39,800 research outputs found
KPP reaction-diffusion equations with a non-linear loss inside a cylinder
We consider in this paper a reaction-diffusion system in presence of a flow
and under a KPP hypothesis. While the case of a single-equation has been
extensively studied since the pioneering Kolmogorov-Petrovski-Piskunov paper,
the study of the corresponding system with a Lewis number not equal to 1 is
still quite open. Here, we will prove some results about the existence of
travelling fronts and generalized travelling fronts solutions of such a system
with the presence of a non-linear spacedependent loss term inside the domain.
In particular, we will point out the existence of a minimal speed, above which
any real value is an admissible speed. We will also give some spreading results
for initial conditions decaying exponentially at infinity
Ratio control in a cascade model of cell differentiation
We propose a kind of reaction-diffusion equations for cell differentiation,
which exhibits the Turing instability. If the diffusivity of some variables is
set to be infinity, we get coupled competitive reaction-diffusion equations
with a global feedback term. The size ratio of each cell type is controlled by
a system parameter in the model. Finally, we extend the model to a cascade
model of cell differentiation. A hierarchical spatial structure appears as a
result of the cell differentiation. The size ratio of each cell type is also
controlled by the system parameter.Comment: 13 pages, 7 figure
Information hiding and retrieval in Rydberg wave packets using half-cycle pulses
We demonstrate an information hiding and retrieval scheme with the relative
phases between states in a Rydberg wave packet acting as the bits of a data
register. We use a terahertz half-cycle pulse (HCP) to transfer phase-encoded
information from an optically accessible angular momentum manifold to another
manifold which is not directly accessed by our laser pulses, effectively hiding
the information from our optical interferometric measurement techniques. A
subsequent HCP acting on these wave packets reintroduces the information back
into the optically accessible data register manifold which can then be `read'
out.Comment: 4 pages, 4 figure
Enhancing single-molecule photostability by optical feedback from quantum-jump detection
We report an optical technique that yields an enhancement of single-molecule
photostability, by greatly suppressing photobleaching pathways which involve
photoexcitation from the triplet state. This is accomplished by dynamically
switching off the excitation laser when a quantum-jump of the molecule to the
triplet state is optically detected. This procedure leads to a lengthened
single-molecule observation time and an increased total number of detected
photons. The resulting improvement in photostability unambiguously confirms the
importance of photoexcitation from the triplet state in photobleaching
dynamics, and may allow the investigation of new phenomena at the
single-molecule level
Plasma Electron Beam Welder for Space Vehicles Final Report
Feasibility of developing plasma electron beam welding system for earth orbiting vehicl
Spatial synchronization and extinction of species under external forcing
We study the interplay between synchronization and extinction of a species.
Using a general model we show that under a common external forcing, the species
with a quadratic saturation term in the population dynamics first undergoes
spatial synchronization and then extinction, thereby avoiding the rescue
effect. This is because the saturation term reduces the synchronization time
scale but not the extinction time scale. The effect can be observed even when
the external forcing acts only on some locations provided there is a
synchronizing term in the dynamics. Absence of the quadratic saturation term
can help the species to avoid extinction.Comment: 4 pages, 2 figure
Unmasking the Active Galactic Nucleus in PKS J2310-437
PKS J2310-437 is an AGN with bright X-ray emission relative to its weak radio
emission and optical continuum. It is believed that its jet lies far enough
from the line of sight that it is not highly relativistically beamed. It thus
provides an extreme test of AGN models. We present new observations aimed at
refining the measurement of the source's properties. In optical photometry with
the NTT we measure a central excess with relatively steep spectrum lying above
the bright elliptical galaxy emission, and we associate the excess wholly or in
part with the AGN. A new full-track radio observation with the ATCA finds that
the core 8.64GHz emission has varied by about 20 per cent over 38 months, and
improves the mapping of the weak jet. With Chandra we measure a
well-constrained power-law spectral index for the X-ray core, uncontaminated by
extended emission from the cluster environment, with a negligible level of
intrinsic absorption. Weak X-ray emission from the resolved radio jet is also
measured. Our analysis suggests that the optical continuum in this radio galaxy
has varied by at least a factor of four over a timescale of about two years,
something that should be testable with further observations. We conclude that
the most likely explanation for the bright central X-ray emission is
synchrotron radiation from high-energy electrons.Comment: 7 pages, 12 figure
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