22,902 research outputs found
Fluctuating Topological Defects in 2D Liquids: Heterogeneous Motion and Noise
We measure the defect density as a function of time at different temperatures
in simulations of a two dimensional system of interacting particles. Just above
the solid to liquid transition temperature, the power spectrum of the defect
fluctuations shows a 1/f signature, which crosses over to a white noise
signature at higher temperatures. When 1/f noise is present, the 5-7 defects
predominately form string like structures, and the particle trajectories show a
1D correlated motion that follows the defect strings. At higher temperatures
this heterogeneous motion is lost. We demonstrate this heterogeneity both in
systems interacting with a short ranged screened Coulomb interaction, as well
as in systems with a long range logarithmic interaction between the particles.Comment: 4 pages, 5 postscript figure
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.Comment: 18 pages, 22 postscript figure
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.Comment: 13 pages, 17 postscript figure
ECONOMIC AND SOCIAL IMPACTS OF PRESERVING ANCIENT FORESTS IN THE PACIFIC NORTHWEST
Resource /Energy Economics and Policy,
Study of an attitude reference system utilizing an electrically suspended gyro final report, 1 aug. 1964 - 31 mar. 1965
Miniature electrically suspended gyroscope for spacecraft attitude reference syste
The development of an advanced system to cool a man in a pressure suit
Conductive cooling system for cooling man in pressurized space sui
Point Defect Dynamics in Two-Dimensional Colloidal Crystals
We study the topological configurations and dynamics of individual point
defect vacancies and interstitials in a two-dimensional colloidal crystal. Our
Brownian dynamics simulations show that the diffusion mechanism for vacancy
defects occurs in two phases. The defect can glide along the crystal lattice
directions, and it can rotate during an excited topological transition
configuration to assume a different direction for the next period of gliding.
The results for the vacancy defects are in good agreement with recent
experiments. For the interstitial point defects, which were not studied in the
experiments, we find several of the same modes of motion as in the vacancy
defect case along with two additional diffusion pathways. The interstitial
defects are more mobile than the vacancy defects due to the more
two-dimensional nature of the diffusion of the interstitial defects.Comment: 8 pages, 9 postscript figures. Version to appear in Phys. Rev.
Silicon purification using a Cu-Si alloy source
Production of 99.9999% pure silicon from 98% pure metallurgical grade (MG) silicon by a vapor transport filtration process (VTP) is described. The VTF process is a cold wall version of an HCl chemical vapor transport technique using a Si:Cu3Si alloy as the silicon source. The concentration, origin, and behavior of the various impurities involved in the process were determined by chemically analyzing alloys of different purity, the slag formed during the alloying process, and the purified silicon. Atomic absorption, emission spectrometry, inductively coupled plasma, spark source mass spectrometry, and secondary ion mass spectroscopy were used for these analyses. The influence of the Cl/H ratio and the deposition temperature on the transport rate was also investigated
Restoration of multichannel microwave radiometric images
A constrained iterative image restoration method is applied to multichannel diffraction-limited imagery. This method is based on the Gerchberg-Papoulis algorithm utilizing incomplete information and partial constraints. The procedure is described using the orthogonal projection operators which project onto two prescribed subspaces iteratively. Some of its properties and limitations are also presented. The selection of appropriate constraints was emphasized in a practical application. Multichannel microwave images, each having different spatial resolution, were restored to a common highest resolution to demonstrate the effectiveness of the method. Both noise-free and noisy images were used in this investigation
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