707 research outputs found
A spot-size transformer for fiber-chip coupling in sensor applications at 633 nm in silicon oxynitride
A mode-size adapter was designed, fabricated in SiON/SiO2 and tested. It consists of a laterally tapered SiON waveguide having a step-wise decrease in thickness towards the taper point which may have up to 0.5 ¿m residual widt
Applicability of the Fisher Equation to Bacterial Population Dynamics
The applicability of the Fisher equation, which combines diffusion with
logistic nonlinearity, to population dynamics of bacterial colonies is studied
with the help of explicit analytic solutions for the spatial distribution of a
stationary bacterial population under a static mask. The mask protects the
bacteria from ultraviolet light. The solution, which is in terms of Jacobian
elliptic functions, is used to provide a practical prescription to extract
Fisher equation parameters from observations and to decide on the validity of
the Fisher equation.Comment: 5 pages, 3 figs. include
New Forms of Deuteron Equations and Wave Function Representations
A recently developed helicity basis for nucleon-nucleon (NN) scattering is
applied to th e deuteron bound state. Here the total spin of the deuteron is
treated in such a helicity representation. For the bound state, two sets of two
coupled eigenvalue equations are developed, where the amplitudes depend on two
and one variable, respectively. Numerical illustrations based on the realistic
Bonn-B NN potential are given. In addition, an `operator form' of the deuteron
wave function is presented, and several momentum dependent spin densities are
derived and shown, in which the angular dependence is given analytically.Comment: 19 pages (Revtex), 9 fig
The Weakly Pushed Nature of "Pulled" Fronts with a Cutoff
The concept of pulled fronts with a cutoff has been introduced to
model the effects of discrete nature of the constituent particles on the
asymptotic front speed in models with continuum variables (Pulled fronts are
the fronts which propagate into an unstable state, and have an asymptotic front
speed equal to the linear spreading speed of small linear perturbations
around the unstable state). In this paper, we demonstrate that the introduction
of a cutoff actually makes such pulled fronts weakly pushed. For the nonlinear
diffusion equation with a cutoff, we show that the longest relaxation times
that govern the convergence to the asymptotic front speed and profile,
are given by , for
.Comment: 4 pages, 2 figures, submitted to Brief Reports, Phys. Rev.
The Spatial Properties of L- and M-Cone Inputs to Electroretinograms That Reflect Different Types of Post-Receptoral Processing
yesWe studied the spatial arrangement of L- and M-cone driven electroretinograms (ERGs) reflecting
the activity of magno- and parvocellular pathways. L- and M-cone isolating sine
wave stimuli were created with a four primary LED stimulator using triple silent substitution
paradigms. Temporal frequencies were 8 and 12 Hz, to reflect cone opponent activity, and
30, 36 and 48 Hz to reflect luminance activity. The responses were measured for full-field
stimuli and for different circular and annular stimuli. The ERG data confirm the presence of
two different mechanisms at intermediate and high temporal frequencies. The responses
measured at high temporal frequencies strongly depended upon spatial stimulus configuration.
In the full-field conditions, the L-cone driven responses were substantially larger than
the full-field M-cone driven responses and also than the L-cone driven responses with
smaller stimuli. The M-cone driven responses at full-field and with 70° diameter stimuli displayed
similar amplitudes. The L- and M-cone driven responses measured at 8 and 12 Hz
were of similar amplitude and approximately in counter-phase. The amplitudes were constant
for most stimulus configurations. The results indicate that, when the ERG reflects luminance
activity, it is positively correlated with stimulus size. Beyond 35° retinal eccentricity,
the retina mainly contains L-cones. Small stimuli are sufficient to obtain maximal ERGs at
low temporal frequencies where the ERGs are also sensitive to cone-opponent processin
Vector meson production and nucleon resonance analysis in a coupled-channel approach for energies m_N < sqrt(s) < 2 GeV I: pion-induced results and hadronic parameters
We present a nucleon resonance analysis by simultaneously considering all
pion- and photon-induced experimental data on the final states gamma N, pi N, 2
pi N, eta N, K Lambda, K Sigma, and omega N for energies from the nucleon mass
up to sqrt(s) = 2 GeV. In this analysis we find strong evidence for the
resonances P_{31}(1750), P_{13}(1900), P_{33}(1920), and D_{13}(1950). The
omega N production mechanism is dominated by large P_{11}(1710) and
P_{13}(1900) contributions. In this first part, we present the results of the
pion-induced reactions and the extracted resonance and background properties
with emphasis on the difference between global and purely hadronic fits.Comment: 54 pages, 26 figures, discussion extended, typos corrected,
references updated, to appear in Phys. Rev.
Bounds for graph regularity and removal lemmas
We show, for any positive integer k, that there exists a graph in which any
equitable partition of its vertices into k parts has at least ck^2/\log^* k
pairs of parts which are not \epsilon-regular, where c,\epsilon>0 are absolute
constants. This bound is tight up to the constant c and addresses a question of
Gowers on the number of irregular pairs in Szemer\'edi's regularity lemma.
In order to gain some control over irregular pairs, another regularity lemma,
known as the strong regularity lemma, was developed by Alon, Fischer,
Krivelevich, and Szegedy. For this lemma, we prove a lower bound of
wowzer-type, which is one level higher in the Ackermann hierarchy than the
tower function, on the number of parts in the strong regularity lemma,
essentially matching the upper bound. On the other hand, for the induced graph
removal lemma, the standard application of the strong regularity lemma, we find
a different proof which yields a tower-type bound.
We also discuss bounds on several related regularity lemmas, including the
weak regularity lemma of Frieze and Kannan and the recently established regular
approximation theorem. In particular, we show that a weak partition with
approximation parameter \epsilon may require as many as
2^{\Omega(\epsilon^{-2})} parts. This is tight up to the implied constant and
solves a problem studied by Lov\'asz and Szegedy.Comment: 62 page
Proton-proton scattering above 3 GeV/c
A large set of data on proton-proton differential cross sections, analyzing
powers and the double polarization parameter A_NN is analyzed employing the
Regge formalism. We find that the data available at proton beam momenta from 3
GeV/c to 50 GeV/c exhibit features that are very well in line with the general
characteristics of Regge phenomenology and can be described with a model that
includes the rho, omega, f_2, and a_2 trajectories and single Pomeron exchange.
Additional data, specifically for spin-dependent observables at forward angles,
would be very helpful for testing and refining our Regge model.Comment: 16 pages, 19 figures; revised version accepted for publication in
EPJ
Prospective comparative study of spiral computer tomography and magnetic resonance imaging for detection of hepatocellular carcinoma
BACKGROUND: Hepatocellular carcinoma (HCC) is often detected at a
relatively late stage when tumour size prohibits curative surgery.
Screening to detect HCC at an early stage is performed for patients at
risk. AIM: The aim of this study was to compare prospectively the
diagnostic accuracy and classification for management of the two state of
the art secondline imaging techniques: triphasic spiral computer
tomography (CT) and super paramagnetic iron oxide (SPIO) enhanced magnetic
resonance imaging (MRI). PATIENTS: Sixty one patients were evaluated
between January 1996 and January 1998. Patients underwent CT and MRI
within a mean interval of 6.75 days. METHODS: CT and MRI were evaluated
blindly for the presence and number of lesions, characterisation of these
lesions, and classification for management. For comparison of the data on
characterisation, the CT and MRI findings were compared with
histopathological studies of the surgical specimens and/or follow up
imaging. Data of patients not lost to follow up were available to January
2001. RESULTS: SPIO enhanced MRI detected more lesions and overall smaller
lesions than triphasic spiral CT (number of lesions 189 v 124; median
diameter 1.0 v 1.8 cm; Spearman rank's correlation coefficient 0.63,
p<0.001). There was no significant difference in accuracy between CT and
MRI for lesion characterisation. The agreement in classification for
management was very good (weighted kappa 0.91, 95% CI 0.83-0.99).
CONCLUSION: SPIO enhanced MRI detects more and smaller lesions, but both
techniques are comparable in terms of classification for management. SPIO
enhanced MRI may be preferred as there is no exposure to ionising
radiation
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