37,635 research outputs found
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
Avalanche Dynamics for Active Matter in Heterogeneous Media
Using numerical simulations, we examine the dynamics of active matter
run-and-tumble disks moving in a disordered array of obstacles. As a function
of increasing active disk density and activity, we find a transition from a
completely clogged state to a continuous flowing phase, and in the large
activity limit, we observe an intermittent state where the motion occurs in
avalanches that are power law distributed in size with an exponent of . In contrast, in the thermal or low activity limit we find bursts of
motion that are not broadly distributed in size. We argue that in the highly
active regime, the system reaches a self-jamming state due to the
activity-induced self-clustering, and that the intermittent dynamics is similar
to that found in the yielding of amorphous solids. Our results show that
activity is another route by which particulate systems can be tuned to a
nonequilibrium critical state.Comment: 11 pages, 7 figure
Velocity Force Curves, Laning, and Jamming for Oppositely Driven Disk Systems
Using simulations we examine a two-dimensional disk system in which two disk
species are driven in opposite directions. We measure the average velocity of
one of the species versus the applied driving force and identify four phases as
function of drive and disk density: a jammed state, a completely phase
separated state, a continuously mixing phase, and a laning phase. The
transitions between these phases are correlated with jumps in the
velocity-force curves that are similar to the behavior observed at dynamical
phase transitions in driven particle systems with quenched disorder such as
vortices in type-II superconductors. In some cases the transitions between
phases are associated with negative differential mobility in which the average
absolute velocity of either species decreases with increasing drive. We also
consider the situation where the drive is applied to only one species as well
as systems in which both species are driven in the same direction with
different drive amplitudes. Finally, we discuss how the transitions we observe
could be related to absorbing phase transitions where a system in a phase
separated or laning regime organizes to a state in which contacts between the
disks no longer occur and dynamical fluctuations are lost.Comment: 8 pages, 10 png figure
Nonlinear Transport, Dynamic Ordering, and Clustering for Driven Skyrmions on Random Pinning
Using numerical simulations, we examine the nonlinear dynamics of skyrmions
driven over random pinning. For weak pinning, the skyrmions depin elastically,
retaining sixfold ordering; however, at the onset of motion there is a dip in
the magnitude of the structure factor peaks due to a decrease in positional
ordering, indicating that the depinning transition can be detected using the
structure factor even within the elastic depinning regime. At higher drives the
moving skyrmion lattice regains full ordering. For increasing pinning strength,
we find a transition from elastic to plastic depinning that is accompanied by a
sharp increase in the depinning threshold due to the proliferation of
topological defects, similar to the peak effect found at the elastic to plastic
depinning transition in superconducting vortex systems. For strong pinning and
strong Magnus force, the skyrmions in the moving phase can form a strongly
clustered or phase separated state with highly modulated skyrmion density,
similar to that recently observed in continuum-based simulations for strong
disorder. As the Magnus force is decreased, the density phase separated state
crosses over to a dynamically phase separated state with uniform density but
with flow localized in bands of motion, while in the strongly damped limit,
both types of phase separated states are lost. In the strong pinning limit, we
find highly nonlinear velocity-force curves in the transverse and longitudinal
directions, along with distinct regions of negative differential conductivity
in the plastic flow regime. The negative differential conductivity is absent in
the overdamped limit. The Magnus force is responsible for both the negative
differential conductivity and the clustering effects, since it causes faster
moving skyrmions to partially rotate around slower moving or pinned skyrmions.Comment: 17 pages, 23 figure
Clogging and Depinning of Ballistic Active Matter Systems in Disordered Media
We numerically examine ballistic active disks driven through a random
obstacle array. Formation of a pinned or clogged state occurs at much lower
obstacle densities for the active disks than for passive disks. As a function
of obstacle density we identify several distinct phases including a depinned
fluctuating cluster state, a pinned single cluster or jammed state, a pinned
multicluster state, a pinned gel state, and a pinned disordered state. At lower
active disk densities, a drifting uniform liquid forms in the absence of
obstacles, but when even a small number of obstacles are introduced, the disks
organize into a pinned phase-separated cluster state in which clusters nucleate
around the obstacles, similar to a wetting phenomenon. We examine how the
depinning threshold changes as a function of disk or obstacle density, and find
a crossover from a collectively pinned cluster state to a disordered plastic
depinning transition as a function of increasing obstacle density. We compare
this to the behavior of nonballistic active particles and show that as we vary
the activity from completely passive to completely ballistic, a clogged
phase-separated state appears in both the active and passive limits, while for
intermediate activity, a readily flowing liquid state appears and there is an
optimal activity level that maximizes the flux through the sample.Comment: 14 pages, 19 postscript figure
Disordering, Clustering, and Laning Transitions in Particle Systems with Dispersion in the Magnus Term
We numerically examine a two-dimensional system of repulsively interacting
particles with dynamics that are governed by both a damping term and a Magnus
term. The magnitude of the Magnus term has one value for half of the particles
and a different value for the other half of the particles. In the absence of a
driving force, the particles form a triangular lattice, while when a driving
force is applied, we find that there is a critical drive above which a
Magnus-induced disordering transition can occur even if the difference in the
Magnus term between the two particle species is as small as one percent. The
transition arises due to the different Hall angles of the two species, which
causes their motion to decouple at the critical drive. At higher drives, the
disordered state can undergo both species and density phase separation into a
density modulated stripe that is oriented perpendicular to the driving
direction. We observe several additional phases that occur as a function of
drive and Magnus force disparity, including a variety of density modulated
diagonal laned phases. In general we find a much richer variety of states
compared to systems of oppositely driven overdamped Yukawa particles. We
discuss the implications of our work for skyrmion systems, where we predict
that even for small skyrmion dispersities, a drive-induced disordering
transition can occur along with clustering phases and pattern forming states.Comment: 14 pages, 24 figure
Controlled Fluidization, Mobility and Clogging in Obstacle Arrays Using Periodic Perturbations
We show that the clogging susceptibility and flow of particles moving through
a random obstacle array can be controlled with a transverse or longitudinal ac
drive. The flow rate can vary over several orders of magnitude, and we find
both an optimal frequency and an optimal amplitude of driving that maximizes
the flow. For dense arrays, at low ac frequencies a heterogeneous creeping
clogged phase appears in which rearrangements between different clogged
configurations occur. At intermediate frequencies a high mobility fluidized
state forms, and at high frequencies the system reenters a heterogeneous frozen
clogged state. These results provide a technique for optimizing flow through
heterogeneous media that could also serve as the basis for a particle
separation method.Comment: 5 pages, 4 postscript figure
A DFT-based Molecular Transport Implementation in ADF/BAND
We present a novel implementation of the first-principles approach to
molecular charge transport using the non-equilibrium Green's function formalism
in combination with the ADF/BAND periodic band-structure DFT code, together
with results for several example systems. As a proof of concept, we first
discuss transport calculations on 1D chains of Li and Al atoms. We then present
a detailed study of BDT and archetypal molecular wires from the OPE-family,
sandwiched between 3D Au contacts, comparing well with results from the
literature. Our implementation further allows us to make a comparison of 3D
contacts with and without periodic boundary conditions, the latter being
particularly useful for modeling the needle-shaped contacts used in
break-junction experiments
Individual Vortex Manipulation and Stick-Slip Motion in Periodic Pinning Arrays
We numerically examine the manipulation of vortices interacting with a moving
trap representing a magnetic force tip translating across a superconducting
sample containing a periodic array of pinning sites. As a function of the tip
velocity and coupling strength, we find five distinct dynamic phases, including
a decoupled regime where the vortices are dragged a short distance within a
pinning site, an intermediate coupling regime where vortices in neighboring
pinning sites exchange places, an intermediate trapping regime where individual
vortices are dragged longer distances and exchange modes of vortices occur in
the surrounding pins, an intermittent multiple trapping regime where the trap
switches between capturing one or two vortices, and a strong couping regime in
which the trap permanently captures and drags two vortices. In some regimes we
observe the counterintuitive behavior that slow moving traps couple less
strongly to vortices than faster moving traps; however, the fastest moving
traps are generally decoupled. The different phases can be characterized by the
distances the vortices are displaced and the force fluctuations exerted on the
trap. We find different types of stick-slip motion depending on whether
vortices are moving into and out of pinning sites, undergoing exchange, or
performing correlated motion induced by vortices outside of the trap. Our
results are general to the manipulation of other types of particle-based
systems interacting with periodic trap arrays, such as colloidal particles or
certain types of frictional systems.Comment: 10 pages, 13 figure
Noise spectra in the reversible-irreversible transition in amorphous solids under oscillatory driving
We study the stress fluctuations in simulations of a two-dimensional
amorphous solid under a cyclic drive. It is known that this system organizes
into a reversible state for small driving amplitudes and remains in an
irreversible state for high driving amplitudes, and that a critical driving
amplitude separates the two regimes. Here we study the time series of the
stress fluctuations below and above the reversible-irreversible transition. In
the irreversible regime above the transition, the power spectrum of the stress
fluctuations is broad and has a shape with . We
find that the low frequency noise power peaks near the stress at which dc
yielding occurs, which is consistent with the behavior expected in systems
undergoing a non-equilibrium phase transition.Comment: 9 pages, 8 figure
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