2,571 research outputs found
Swelling and shrinking kinetics of a lamellar gel phase
We investigate the swelling and shrinking of L_beta lamellar gel phases
composed of surfactant and fatty alcohol after contact with aqueous
poly(ethylene-glycol) solutions. The height change is
diffusion-like with a swelling coefficient, S: . On
increasing polymer concentration we observe sequentially slower swelling,
absence of swelling, and finally shrinking of the lamellar phase. This behavior
is summarized in a non-equilibrium diagram and the composition dependence of S
quantitatively described by a generic model. We find a diffusion coefficient,
the only free parameter, consistent with previous measurements.Comment: 3 pages, 4 figures to appear in Applied Physics Letter
State-space solutions to the dynamic magnetoencephalography inverse problem using high performance computing
Determining the magnitude and location of neural sources within the brain
that are responsible for generating magnetoencephalography (MEG) signals
measured on the surface of the head is a challenging problem in functional
neuroimaging. The number of potential sources within the brain exceeds by an
order of magnitude the number of recording sites. As a consequence, the
estimates for the magnitude and location of the neural sources will be
ill-conditioned because of the underdetermined nature of the problem. One
well-known technique designed to address this imbalance is the minimum norm
estimator (MNE). This approach imposes an regularization constraint that
serves to stabilize and condition the source parameter estimates. However,
these classes of regularizer are static in time and do not consider the
temporal constraints inherent to the biophysics of the MEG experiment. In this
paper we propose a dynamic state-space model that accounts for both spatial and
temporal correlations within and across candidate intracortical sources. In our
model, the observation model is derived from the steady-state solution to
Maxwell's equations while the latent model representing neural dynamics is
given by a random walk process.Comment: Published in at http://dx.doi.org/10.1214/11-AOAS483 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
A Causal Source which Mimics Inflation
How unique are the inflationary predictions for the cosmic microwave
anisotropy pattern? In this paper, it is asked whether an arbitrary causal
source for perturbations in the standard hot big bang could effectively mimic
the predictions of the simplest inflationary models. A surprisingly simple
example of a `scaling' causal source is found to closely reproduce the
inflationary predictions. This letter extends the work of a previous paper
(ref. 6) to a full computation of the anisotropy pattern, including the Sachs
Wolfe integral. I speculate on the possible physics behind such a source.Comment: 4 pages, RevTex, 3 figure
Power Spectra in Global Defect Theories of Cosmic Structure Formation
An efficient technique for computing perturbation power spectra in field
ordering theories of cosmic structure formation is introduced, enabling
computations to be carried out with unprecedented precision. Large scale
simulations are used to measure unequal time correlators of the source stress
energy, taking advantage of scaling during matter and radiation domination, and
causality, to make optimal use of the available dynamic range. The correlators
are then re-expressed in terms of a sum of eigenvector products, a
representation which we argue is optimal, enabling the computation of the final
power spectra to be performed at high accuracy. Microwave anisotropy and matter
perturbation power spectra for global strings, monopoles, textures and
non-topological textures are presented and compared with recent observations.Comment: 4 pages, compressed and uuencoded RevTex file and postscript figure
The Doppler Peaks from Cosmic Texture
We compute the angular power spectrum of temperature anisotropies on the
microwave sky in the cosmic texture theory, with standard recombination
assumed. The spectrum shows `Doppler' peaks analogous to those in scenarios
based on primordial adiabatic fluctuations such as `standard CDM', but at quite
different angular scales. There appear to be excellent prospects for using this
as a discriminant between inflationary and cosmic defect theories.Comment: 14 pages, latex, 3 figures, compressed and uuencoded, replaced
version has minor typographical correction
Polarization of the Microwave Background in Defect Models
We compute the polarization power spectra for global strings, monopoles,
textures and nontopological textures, and compare them to inflationary models.
We find that topological defect models predict a significant (1 microK)
contribution to magnetic type polarization on degree angular scales, which is
produced by the large vector component of the defect source. We also
investigate the effect of decoherence on polarization. It leads to a smoothing
of acoustic oscillations both in temperature and polarization power spectra and
strongly suppresses the cross-correlation between temperature and polarization
relative to inflationary models. Presence or absence of magnetic polarization
or cross-correlation would be a strong discriminator between the two theories
of structure formation and will be testable with the next generation of CMB
satellites.Comment: 4 pages, 3 figures, RevTeX fil
Cosmic Texture from a Broken Global SU(3) Symmetry
We investigate the observable consequences of creating cosmic texture by
breaking a global SU(3) symmetry, rather than the SU(2) case which is generally
studied. To this end, we study the nonlinear sigma model for a totally broken
SU(3) symmetry, and develop a technique for numerically solving the classical
field equations. This technique is applied in a cosmological context: the
energy of the collapsing SU(3) texture field is used as a gravitational source
for the production of perturbations in the primordial fluids of the early
universe. From these calculations, we make predictions about the appearance of
the anisotropies in the cosmic microwave background radiation (CMBR) which
would be present if the large scale structure of the universe was
gravitationally seeded by the collapse of SU(3) textures.Comment: 28 pages, latex, 11 figures, submitted to Phys. Rev.
Stealth dicing of sapphire wafers with near infra-red femtosecond pulses
The quality of the reflecting faces after dicing is critical for the fabrication of efficient and stable laser diodes emitting in the greenâviolet region. However, high-quality faces can be difficult to achieve for devices grown on a sapphire substrate as this material is difficult to cleave cleanly. We have therefore investigated a technology known as âstealth dicingâ. The technology uses a pulsed laser to damage a plane of material inside of the wafer due to multi-photon absorption instead of cutting through the wafer surface. If the damage is induced in a line of stress points, the sample can then be cleaved easily along the damaged plane to leave a high-quality surface. The use of this technique also reduces thermal damage and debris
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