16,682 research outputs found
Extinction in neutrally stable stochastic Lotka-Volterra models
Populations of competing biological species exhibit a fascinating interplay
between the nonlinear dynamics of evolutionary selection forces and random
fluctuations arising from the stochastic nature of the interactions. The
processes leading to extinction of species, whose understanding is a key
component in the study of evolution and biodiversity, are influenced by both of
these factors.
In this paper, we investigate a class of stochastic population dynamics
models based on generalized Lotka-Volterra systems. In the case of neutral
stability of the underlying deterministic model, the impact of intrinsic noise
on the survival of species is dramatic: it destroys coexistence of interacting
species on a time scale proportional to the population size. We introduce a new
method based on stochastic averaging which allows one to understand this
extinction process quantitatively by reduction to a lower-dimensional effective
dynamics. This is performed analytically for two highly symmetrical models and
can be generalized numerically to more complex situations. The extinction
probability distributions and other quantities of interest we obtain show
excellent agreement with simulations.Comment: 14 pages, 7 figure
Phase perturbation measurements through a heated ionosphere
High frequency radiowaves incident on an overdense (i.e., HF-frequency penetration frequency) ionosphere produce electron density irregularities. The effect of such ionospheric irregularities on the phase of UHF-radiowaves was determined. For that purpose the phase of radiowaves originating from celestial radio sources was observed with two antennas. The radiosources were chosen such that the line of sight to at least one of the antennas (usually both) passed through the modified volume of the ionosphere. Observations at 430 MHz and at 2380 MHz indicate that natural irregularities have a much stronger effect on the UHF phase fluctuations than the HF-induced irregularities for presently achieved HF-power densities of 20-80 uW/sq m. It is not clear whether some of the effects observed are the result of HF-modification of the ionosphere. Upper limits on the phase perturbations produced by HF-modification are 10 deg at 2380 MHz and 80 deg at 430 MHz
Mechanics of bundled semiflexible polymer networks
While actin bundles are used by living cells for structural fortification,
the microscopic origin of the elasticity of bundled networks is not understood.
Here, we show that above a critical concentration of the actin binding protein
fascin, a solution of actin filaments organizes into a pure network of bundles.
While the elasticity of weakly crosslinked networks is dominated by the affine
deformation of tubes, the network of bundles can be fully understood in terms
of non-affine bending undulations.Comment: 5 pages, 3 figures, final version as publishe
Current reversal and exclusion processes with history-dependent random walks
A class of exclusion processes in which particles perform history-dependent
random walks is introduced, stimulated by dynamic phenomena in some biological
and artificial systems. The particles locally interact with the underlying
substrate by breaking and reforming lattice bonds. We determine the
steady-state current on a ring, and find current-reversal as a function of
particle density. This phenomenon is attributed to the non-local interaction
between the walkers through their trails, which originates from strong
correlations between the dynamics of the particles and the lattice. We
rationalize our findings within an effective description in terms of
quasi-particles which we call front barriers. Our analytical results are
complemented by stochastic simulations.Comment: 5 pages, 6 figure
Ultrafast control of inelastic tunneling in a double semiconductor quantum
In a semiconductor-based double quantum well (QW) coupled to a degree of
freedom with an internal dynamics, we demonstrate that the electronic motion is
controllable within femtoseconds by applying appropriately shaped
electromagnetic pulses. In particular, we consider a pulse-driven AlxGa1-xAs
based symmetric double QW coupled to uniformly distributed or localized
vibrational modes and present analytical results for the lowest two levels.
These predictions are assessed and generalized by full-fledged numerical
simulations showing that localization and time-stabilization of the driven
electron dynamics is indeed possible under the conditions identified here, even
with a simultaneous excitations of vibrational modes.Comment: to be published in Appl.Phys.Let
Are Noachian-age ridged plains (Nplr) actually early Hesperian in age
Whether or not the Nplr units in Memnonia and Argyre truly represent ridged plains volcanism of Noachian age or are simply areas of younger (Early Hesperian age) volcanism which failed to bury older craters and therefore have a greater total crater age than really applies to the ridged plains portion of those terrains is examined. The Nuekum and Hiller technique is used to determine the number of preserved crater retention surfaces in the Memnonia and Argyre regions where Scott and Tanaka show Nplr units to be common. The results for cratered terrain (Npl) in Memnonia is summarized along with those for ridged plains (Nplr) in both Memnonia and Argyre, and they are compared with similar results obtained for Tempe Terra and Lunae Plunum
High-resolution radio imaging of two luminous quasars beyond redshift 4.5
Context. Radio-loud active galactic nuclei in the early Universe are rare.
The quasars J0906+6930 at redshift z=5.47 and J2102+6015 at z=4.57 stand out
from the known sample with their compact emission on milliarcsecond (mas)
angular scale with high (0.1-Jy level) flux densities measured at GHz radio
frequencies. This makes them ideal targets for very long baseline
interferometry (VLBI) observations. Aims. By means of VLBI imaging we can
reveal the inner radio structure of quasars and model their brightness
distribution to better understand the geometry of the jet and the physics of
the sources. Methods. We present sensitive high-resolution VLBI images of
J0906+6930 and J2102+6015 at two observing frequencies, 2.3 and 8.6 GHz. The
data were taken in an astrometric observing programme involving a global
five-element radio telescope array. We combined the data from five different
epochs from 2017 February to August. Results. For one of the highest redshift
blazars known, J0906+6930, we present the first-ever VLBI image obtained at a
frequency below 8 GHz. Based on our images at 2.3 and 8.6 GHz, we confirm that
this source has a sharply bent helical inner jet structure within ~3 mas from
the core. The quasar J2102+6015 shows an elongated radio structure in the
east-west direction within the innermost ~2 mas that can be described with a
symmetric three-component brightness distribution model at 8.6 GHz. Because of
their non-pointlike mas-scale structure, these sources are not ideal as
astrometric reference objects. Our results demonstrate that VLBI observing
programmes conducted primarily with astrometric or geodetic goals can be
utilized for astrophysical purposes as well.Comment: 8 pages, 3 figures, accepted for publication in Astronomy &
Astrophysic
Scaling and universality in coupled driven diffusive models
Inspired by the physics of magnetohydrodynamics (MHD) a simplified coupled
Burgers-like model in one dimension (1d), a generalization of the Burgers model
to coupled degrees of freedom, is proposed to describe 1dMHD. In addition to
MHD, this model serves as a 1d reduced model for driven binary fluid mixtures.
Here we have performed a comprehensive study of the universal properties of the
generalized d-dimensional version of the reduced model. We employ both
analytical and numerical approaches. In particular, we determine the scaling
exponents and the amplitude-ratios of the relevant two-point time-dependent
correlation functions in the model. We demonstrate that these quantities vary
continuously with the amplitude of the noise cross-correlation. Further our
numerical studies corroborate the continuous dependence of long wavelength and
long time-scale physics of the model on the amplitude of the noise
cross-correlations, as found in our analytical studies. We construct and
simulate lattice-gas models of coupled degrees of freedom in 1d, belonging to
the universality class of our coupled Burgers-like model, which display similar
behavior. We use a variety of numerical (Monte-Carlo and Pseudospectral
methods) and analytical (Dynamic Renormalization Group, Self-Consistent
Mode-Coupling Theory and Functional Renormalization Group) approaches for our
work. The results from our different approaches complement one another.
Possible realizations of our results in various nonequilibrium models are
discussed.Comment: To appear in JSTAT (2009); 52 pages in JSTAT format. Some figure
files have been replace
Stiff Polymers, Foams and Fiber Networks
We study the elasticity of fibrous materials composed of generalized stiff
polymers. It is shown that in contrast to cellular foam-like structures affine
strain fields are generically unstable. Instead, a subtle interplay between the
architecture of the network and the elastic properties of its building blocks
leads to intriguing mechanical properties with intermediate asymptotic scaling
regimes. We present exhaustive numerical studies based on a finite element
method complemented by scaling arguments.Comment: 4 pages, 5 figure
Zero field muon spin lattice relaxation rate in a Heisenberg ferromagnet at low temperature
We provide a theoretical framework to compute the zero field muon spin
relaxation rate of a Heisenberg ferromagnet at low temperature. We use the
linear spin wave approximation. The rate, which is a measure of the spin
lattice relaxation induced by the magnetic fluctuations along the easy axis,
allows one to estimate the magnon stiffness constant.Comment: REVTeX 3.0 manuscript, 5 pages, no figure. Published in Phys. Rev. B
52, 9155 (1995
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