43,535 research outputs found
Clusters and Fluctuations at Mean-Field Critical Points and Spinodals
We show that the structure of the fluctuations close to spinodals and
mean-field critical points is qualitatively different than the structure close
to non-mean-field critical points. This difference has important implications
for many areas including the formation of glasses in supercooled liquids. In
particular, the divergence of the measured static structure function in
near-mean-field systems close to the glass transition is suppressed relative to
the mean-field prediction in systems for which a spatial symmetry is broken.Comment: 5 pages, 1 figur
Birefringence and Dichroism of the QED Vacuum
We use an analytic form for the Heisenberg-Euler Lagrangian to calculate the
birefringent and dichroic properties of the vacuum for arbitrarily strong
wrenchless fields.
PACS : 12.20.Ds, 42.25.Lc 97.60.Jd, 98.70.RzComment: 8 pages, 2 figures, to appear in Journal of Physics
One way Doppler extractor. Volume 1: Vernier technique
A feasibility analysis, trade-offs, and implementation for a One Way Doppler Extraction system are discussed. A Doppler error analysis shows that quantization error is a primary source of Doppler measurement error. Several competing extraction techniques are compared and a Vernier technique is developed which obtains high Doppler resolution with low speed logic. Parameter trade-offs and sensitivities for the Vernier technique are analyzed, leading to a hardware design configuration. A detailed design, operation, and performance evaluation of the resulting breadboard model is presented which verifies the theoretical performance predictions. Performance tests have verified that the breadboard is capable of extracting Doppler, on an S-band signal, to an accuracy of less than 0.02 Hertz for a one second averaging period. This corresponds to a range rate error of no more than 3 millimeters per second
25 kHz narrow spectral bandwidth of a wavelength tunable diode laser with a short waveguide-based external cavity
We report on the spectral properties of a diode laser with a tunable external
cavity in integrated optics. Even though the external cavity is short compared
to other small-bandwidth external cavity lasers, the spectral bandwidth of this
tunable laser is as small as 25 kHz (FWHM), at a side-mode suppression ratio
(SMSR) of 50 dB. Our laser is also able to access preset wavelengths in as
little as 200 us and able to tune over the full telecom C-band (1530 nm - 1565
nm).Comment: 8 pages, 7 figure
Particle-scale structure in frozen colloidal suspensions from small angle X-ray scattering
During directional solidification of the solvent in a colloidal suspension, the colloidal particles segregate from the growing solid, forming high-particle-density regions with structure on a hierarchy of length scales ranging from that of the particle-scale packing to the large-scale spacing between these regions. Previous work has mostly concentrated on the medium- to large-length scale structure, as it is the most accessible and thought to be more technologically relevant. However, the packing of the colloids at the particle-scale is an important component not only in theoretical descriptions of the segregation process, but also to the utility of freeze-cast materials for new applications. Here we present the results of experiments in which we investigated this structure across a wide range of length scales using a combination of small angle X-ray scattering and direct optical imaging. As expected, during freezing the particles were concentrated into regions between ice dendrites forming a microscopic pattern of high- and low-particle-density regions. X-ray scattering indicates that the particles in the high density regions were so closely packed as to be touching. However, the arrangement of the particles does not conform to that predicted by any standard inter-particle pair potentials, suggesting that the particle packing induced by freezing differs from that formed during equilibrium or steady-state densification processes
Relativistic Static Thin Disks: The Counter-Rotating Model
A detailed study of the Counter-Rotating Model (CRM) for generic finite
static axially symmetric thin disks with nonzero radial pressure is presented.
We find a general constraint over the counter-rotating tangential velocities
needed to cast the surface energy-momentum tensor of the disk as the
superposition of two counter-rotating perfect fluids. We also found expressions
for the energy density and pressure of the counter-rotating fluids. Then we
shown that, in general, there is not possible to take the two counter-rotating
fluids as circulating along geodesics neither take the two counter-rotating
tangential velocities as equal and opposite. An specific example is studied
where we obtain some CRM with well defined counter-rotating tangential
velocities and stable against radial perturbations. The CRM obtained are in
agree with the strong energy condition, but there are regions of the disks with
negative energy density, in violation of the weak energy condition.Comment: 19 pages, 6 figures. Submitted to Physical Review
Kinetic Scale Density Fluctuations in the Solar Wind
We motivate the importance of studying kinetic scale turbulence for
understanding the macroscopic properties of the heliosphere, such as the
heating of the solar wind. We then discuss the technique by which kinetic scale
density fluctuations can be measured using the spacecraft potential, including
a calculation of the timescale for the spacecraft potential to react to the
density changes. Finally, we compare the shape of the density spectrum at ion
scales to theoretical predictions based on a cascade model for kinetic
turbulence. We conclude that the shape of the spectrum, including the ion scale
flattening, can be captured by the sum of passive density fluctuations at large
scales and kinetic Alfven wave turbulence at small scales
Slowly rotating voids in cosmology
We consider a spacetime consisting of an empty void separated from an almost
Friedmann-Lema\^\i tre-Robertson-Walker (FLRW) dust universe by a spherically
symmetric, slowly rotating shell which is comoving with the cosmic dust. We
treat in a unified manner all types of the FLRW universes. The metric is
expressed in terms of a constant characterizing the angular momentum of the
shell, and parametrized by the comoving radius of the shell. Treating the
rotation as a first order perturbation, we compute the dragging of inertial
frames as well as the apparent motion of distant stars within the void.
Finally, we discuss, in terms of in principle measurable quantities, 'Machian'
features of the model.Comment: 21 pages, 5 figures, REVTex, accepted for publication in
Class.Quant.Gravit
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