941 research outputs found
Laser scattering by density fluctuations of ultra-cold atoms in a magneto-optical trap
We study the spectrum of density fluctuations in the ultra-cold gas of
neutral atoms, confined in a magneto-optical trap. We determine the
corresponding amplitude and spectra of laser light scattered by this medium. We
derive an expression for the dynamical structure function, by using a test
particle method. We propose to use the collective laser scattering as a
diagnostic method for the microscopic properties of the ultra-cold matter. This
will also allow us to check on the atomic correlations which are mediated by
the collective mean field inside the gas.Comment: J. Phys. B (in press
A Self-Consistent Marginally Stable State for Parallel Ion Cyclotron Waves
We derive an equation whose solutions describe self-consistent states of
marginal stability for a proton-electron plasma interacting with
parallel-propagating ion cyclotron waves. Ion cyclotron waves propagating
through this marginally stable plasma will neither grow nor damp. The
dispersion relation of these waves, {\omega} (k), smoothly rises from the usual
MHD behavior at small |k| to reach {\omega} = {\Omega}p as k \rightarrow
\pm\infty. The proton distribution function has constant phase-space density
along the characteristic resonant surfaces defined by this dispersion relation.
Our equation contains a free function describing the variation of the proton
phase-space density across these surfaces. Taking this free function to be a
simple "box function", we obtain specific solutions of the marginally stable
state for a range of proton parallel betas. The phase speeds of these waves are
larger than those given by the cold plasma dispersion relation, and the
characteristic surfaces are more sharply peaked in the v\bot direction. The
threshold anisotropy for generation of ion cyclotron waves is also larger than
that given by estimates which assume bi-Maxwellian proton distributions.Comment: in press in Physics of Plasma
Dynamic Light Scattering From Colloidal Gels
We present a brief, preliminary account of the interpretation of dynamic light scattering from fractal colloidal gels. For small scattering angles, and for high initial colloid particle volume fractions, the correlation functions exhibit arrested decay, reflecting the non-ergodic nature of these systems and allowing us to directly determine the elastic modulus of the gels. For smaller initial volume fractions, the correlation functions decay completely. In all cases, the initial decay is not exponential, but is instead described by a stretched exponential. We summarize the principles of a model that accounts for these data and discuss the scaling behavior of the measured parameters
RISK-RETURN ANALYSIS OF INCORPORATING ANNUAL LEGUMES AND LAMB GRAZING WITH DRYLAND CROP ROTATIONS
Profitability and risk, 1988-2001, are examined for lamb-grazed field pea as a fallow alternative with wheat, or an extended wheat-sunflower-millet rotation. Switching from conventional wheat-fallow to an extended rotation with grazed-peas increases profitability (2.3% to 7.3%), and reduces risk (below 0% target in only 2 versus 7 of 14 years).Crop Production/Industries,
Attracted Diffusion-Limited Aggregation
In this paper, we present results of extensive Monte Carlo simulations of
diffusion-limited aggregation (DLA) with a seed placed on an attractive plane
as a simple model in connection with the electrical double layers. We compute
the fractal dimension of the aggregated patterns as a function of the
attraction strength \alpha. For the patterns grown in both two and three
dimensions, the fractal dimension shows a significant dependence on the
attraction strength for small values of \alpha, and approaches to that of the
ordinary two-dimensional (2D) DLA in the limit of large \alpha. For
non-attracting case with \alpha=1, our results in three dimensions reproduce
the patterns of 3D ordinary DLA, while in two dimensions our model leads to
formation of a compact cluster with dimension two. For intermediate \alpha, the
3D clusters have quasi-2D structure with a fractal dimension very close to that
of the ordinary 2D-DLA. This allows one to control morphology of a growing
cluster by tuning a single external parameter \alpha.Comment: 6 pages, 6 figures, to appear in Phys. Rev. E (2012
Heavy-Fermion Instability in Double-Degenerate Plasmas
In this work we study the propagations of normal frequency modes for quantum
hydrodynamic (QHD) waves in the linear limit and introduce a new kind of
instability in a double-degenerate plasma. Three different regimes, namely,
low, intermediate and high magnetic field strengths are considered which span
the applicability of the work to a wide variety of environments. Distinct
behavior is observed for different regimes, for instance, in the
laboratory-scale field regime no frequency-mode instability occurs unlike those
of intermediate and high magnetic-field strength regimes. It is also found that
the instability of this kind is due to the heavy-fermions which appear below a
critical effective-mass parameter () and that the responses
of the two (lower and upper frequency) modes to fractional effective-mass
change in different effective-mass parameter ranges (below and above the
critical value) are quite opposite to each other. It is shown that, the
heavy-fermion instability due to extremely high magnetic field such as that
encountered for a neutron-star crust can lead to confinement of stable
propagations in both lower and upper frequency modes to the magnetic poles.
Current study can have important implications for linear wave dynamics in both
laboratory and astrophysical environments possessing high magnetic fields
Parameter dependence of magnetized CMB observables
Pre-decoupling magnetic fields affect the scalar modes of the geometry and
produce observable effects which can be constrained also through the use of
current (as opposed to forthcoming) data stemming from the Cosmic Microwave
Background observations. The dependence of the temperature and polarization
angular power spectra upon the parameters of an ambient magnetic field is
encoded in the scaling properties of a set of basic integrals whose derivation
is simplified in the limit of small angular scales. The magnetically-induced
distortions patterns of the relevant observables can be computed analytically
by employing scaling considerations which are corroborated by numerical
results.Comment: 48 pages, 11 figures; corrected minor typos; discussions added; to
appear in Physical Revie
New techniques for diffusing-wave spectroscopy
We present two new types of measurements that can be made with diffusing-wave spectroscopy (DWS), a form of dynamic light scattering that applies in limit of strong multiple scattering. The first application is to measure the frequency-dependent linear viscoelastic moduli of complex fluids using light scattering. This is accomplished by measuring the mean square displacement of probe particles using DWS. Their response to thermal fluctuations is determined by the fluctuation-dissipation relation, and is controlled by the response of the surrounding complex fluid. This response can be described in terms of a memory function, which is directly related to the complex elastic modulus of the system. Thus by measuring the mean square displacement, we are able to determine the frequency dependent modulus. The second application is the measurement of shape fluctuations of scattering particles. This is achieved by generalizing the theory for DWS to incorporate the effects if amplitude fluctuations in the scattering intensity of the particles. We apply this new method to study the thermally induced fluctuations in the shape of spherical emulsion droplets whose geometry is controlled by surface tension
Space-filter techniques for quasi-neutral hybrid-kinetic models
The space-filter approach has proved a fundamental tool in studying
turbulence in neutral fluids, providing the ability to analyze scale-to-scale
energy transfer in configuration space. It is well known that turbulence in
plasma presents challenges different from neutral fluids, especially when the
scale of interests include kinetic effects. The space-filter approach is still
largely unexplored for kinetic plasma. Here we derive the space-filtered (or,
equivalently "coarse-grained") equations in configuration space for a
quasi-neutral hybrid-kinetic plasma model, in which ions are fully kinetic and
electrons are a neutralizing fluid. Different models and closures for the
electron fluid are considered, including finite electron-inertia effects and
full electrons' pressure-tensor dynamics. Implications for the cascade of
turbulent fluctuations in real space depending on different approximations are
discussed.Comment: 43 pages, 2 figure
Physical applications of second-order linear differential equations that admit polynomial solutions
Conditions are given for the second-order linear differential equation P3 y"
+ P2 y'- P1 y = 0 to have polynomial solutions, where Pn is a polynomial of
degree n. Several application of these results to Schroedinger's equation are
discussed. Conditions under which the confluent, biconfluent, and the general
Heun equation yield polynomial solutions are explicitly given. Some new classes
of exactly solvable differential equation are also discussed. The results of
this work are expressed in such way as to allow direct use, without preliminary
analysis.Comment: 13 pages, no figure
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