215 research outputs found
Driven Disordered Periodic Media with an Underlying Structural Phase Transition
We investigate the driven states of a two-dimensional crystal whose ground
state can be tuned through a square-triangular transition. The depinning of
such a system from a quenched random background potential occurs via a complex
sequence of dynamical states, which include plastic flow states, hexatics,
dynamically stabilized triangle and square phases and intermediate regimes of
phase coexistence. These results are relevant to transport experiments in the
mixed phase of several superconductors which exhibit such structural
transitions as well as to driven colloidal systems whose interactions can be
tuned via surface modifications.Comment: Two-column, 4 pages, figures include
Diffusion of Point Defects in Two-Dimensional Colloidal Crystals
We report the first study of the dynamics of point defects, mono and
di-vacancies, in a confined 2-D colloidal crystal in real space and time using
digital video microscopy. The defects are introduced by manipulating individual
particles with optical tweezers. The diffusion rates are measured to be
Hz for mono-vacancies and
Hz for di-vacancies. The elementary diffusion
processes are identified and it is found that the diffusion of di-vacancies is
enhanced by a \textit{dislocation dissociation-recombination} mechanism.
Furthermore, the defects do not follow a simple random walk but their hopping
exhibits memory effects, due to the reduced symmetry (compared to the
triangular lattice) of their stable configurations, and the slow relaxation
rates of the lattice modes.Comment: 6 pages (REVTEX), 5 figures (PS
Interstitial Fractionalization and Spherical Crystallography
Finding the ground states of identical particles packed on spheres has
relevance for stabilizing emulsions and a venerable history in the literature
of theoretical physics and mathematics. Theory and experiment have confirmed
that defects such as disclinations and dislocations are an intrinsic part of
the ground state. Here we discuss the remarkable behavior of vacancies and
interstitials in spherical crystals. The strain fields of isolated
disclinations forced in by the spherical topology literally rip interstitials
and vacancies apart, typically into dislocation fragments that combine with the
disclinations to create small grain boundary scars. The fractionation is often
into three charge-neutral dislocations, although dislocation pairs can be
created as well. We use a powerful, freely available computer program to
explore interstitial fractionalization in some detail, for a variety of power
law pair potentials. We investigate the dependence on initial conditions and
the final state energies, and compare the position dependence of interstitial
energies with the predictions of continuum elastic theory on the sphere. The
theory predicts that, before fragmentation, interstitials are repelled from
5-fold disclinations and vacancies are attracted. We also use vacancies and
interstitials to study low energy states in the vicinity of "magic numbers"
that accommodate regular icosadeltahedral tessellations.Comment: 21 pages, 9 figure
Optical Tweezers as a Micromechanical Tool for Studying Defects in 2D Colloidal Crystals
This paper reports on some new results from the analyses of the video
microscopy data obtained in a prior experiment on two-dimensional (2D)
colloidal crystals. It was reported previously that optical tweezers can be
used to create mono- and di-vacancies in a 2D colloidal crystal. Here we report
the results on the creation of a vacancy-interstitial pair, as well as
tri-vacancies. It is found that the vacancy-interstitial pair can be
long-lived, but they do annihilate each other. The behavior of tri-vacancies is
most intriguing, as it fluctuates between a configuration of bound pairs of
dislocations and that of a locally amorphous state. The relevance of this
observation to the issue of the nature of 2D melting is discussed.Comment: 6 pages, 4 figure
Dynamics of Vortex Shells in Mesoscopic Superconducting Corbino Disks
In mesoscopic superconducting disks vortices form shell structures as
recently observed in Nb disks. We study the dynamics of such vortices, driven
by an external current I_0, in a Corbino setup. At very low I_0, the system
exhibits rigid body rotation while at some critical current I_c,i vortex shells
rotate separately with angular velocities omega_i. This critical current I_c,i
has a remarkable non-monotonous dependence on the applied magnetic field which
is due to a dynamically-induced structural transition with a rearrangement of
vortices over the shells similar to the Coster-Kronig transition in hollow
atoms. Thermally-activated externally-driven flux motion in a disk with pinning
centers explains experimentally observed omega_i as a function of I_0 and T and
the dynamically-induced melting transition.Comment: 5 pages, 5 figure
Colloidal Dynamics on Disordered Substrates
Using Langevin simulations we examine driven colloids interacting with
quenched disorder. For weak substrates the colloids form an ordered state and
depin elastically. For increasing substrate strength we find a sharp crossover
to inhomogeneous depinning and a substantial increase in the depinning force,
analogous to the peak effect in superconductors. The velocity versus driving
force curve shows criticality at depinning, with a change in scaling exponent
occuring at the order to disorder crossover. Upon application of a sudden pulse
of driving force, pronounced transients appear in the disordered regime which
are due to the formation of long-lived colloidal flow channels.Comment: 4 pages, 4 postscript figure
Pinning and depinning of a classic quasi-one-dimensional Wigner crystal in the presence of a constriction
We studied the dynamics of a quasi-one-dimensional chain-like system of
charged particles at low temperature, interacting through a screened Coulomb
potential in the presence of a local constriction. The response of the system
when an external electric field is applied was investigated. We performed
Langevin molecular dynamics simulations for different values of the driving
force and for different temperatures. We found that the friction together with
the constriction pins the particles up to a critical value of the driving
force. The system can depin \emph{elastically} or \emph{quasi-elastically}
depending on the strength of the constriction. The elastic (quasi-elastic)
depinning is characterized by a critical exponent
(). The dc conductivity is zero in the pinned regime, it has
non-ohmic characteristics after the activation of the motion and then it is
constant. Furthermore, the dependence of the conductivity with temperature and
strength of the constriction was investigated in detail. We found interesting
differences between the single and the multi-chain regimes as the temperature
is increased.Comment: 18 pages, 16 figures, accepted for publication in PR
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