5,700 research outputs found
Relativistic electronic dressing in laser-assisted ionization of atomic hydrogen by electron impact
Within the framework of the coplanar binary geometry where it is justified to
use plane wave solutions for the study of the reaction and in the
presence of a circularly polarized laser field, we introduce as a first step
the DVRPWBA1 (Dirac-Volkov Plane Wave Born Approximation1) where we take into
account only the relativistic dressing of the incident and scattered electrons.
Then, we introduce the DVRPWBA2 (Dirac-Volkov Plane Wave Born Approximation2)
where we take totally into account the relativistic dressing of the incident,
scattered and ejected electrons. We then compare the corresponding triple
differential cross sections for laser-assisted ionization of atomic hydrogen by
electron impact both for the non relativistic and the relativistic regime.Comment: 18 pages, Latex, 7 figure
On Random Field Induced Ordering in the Classical XY Model
Consider the classical XY model in a weak random external field pointing
along the axis with strength . We study the behavior of this
model as the range of the interaction is varied. We prove that in any dimension
and for all sufficiently small, there is a range
so that whenever the inverse temperature is larger than
some , there is strong residual ordering along the
direction.Comment: 30 page
The relationship between the optical Halpha filaments and the X-ray emission in the core of the Perseus cluster
NGC 1275 in the centre of the Perseus cluster of galaxies, Abell 426, is
surrounded by a spectacular filamentary Halpha nebula. Deep Chandra X-ray
imaging has revealed that the brighter outer filaments are also detected in
soft X-rays. This can be due to conduction and mixing of the cold gas in the
filaments with the hot, dense intracluster medium. We show the correspondence
of the filaments in both wavebands and draw attention to the relationship of
two prominent curved NW filaments to an outer, buoyant radio bubble seen as a
hole in the X-ray image. There is a strong resemblance in the shape of the hole
and the disposition of the filaments to the behaviour of a large air bubble
rising in water. If this is a correct analogy, then the flow is laminar and the
intracluster gas around this radio source is not turbulent. We obtain a limit
on the viscosity of this gas.Comment: Accepted for publication in MNRA
The marine epilithic diatom Melosira brandinii sp. nov. (Bacillariophyta) from Elephant Island, Antarctic Peninsula, with comments on some related species
Uma nova espĂ©cie de diatomĂĄcea eplĂtica Ă© descrita sob microscopia eletrĂŽnica, a partir de amostras coletadas prĂłximo Ă Ilha Elefante, PenĂnsula AntĂĄrtica. As cĂ©lulas de Melosira brandinii sp. nov. encontram-se reunidas em cadeias filamentosas atravĂ©s de longos tubos mucilaginosos. As valvas sĂŁo circulares, com superfĂcie valvar provida de lĂłculos pentagonais arranjados irregularmente. Cada lĂłculo possui 6-12 poros na superfĂcie externa, abrindo-se para o interior atravĂ©s de arĂ©olas do tipo rota. A corona Ă© composta exclusivamente por grĂąnulos grosseiros, os quais estĂŁo mais concentrados na borda do manto. ComparaçÔes com as espĂ©cies prĂłximas Melosira arctica, M. moniliformis e M. nummuloides foram realizadas. Adicionalmente, sĂŁo fornecidas fotomicrografias de M. arctica provenientes do material tipo e do Mar de Barents (Ărtico), e de M. moniliformis de estuĂĄrios do Sul do Brasil.A new species of epilithic diatom is described from samples collected near Elephant Island, Antarctic Peninsula. The cells of Melosira brandinii sp. nov. are joined in filamentous chains and attached by means of long mucilaginous stalks. The valves are circular with the valvar surface composed of irregularly arranged pentagonal Ioculi. Each loculus bears 6-12 pores on the external surface, opening to the innerside through rotae. The corona is only composed of coarse granules. A mantle is well developed, presenting rimoportulae and bearing coarse granules, which are more concentrated at the mantle edge. Comparisons with the related species Melosira arctica, M. moniliformis and M. nummuloides are made. Additionally, photomicrographs of M. arctica from the type material and Barents Sea, and of M. moniliformis from estuaries of Southern Brazil are included
Geometric scaling in the spectrum of an electron captured by a stationary finite dipole
We examine the energy spectrum of a charged particle in the presence of a
{\it non-rotating} finite electric dipole. For {\emph{any}} value of the dipole
moment above a certain critical value p_{\mathrm{c}}$ an infinite series of
bound states arises of which the energy eigenvalues obey an Efimov-like
geometric scaling law with an accumulation point at zero energy. These
properties are largely destroyed in a realistic situation when rotations are
included. Nevertheless, our analysis of the idealised case is of interest
because it may possibly be realised using quantum dots as artificial atoms.Comment: 5 figures; references added, outlook section reduce
Bottom-up effects of a no-take zone on endangered penguin demographics.
This is the author accepted manuscript. The final version is available from The Royal Society via the DOI in this record.Marine no-take zones can have positive impacts for target species and are increasingly important management tools. However, whether they indirectly benefit higher order predators remains unclear. The endangered African penguin (Spheniscus demersus) depends on commercially exploited forage fish. We examined how chick survival responded to an experimental 3-year fishery closure around Robben Island, South Africa, controlling for variation in prey biomass and fishery catches. Chick survival increased by 18% when the closure was initiated, which alone led to a predicted 27% higher population compared with continued fishing. However, the modelled population continued to decline, probably because of high adult mortality linked to poor prey availability over larger spatial scales. Our results illustrate that small no-take zones can have bottom-up benefits for highly mobile marine predators, but are only one component of holistic, ecosystem-based management regimes.We thank the Earthwatch Institute, Bristol Zoological Society, Leiden Conservation Foundation and National Research Foundation
A moment based approach to the dynamical solution of the Kuramoto model
We examine the dynamics of the Kuramoto model with a new analytical approach.
By defining an appropriate set of moments the dynamical equations can be
exactly closed. We discuss some applications of the formalism like the
existence of an effective Hamiltonian for the dynamics. We also show how this
approach can be used to numerically investigate the dynamical behavior of the
model without finite size effects.Comment: 6 pages, 5 figures, Revtex file, to appear in J. Phys.
Pretransitional behavior in a water-DDAB-5CB microemulsion close to the demixing transition. Evidence for intermicellar attraction mediated by paranematic fluctuations
We present a study of a water-in-oil microemulsion in which surfactant coated
water nanodroplets are dispersed in the isotropic phase of the thermotropic
liquid crystal 5CB. As the temperature is lowered below the isotropic to
nematic phase transition of pure 5CB, the system displays a demixing transition
leading to a coexistence of a droplet rich isotropic phase with a droplet poor
nematic. The transition is anticipated, in the high T side, by increasing
pretransitional fluctuations in 5CB molecular orientation and in the
nanodroplet concentration. The observed phase behavior supports the notion that
the nanosized droplets, while large enough for their statistical behavior to be
probed via light scattering, are also small enough to act as impurities,
disturbing the local orientational ordering of the liquid crystal and thus
experiencing pretransitional attractive interaction mediated by paranematic
fluctuations. The pretransitional behavior, together with the topology of the
phase diagram, can be understood on the basis of a diluted Lebwohl-Lasher model
which describes the nanodroplets simply as holes in the liquid crystal.Comment: 64 pages, 16 figures, J. Chem. Phys. in pres
Amplitude Expansions for Instabilities in Populations of Globally-Coupled Oscillators
We analyze the nonlinear dynamics near the incoherent state in a mean-field
model of coupled oscillators. The population is described by a Fokker-Planck
equation for the distribution of phases, and we apply center-manifold reduction
to obtain the amplitude equations for steady-state and Hopf bifurcation from
the equilibrium state with a uniform phase distribution. When the population is
described by a native frequency distribution that is reflection-symmetric about
zero, the problem has circular symmetry. In the limit of zero extrinsic noise,
although the critical eigenvalues are embedded in the continuous spectrum, the
nonlinear coefficients in the amplitude equation remain finite in contrast to
the singular behavior found in similar instabilities described by the
Vlasov-Poisson equation. For a bimodal reflection-symmetric distribution, both
types of bifurcation are possible and they coincide at a codimension-two Takens
Bogdanov point. The steady-state bifurcation may be supercritical or
subcritical and produces a time-independent synchronized state. The Hopf
bifurcation produces both supercritical stable standing waves and supercritical
unstable travelling waves. Previous work on the Hopf bifurcation in a bimodal
population by Bonilla, Neu, and Spigler and Okuda and Kuramoto predicted stable
travelling waves and stable standing waves, respectively. A comparison to these
previous calculations shows that the prediction of stable travelling waves
results from a failure to include all unstable modes.Comment: 46 pages (Latex), 4 figures available in hard copy from the author
([email protected]); paper submitted to the Journal of Statistical
Physic
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