32,458 research outputs found
Frequency domain laser velocimeter signal processor: A new signal processing scheme
A new scheme for processing signals from laser velocimeter systems is described. The technique utilizes the capabilities of advanced digital electronics to yield a smart instrument that is able to configure itself, based on the characteristics of the input signals, for optimum measurement accuracy. The signal processor is composed of a high-speed 2-bit transient recorder for signal capture and a combination of adaptive digital filters with energy and/or zero crossing detection signal processing. The system is designed to accept signals with frequencies up to 100 MHz with standard deviations up to 20 percent of the average signal frequency. Results from comparative simulation studies indicate measurement accuracies 2.5 times better than with a high-speed burst counter, from signals with as few as 150 photons per burst
Structure-property study of keto-ether polyimides
As part of an on-going effort to develop an understanding of how changes in the chemical structure affect polymer properties, an empirical study was performed on polyimides containing only ether and/or carbonyl connecting groups in the polymer backbone. During the past two decades the structure-property relationships in linear aromatic polyimides have been extensively investigated. More recently, work has been performed to study the effect of isomeric attachment of keto-ether polyimides on properties such as glass transition temperature and solubility. However, little work has been reported on the relation of polyimide structure to mechanical properties. The purpose of this study was to determine the effect of structural changes in the backbone of keto-ether polyimides on their mechanical properties, specifically, unoriented thin film tensile properties. This study was conducted in two stages. The purpose of the initial stage was to examine the physical and mechanical properties of a representative group (four) of polyimide systems to determine the optimum solvent and cure cycle requirements. These optimum conditions were then utilized in the second stage to prepare films of keto-ether polyimides which were evaluated for mechanical and physical properties. All of the polyimides were prepared using isomers of oxydianiline (ODA) and diaminobenzophenone (DABP) in combination with 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA) and 4,4'-oxydiphthalic anhydride (ODPA)
Nucleation of colloids and macromolecules: does the nucleation pathway matter?
A recent description of diffusion-limited nucleation based on fluctuating
hydrodynamics that extends classical nucleation theory predicts a very
non-classical two-step scenario whereby nucleation is most likely to occur in
spatially-extended, low-amplitude density fluctuations. In this paper, it is
shown how the formalism can be used to determine the maximum probability of
observing \emph{any} proposed nucleation pathway, thus allowing one to address
the question as to their relative likelihood, including of the newly proposed
pathway compared to classical scenarios. Calculations are presented for the
nucleation of high-concentration bubbles in a low-concentration solution of
globular proteins and it is found that the relative probabilities (new theory
compared to classical result) for reaching a critical nucleus containing
molecules scales as thus indicating that for all but the smallest
nuclei, the classical scenario is extremely unlikely.Comment: 7 pages, 5 figure
Intrinsic optical dichroism in the chiral superconducting state of SrRuO
We present an analysis of the Hall conductivity in
time reversal symmetry breaking states of exotic superconductors. We find that
the dichroic signal is non-zero in systems with inter-band order parameters.
This new intrinsic mechanism may explain the Kerr effect observed in strontium
ruthenate and possibly other superconductors. We predict coherence factor
effects in the temperature dependence of the imaginary part of the ac Hall
conductivity , which can be tested experimentally.Comment: 4+ pages, 4 figures, published versio
The Kerr rotation in the unconventional superconductor SrRuO
The interpretation of Kerr rotation measurements in the superconducting phase
of SrRuO is a controversial topic. Both intrinsic and extrinsic
mechanisms have been proposed, and it has been argued that the intrinsic
response vanishes by symmetry. We focus on the intrinsic contribution and
clarify several conflicting results in the literature. On the basis of symmetry
considerations and detailed calculations we show that the intrinsic Kerr signal
is not forbidden in a general multi- band system but has a rich structure in
the near infrared regime. We distinguish different optical transitions
determined by the superconducting gap (far infrared) and the inter orbital
coupling of the normal state (near infrared). We argue that the low frequency
transitions do not contribute to the Hall conductivity while only the
inter-orbital transitions in the near infrared regime contribute. Finally, we
discuss the difficulties to connect the calculations for the optical Hall
conductivity to the experimental measurement of the Kerr angle. We will compare
different approximations which might lead to conflicting results.Comment: 9 pages, 8 figures, 1 tabl
Gap Nodes and Time Reversal Symmetry Breaking in Strontium Ruthenate
We study the superconducting state of SrRuO on the bases of a
phenomenological but orbital specific description of the electron-electron
attraction and a realistic quantitative account of the electronic structure in
the normal state. We found that a simple model which features both `in plane'
and `out of plane' coupling with strengths meV and
meV respectively reproduced the experimentally observed power law
behaviour of the low temperature specific heat , superfluid density
and thermal conductivity in quantitative detail. Moreover, it predicts
that the quasi-particle spectrum on the -sheet is fully gaped and the
corresponding order parameter breaks the time reversal symmetry. We have also
investigated the stability of this model to inclusion of further interaction
constants in particular %those which describe `proximity coupling' between
orbitals contributing to the sheet of the Fermi surface and the
and sheets. We found that the predictions of the model are
robust under such changes. Finally, we have incorporated a description of weak
disorder into the model and explored some of its consequences. For example we
demonstrated that the disorder has a more significant effect on the -wave
component of the order parameter than on the p-wave one.Comment: EPJ B submitte
Stability of the Ground State of a Harmonic Oscillator in a Monochromatic Wave
Classical and quantum dynamics of a harmonic oscillator in a monochromatic
wave is studied in the exact resonance and near resonance cases. This model
describes, in particular, a dynamics of a cold ion trapped in a linear ion trap
and interacting with two lasers fields with close frequencies. Analytically and
numerically a stability of the ``classical ground state'' (CGS) -- the vicinity
of the point () -- is analyzed. In the quantum case, the method for
studying a stability of the quantum ground state (QGS) is suggested, based on
the quasienergy representation. The dynamics depends on four parameters: the
detuning from the resonance, , where and
are, respectively, the wave and the oscillator's frequencies; the
positive integer (resonance) number, ; the dimensionless Planck constant,
, and the dimensionless wave amplitude, . For , the CGS
and the QGS are unstable for resonance numbers . For small
, the QGS becomes more stable with increasing and decreasing
. When increases, the influence of chaos on the stability of the
QGS is analyzed for different parameters of the model, , and
.Comment: RevTeX, 38 pages, 24 figure
An Ontology-based Image Repository for a Biomedical Research Lab
We have developed a prototype web-based database for managing images acquired during experiments in a biomedical research lab studying the factors controlling cataract development. Based on an evolving ontology we are developing for describing the experimental data and protocols used in the lab, the image repository allows lab members to organize image data by multiple attributes. The use of an ontology for developing this and other tools will facilitate intercommunication among tools, and eventual data sharing with other researchers
State-space based mass event-history model I: many decision-making agents with one target
A dynamic decision-making system that includes a mass of indistinguishable
agents could manifest impressive heterogeneity. This kind of nonhomogeneity is
postulated to result from macroscopic behavioral tactics employed by almost all
involved agents. A State-Space Based (SSB) mass event-history model is
developed here to explore the potential existence of such macroscopic
behaviors. By imposing an unobserved internal state-space variable into the
system, each individual's event-history is made into a composition of a common
state duration and an individual specific time to action. With the common state
modeling of the macroscopic behavior, parametric statistical inferences are
derived under the current-status data structure and conditional independence
assumptions. Identifiability and computation related problems are also
addressed. From the dynamic perspectives of system-wise heterogeneity, this SSB
mass event-history model is shown to be very distinct from a random effect
model via the Principle Component Analysis (PCA) in a numerical experiment.
Real data showing the mass invasion by two species of parasitic nematode into
two species of host larvae are also analyzed. The analysis results not only are
found coherent in the context of the biology of the nematode as a parasite, but
also include new quantitative interpretations.Comment: Published in at http://dx.doi.org/10.1214/08-AOAS189 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
A Local One-Zone Model of MHD Turbulence in Dwarf Nova Disks
The evolution of the magnetorotational instability (MRI) during the
transition from outburst to quiescence in a dwarf nova disk is investigated
using three-dimensional MHD simulations. The shearing box approximation is
adopted for the analysis, so that the efficiency of angular momentum transport
is studied in a small local patch of the disk: this is usually referred as to a
one-zone model. To take account of the low ionization fraction of the disk, the
induction equation includes both ohmic dissipation and the Hall effect. We
induce a transition from outburst to quiescence by an instantaneous decrease of
the temperature. The evolution of the MRI during the transition is found to be
very sensitive to the temperature of the quiescent disk. As long as the
temperature is higher than a critical value of about 2000 K, MHD turbulence and
angular momentum transport is sustained by the MRI. However, MHD turbulence
dies away within an orbital time if the temperature falls below this critical
value. In this case, the stress drops off by more than 2 orders of magnitude,
and is dominated by the Reynolds stress associated with the remnant motions
from the outburst. The critical temperature depends slightly on the distance
from the central star and the local density of the disk.Comment: 20 pages, 2 tables, 6 figures, accepted for publication in Ap
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