246 research outputs found
A class of colliding waves in metric-affine gravity, nonmetricity and torsion shock waves
By using our recent generalization of the colliding waves concept to
metric-affine gravity theories, and also our generalization of the advanced and
retarded time coordinate representation in terms of Jacobi functions, we find a
general class of colliding wave solutions with fourth degree polynomials in
metric-affine gravity. We show that our general approach contains the standard
second degree polynomials colliding wave solutions as a particular case.Comment: 13 pages, latex, to appear in J.Math.Phy
The dynamics of vortices on S^2 near the Bradlow limit
The explicit solutions of the Bogomolny equations for N vortices on a sphere
of radius R^2 > N are not known. In particular, this has prevented the use of
the geodesic approximation to describe the low energy vortex dynamics. In this
paper we introduce an approximate general solution of the equations, valid for
R^2 close to N, which has many properties of the true solutions, including the
same moduli space CP^N. Within the framework of the geodesic approximation, the
metric on the moduli space is then computed to be proportional to the Fubini-
Study metric, which leads to a complete description of the particle dynamics.Comment: 17 pages, 9 figure
Transition from Diffusive to Localized Regimes in Surface Corrugated Optical Waveguides
Exact calculations of the transmittance of surface corrugated optical
waveguides are presented. The elastic scattering of diffuse light or other
electromagnetic waves from a rough surface induces a diffusive transport along
the waveguide axis. As the length of the corrugated part of the waveguide
increases, a transition from the diffusive to the localized regime is observed.
This involves an analogy with electron conduction in nanowires, and hence, a
concept analogous to that of ``resistance'' can be introduced. We show an
oscillatory behavior of both the elastic mean free path and the localization
length versus the wavelength.Comment: 3 pages, REVTEX, 3 PS figure
Distributed Algorithms for Consensus and Coordination in the Presence of Packet-Dropping Communication Links - Part II: Coefficients of Ergodicity Analysis Approach
In this two-part paper, we consider multicomponent systems in which each
component can iteratively exchange information with other components in its
neighborhood in order to compute, in a distributed fashion, the average of the
components' initial values or some other quantity of interest (i.e., some
function of these initial values). In particular, we study an iterative
algorithm for computing the average of the initial values of the nodes. In this
algorithm, each component maintains two sets of variables that are updated via
two identical linear iterations. The average of the initial values of the nodes
can be asymptotically computed by each node as the ratio of two of the
variables it maintains. In the first part of this paper, we show how the update
rules for the two sets of variables can be enhanced so that the algorithm
becomes tolerant to communication links that may drop packets, independently
among them and independently between different transmission times. In this
second part, by rewriting the collective dynamics of both iterations, we show
that the resulting system is mathematically equivalent to a finite inhomogenous
Markov chain whose transition matrix takes one of finitely many values at each
step. Then, by using e a coefficients of ergodicity approach, a method commonly
used for convergence analysis of Markov chains, we prove convergence of the
robustified consensus scheme. The analysis suggests that similar convergence
should hold under more general conditions as well.Comment: University of Illinois at Urbana-Champaign. Coordinated Sciences
Laboratory technical repor
Time-gated transillumination and reflection by biological tissues and tissuelike phantoms: simulation versus experiment
A numerical method is presented to solve exactly the time-dependent diffusion equation that describes light transport in turbid media. The simulation takes into account spatial variations of the scattering and absorption factors of the medium and the objects as well as random fluctuations of these quantities. The technique is employed to explore the possibility of locating millimeter-sized objects immersed in turbid media from time-gated measurements of the transmitted or reflected (near-infrared) light. The simulation results for tissue-like phantoms are compared with experimental transillumination data, and excellent agreement is found. Simulations of time-gated reflection experiments indicate that it may be possible to detect objects of 1-mm radius.
Growth and optical characterization of indirect-gap AlxGa1−xAs alloys
Nonintentionally doped AlxGa1−xAs layers with 0.38 x 0.84 were grown on (100) GaAs substrates by liquid phase epitaxy (LPE) under near-equilibrium conditions. The crystalline quality of the samples was studied by photoluminescence at 2 K and room temperature Raman spectroscopy. The peculiar behavior in the photoluminescence intensities of the indirect bound exciton line and the donor–acceptor pair transition is explained from the evolution of the silicon donor binding energy according to the aluminum composition. It was also possible to observe the excitonic transition corresponding to the AlxGa1−xAs/GaAs interface, despite the disorder and other factors which are normally involved when growing high-aluminum-content layers by this technique. Furthermore, Raman measurements show the quadratic variations of longitudinal optical phonon frequencies with aluminum concentration in good agreement with previous experimental results. In this work we show that high quality indirect-gap AlxGa1−xAs samples can be grown by LPE under near-equilibrium [email protected]
The ZEUS Forward Plug Calorimeter with Lead-Scintillator Plates and WLS Fiber Readout
A Forward Plug Calorimeter (FPC) for the ZEUS detector at HERA has been built
as a shashlik lead-scintillator calorimeter with wave length shifter fiber
readout. Before installation it was tested and calibrated using the X5 test
beam facility of the SPS accelerator at CERN. Electron, muon and pion beams in
the momentum range of 10 to 100 GeV/c were used. Results of these measurements
are presented as well as a calibration monitoring system based on a Co
source.Comment: 38 pages (Latex); 26 figures (ps
Complex dynamics in a simple model of pulsations for Super-Asymptotic Giant Branch Stars
When intermediate mass stars reach their last stages of evolution they show
pronounced oscillations. This phenomenon happens when these stars reach the
so-called Asymptotic Giant Branch (AGB), which is a region of the
Hertzsprung-Russell diagram located at about the same region of effective
temperatures but at larger luminosities than those of regular giant stars. The
period of these oscillations depends on the mass of the star. There is growing
evidence that these oscillations are highly correlated with mass loss and that,
as the mass loss increases, the pulsations become more chaotic. In this paper
we study a simple oscillator which accounts for the observed properties of this
kind of stars. This oscillator was first proposed and studied by Icke et al.
[Astron.Astrophys. 258, 341 (1992)] and we extend their study to the region of
more massive and luminous stars - the region of Super-AGB stars. The oscillator
consists of a periodic nonlinear perturbation of a linear Hamiltonian system.
The formalism of dynamical systems theory has been used to explore the
associated Poincare map for the range of parameters typical of those stars. We
have studied and characterized the dynamical behaviour of the oscillator as the
parameters of the model are varied, leading us to explore a sequence of local
and global bifurcations. Among these, a tripling bifurcation is remarkable,
which allows us to show that the Poincare map is a nontwist area preserving
map. Meandering curves, hierarchical-islands traps and sticky orbits also show
up. We discuss the implications of the stickiness phenomenon in the evolution
and stability of the Super-AGB stars.Comment: 13 pages, 9 figure
Investigation on the aerosol performance of dry powder inhalation hypromellose capsules with different lubricant levels
HPMC capsules are made by a dipping process and a surface lubricant for the mould pins is an essential processing aid for removing dried capsules shells. For the purpose of this study, the level was determined by quantifying methyloleate (MO) a component found in the lubricant but not in the hypromellose capsules. Here we investigated the influence of the lubricant, low (10.81 μg/capsule = 60 mg/kg MO), medium (15.97 μg/capsule = 90 mg/kg MO) and high (23.23 μg/capsule = 127 mg/kg MO) content on powder (binary mixture of salbutamol: lactose, 1:50 w/w) aerosolization properties was investigated. Results indicated significantly lower emitted dose from capsules with 60 mg/kg MO. Furthermore, the 90 and 127 mg/kg MO level of lubricant capsules produced almost double the Fine Particle Dose & Fine Particle Fraction compared with the low level of lubricant. The data indicates that lubricant level within capsules has an influence on deposition profiles and amount of drug remaining in capsule and inhaler device after actuation. It is suggested lubricant levels greater than 60 mg/kg MO per capsule are required to minimise powder retention within capsules and maximise deposition profiles. AFM (atomic force microscopy) data suggest that internal surface roughness may be related with this phenomena
Universal scaling of magnetoconductance in magnetic nanocontacts (Invited)
We present results of half-metallic ferromagnets formed by atomic nanocontact of CrO2– CrO2 and
CrO2–Ni that show as much as 400% magnetoconductance. Analysis of the magnetoconductance
versus conductance data for all materials known to exhibit so-called ballistic magnetoresistance
strongly suggests that the magnetoconductance of nanocontacts follows universal scaling. If the
maximum magnetoconductance is normalized to unity and the conductance is scaled to the
resistivity of the material, then all data points fall into a universal curve that is independent of the
contact material and the transport mechanism. The analysis was applied to all available
magnetoconductance data of magnetic nanocontacts in the literature, and the results agree with
theory that takes into account the spin scattering within a magnetic domain wall
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