8,181 research outputs found
Nickel hydrogen capacity loss
The results of tests to assess capacity loss in nickel hydrogen cells are presented in outline form. The effects of long storage (greater than 1 month), high hydrogen pressure storage, high cobalt content, and recovery actions are addressed
Fast directional correlation on the sphere with steerable filters
A fast algorithm is developed for the directional correlation of scalar
band-limited signals and band-limited steerable filters on the sphere. The
asymptotic complexity associated to it through simple quadrature is of order
O(L^5), where 2L stands for the square-root of the number of sampling points on
the sphere, also setting a band limit L for the signals and filters considered.
The filter steerability allows to compute the directional correlation uniquely
in terms of direct and inverse scalar spherical harmonics transforms, which
drive the overall asymptotic complexity. The separation of variables technique
for the scalar spherical harmonics transform produces an O(L^3) algorithm
independently of the pixelization. On equi-angular pixelizations, a sampling
theorem introduced by Driscoll and Healy implies the exactness of the
algorithm. The equi-angular and HEALPix implementations are compared in terms
of memory requirements, computation times, and numerical stability. The
computation times for the scalar transform, and hence for the directional
correlation, of maps of several megapixels on the sphere (L~10^3) are reduced
from years to tens of seconds in both implementations on a single standard
computer. These generic results for the scale-space signal processing on the
sphere are specifically developed in the perspective of the wavelet analysis of
the cosmic microwave background (CMB) temperature (T) and polarization (E and
B) maps of the WMAP and Planck experiments. As an illustration, we consider the
computation of the wavelet coefficients of a simulated temperature map of
several megapixels with the second Gaussian derivative wavelet.Comment: Version accepted in APJ. 14 pages, 2 figures, Revtex4 (emulateapj).
Changes include (a) a presentation of the algorithm as directly built on
blocks of standard spherical harmonics transforms, (b) a comparison between
the HEALPix and equi-angular implementation
Collective synchronization in populations of globally coupled phase oscillators with drifting frequencies
We generalize the Kuramoto model for coupled phase oscillators by allowing
the frequencies to drift in time according to Ornstein-Uhlenbeck dynamics. Such
drifting frequencies were recently measured in cellular populations of
circadian oscillator and inspired our work. Linear stability analysis of the
Fokker-Planck equation for an infinite population is amenable to exact solution
and we show that the incoherent state is unstable passed a critical coupling
strength K_c(\ga, \sigf), where \ga is the inverse characteristic drifting
time and \sigf the asymptotic frequency dispersion. Expectedly agrees
with the noisy Kuramoto model in the large \ga (Schmolukowski) limit but
increases slower as \ga decreases. Asymptotic expansion of the solution for
\ga\to 0 shows that the noiseless Kuramoto model with Gaussian frequency
distribution is recovered in that limit. Thus varying a single parameter allows
to interpolate smoothly between two regimes: one dominated by the frequency
dispersion and the other by phase diffusion.Comment: 5 pages, 5 figures, accepted in Phys. Rev.
Tunable Functionality and toxicity studies of Titanium Dioxide Nanotube Layers
In this work, we have developed economic process to elaborate scalable
titanium dioxide nanotube layers which show a tunable functionality. The
titanium dioxide nanotube layers was prepared by electrochemical anodization of
Ti foil in 0.4 wt% hydrofluoric acid solution. The nanotube layers structure
and morphology were characterized using x-ray diffraction and scanning electron
microscopy. The surface topography and wettability was studied according to the
anodization time. The sample synthesized while the current density reached a
local minimum displayed higher contact angle. Beyond this point, the contact
angles decrease with the anodization time. Photo-degradation of acid orange 7
in aqueous solution was used as a probe to assess the photo-catalytic activity
of titanium dioxide nanotube layers under UV irradiation. We obtained better
photocatalitic activity for the sample elaborate at higher current density.
Finally we use the Ciliated Protozoan T. pyriformis, an alternative cell model
used for in vitro toxicity studies, to predict the toxicity of titanium dioxide
nanotube layers in biological system. We did not observe any characteristic
effect in the presence of the titanium dioxide nanotube layers on two
physiological parameters related to this organism, non-specific esterases
activity and population growth rate
QCD and total cross-sections: photons and hadrons
In this contribution, we discuss a total cross-section model which can be
applied to both photon and purely hadronic processes. We find that the model
can reproduce photo-production cross-sections, as well as extrapolations of
gamma* p processes to gamma p using vector meson dominance models, with minimal
modifications from the proton case.Comment: 5 pages, 1 figure, to be published in Proceedings of Diffraction
2008, September 9-14 2008, Marseille, Franc
Correspondence principle between spherical and Euclidean wavelets
Wavelets on the sphere are reintroduced and further developed independently
of the original group theoretic formalism, in an equivalent, but more
straightforward approach. These developments are motivated by the interest of
the scale-space analysis of the cosmic microwave background (CMB) anisotropies
on the sky. A new, self-consistent, and practical approach to the wavelet
filtering on the sphere is developed. It is also established that the inverse
stereographic projection of a wavelet on the plane (i.e. Euclidean wavelet)
leads to a wavelet on the sphere (i.e. spherical wavelet). This new
correspondence principle simplifies the construction of wavelets on the sphere
and allows to transfer onto the sphere properties of wavelets on the plane. In
that regard, we define and develop the notions of directionality and
steerability of filters on the sphere. In the context of the CMB analysis,
these notions are important tools for the identification of local directional
features in the wavelet coefficients of the signal, and for their
interpretation as possible signatures of non-gaussianity, statistical
anisotropy, or foreground emission. But the generic results exposed may find
numerous applications beyond cosmology and astrophysics.Comment: Version accepted in ApJ. Clarified discussion on the unicity of the
wavelet formalism on the sphere and on the related correspondence principle.
14 pages, 8 figures, Revtex4 (emulateapj
Effects of cluster diffusion on the island density and size distribution in submonolayer island growth
The effects of cluster diffusion on the submonolayer island density and
island-size distribution are studied for the case of irreversible growth of
compact islands on a 2D substrate. In our model, we assume instantaneous
coalescence of circular islands, while the cluster mobility is assumed to
exhibit power-law decay as a function of island-size with exponent mu. Results
are presented for mu = 1/2, 1, and 3/2 corresponding to cluster diffusion via
Brownian motion, correlated evaporation-condensation, and edge-diffusion
respectively, as well as for higher values including mu = 2,3, and 6. We also
compare our results with those obtained in the limit of no cluster mobility
corresponding to mu = infinity. In agreement with theoretical predictions of
power-law behavior of the island-size distribution (ISD) for mu < 1, for mu =
1/2 we find Ns({\theta}) ~ s^{-\tau} (where Ns({\theta}) is the number of
islands of size s at coverage {\theta}) up to a cross-over island-size S_c.
However, the value of {\tau} obtained in our simulations is higher than the
mean-field (MF) prediction {\tau} = (3 - mu)/2. Similarly, the value of the
exponent {\zeta} corresponding to the dependence of S_c on the average
island-size S (e.g. S_c ~ S^{\zeta}) is also significantly higher than the MF
prediction {\zeta} = 2/(mu+1). A generalized scaling form for the ISD is also
proposed for mu < 1, and using this form excellent scaling is found for mu =
1/2. However, for finite mu >= 1 neither the generalized scaling form nor the
standard scaling form Ns({\theta}) = {\theta} /S^2 f(s/S) lead to scaling of
the entire ISD for finite values of the ratio R of the monomer diffusion rate
to deposition flux. Instead, the scaled ISD becomes more sharply peaked with
increasing R and coverage. This is in contrast to models of epitaxial growth
with limited cluster mobility for which good scaling occurs over a wide range
of coverages.Comment: 12 pages, submitted to Physical Review
Storage and Retrieval of a Microwave Field in a Spin Ensemble
We report the storage and retrieval of a small microwave field from a
superconducting resonator into collective excitations of a spin ensemble. The
spins are nitrogen-vacancy centers in a diamond crystal. The storage time of
the order of 30 ns is limited by inhomogeneous broadening of the spin ensemble.Comment: 4 pages + supplementary material. Submitted to PR
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