving vortices are locked into one direction, the motion can be switched into a different direction by applying an additional bias drive, producing sharp jumps in the transverse and longitudinal velocities. Further, the dc vortex motion in one direction can be controlled directly by application of a force in the perpendicular direction. When the moving dimers reorient, we find a remarkable dynamical pinning effect in which the dimers jam when they become perpendicular to the easy flow direction of the pinning lattice. Since application of an external field can be used to switch off the vortex flow, we term this a jamming transistor effect. These effects do not occur in triangular pinning arrays due to the lack of the n-merization of the vortices in this case. The switching and dynamical pinning effects demonstrated here may be useful for the creation of new types of fluxtronic devices.

cond-mat.supr-con↗

arXiv2009

Creating Artificial Ice States Using Vortices in Nanostructured Superconductors

We demonstrate that it is possible to realize vortex ice states that are analogous to square and kagome ice. With numerical simulations, we show that the system can be brought into a state that obeys either global or local ice rules by applying an external current according to an annealing protocol. We explore the breakdown of the ice rules due to disorder in the nanostructure array and show that in square ice, topological defects appear along grain boundaries, while in kagome ice, individual defects appear. We argue that the vortex system offers significant advantages over other artificial ice systems.

cond-mat.supr-con↗

arXiv2009

Pattern Switching and Polarizability for Colloids in Optical Trap Arrays

We show that colloidal molecular crystal states interacting with a periodic substrate, such as an optical trap array, and a rotating external field can undergo a rapid pattern switching in which the orientation of the crystal changes. In some cases, a martensitic-like symmetry switching occurs. It is also possible to create a polarized state where the colloids in each substrate minima develop a director field which smoothly rotates with the external drive, similar to liquid crystal behavior. These results open the possibility for creating novel types of devices using photonic band gap materials, and should be generalizable to a variety of other condensed matter systems with multiple particle trapping.

cond-mat.soft↗

arXiv2008

Collective Sliding States for Colloidal Molecular Crystals

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.

cond-mat.soft↗