434 research outputs found
High-harmonic generation from a confined atom
The order of high harmonics emitted by an atom in an intense laser field is
limited by the so-called cutoff frequency. Solving the time-dependent
Schr\"odinger equation, we show that this frequency can be increased
considerably by a parabolic confining potential, if the confinement parameters
are suitably chosen.
Furthermore, due to confinement, the radiation intensity remains high
throughout the extended emission range. All features observed can be explained
with classical arguments.Comment: 4 pages(tex files), 4 figures(eps files); added references and
comment
Cosmological Magnetogenesis driven by Radiation Pressure
The origin of large scale cosmological magnetic fields remains a mystery,
despite the continuous efforts devoted to that problem. We present a new model
of magnetic field generation, based on local charge separation provided by an
anisotropic and inhomogeneous radiation pressure. In the cosmological context,
the processes we explore take place at the epoch of the reionisation of the
Universe. Under simple assumptions, we obtain results (i) in terms of the order
of magnitude of the field generated at large scales and (ii) in terms of its
power spectrum. The amplitudes obtained (B ~ 8.10^(-6) micro-Gauss) are
considerably higher than those obtained in usual magnetogenesis models and
provide suitable seeds for amplification by adiabatic collapse and/or dynamo
during structure formation.Comment: 9 pages, 2 figure
Restructuring of colloidal aggregates in shear flow: Coupling interparticle contact models with Stokesian dynamics
A method to couple interparticle contact models with Stokesian dynamics (SD)
is introduced to simulate colloidal aggregates under flow conditions. The
contact model mimics both the elastic and plastic behavior of the cohesive
connections between particles within clusters. Owing to this, clusters can
maintain their structures under low stress while restructuring or even breakage
may occur under sufficiently high stress conditions. SD is an efficient method
to deal with the long-ranged and many-body nature of hydrodynamic interactions
for low Reynolds number flows. By using such a coupled model, the restructuring
of colloidal aggregates under stepwise increasing shear flows was studied.
Irreversible compaction occurs due to the increase of hydrodynamic stress on
clusters. Results show that the greater part of the fractal clusters are
compacted to rod-shaped packed structures, while the others show isotropic
compaction.Comment: A simulation movie be found at
http://www-levich.engr.ccny.cuny.edu/~seto/sites/colloidal_aggregates_shearflow.htm
Uncertainty measurement in the homogenization and sample reduction in the physical classification of rice and beans
Can Europe recover without credit?
Data from 135 countries covering five decades suggests that creditless recoveries, in which
the stock of real credit does not return to the pre-crisis level for three years after the GDP
trough, are not rare and are characterised by remarkable real GDP growth rates: 4.7 percent
per year in middle-income countries and 3.2 percent per year in high-income countries.
However, the implications of these historical episodes for the current European situation are
limited, for two main reasons. First, creditless recoveries are much less common in highincome
countries, than in low-income countries which are financially undeveloped. European
economies heavily depend on bank loans and research suggests that loan supply played a
major role in the recent weak credit performance of Europe. There are reasons to believe that,
despite various efforts, normal lending has not yet been restored. Limited loan supply could
be disruptive for the European economic recovery and there has been only a minor
substitution of bank loans with debt securities. Second, creditless recoveries were associated
with significant real exchange rate depreciation, which has hardly occurred so far in most of
Europe. This stylised fact suggests that it might be difficult to re-establish economic growth
in the absence of sizeable real exchange rate depreciation, if credit growth does not return
Ab initio density functional investigation of B_24 cluster: Rings, Tubes, Planes, and Cages
We investigate the equilibrium geometries and the systematics of bonding in
various isomers of a 24-atom boron cluster using Born-Oppenheimer molecular
dynamics within the framework of density functional theory. The isomers studied
are the rings, the convex and the quasiplanar structures, the tubes and, the
closed structures. A staggered double-ring is found to be the most stable
structure amongst the isomers studied. Our calculations reveal that a 24-atom
boron cluster does form closed 3-d structures. All isomers show staggered
arrangement of nearest neighbor atoms. Such a staggering facilitates
hybridization in boron cluster. A polarization of bonds between the peripheral
atoms in the ring and the planar isomers is also seen. Finally, we discuss the
fusion of two boron icosahedra. We find that the fusion occurs when the
distance between the two icosahedra is less than a critical distance of about
6.5a.u.Comment: 8 pages, 9 figures in jpeg format Editorially approved for
publication in Phys. Rev.
Enhancement of bichromatic high-harmonic generation with a high-frequency field
Using a high-frequency field superposed to a linearly polarized bichromatic
laser field composed by a wave with frequency and a wave with
frequency , we show it is possible to enhance the intensity of a
group of high harmonics in orders of magnitude. These harmonics have
frequencies about 30% higher than the monochromatic-cutoff frequency, and,
within the three-step-model framework, correspond to a set of electron
trajectories for which tunneling ionization is strongly suppressed. Particular
features in the observed enhancement suggest that the high-frequency field
provides an additional mechanism for the electron to reach the continuum. This
interpretation is supported by a time-frequency analysis of the harmonic yield.
The additional high frequency field permits the control of this group of
harmonics leaving all other sets of harmonics practically unchanged, which is
an advantage over schemes involving only bichromatic fields.Comment: 6 pages RevTex, 5 figures (ps files), Changes in text, figures,
references and equations include
Active Galactic Nuclei at the Crossroads of Astrophysics
Over the last five decades, AGN studies have produced a number of spectacular
examples of synergies and multifaceted approaches in astrophysics. The field of
AGN research now spans the entire spectral range and covers more than twelve
orders of magnitude in the spatial and temporal domains. The next generation of
astrophysical facilities will open up new possibilities for AGN studies,
especially in the areas of high-resolution and high-fidelity imaging and
spectroscopy of nuclear regions in the X-ray, optical, and radio bands. These
studies will address in detail a number of critical issues in AGN research such
as processes in the immediate vicinity of supermassive black holes, physical
conditions of broad-line and narrow-line regions, formation and evolution of
accretion disks and relativistic outflows, and the connection between nuclear
activity and galaxy evolution.Comment: 16 pages, 5 figures; review contribution; "Exploring the Cosmic
Frontier: Astrophysical Instruments for the 21st Century", ESO Astrophysical
Symposia Serie
Stability and collapse of localized solutions of the controlled three-dimensional Gross-Pitaevskii equation
On the basis of recent investigations, a newly developed analytical procedure
is used for constructing a wide class of localized solutions of the controlled
three-dimensional (3D) Gross-Pitaevskii equation (GPE) that governs the
dynamics of Bose-Einstein condensates (BECs). The controlled 3D GPE is
decomposed into a two-dimensional (2D) linear Schr\"{o}dinger equation and a
one-dimensional (1D) nonlinear Schr\"{o}dinger equation, constrained by a
variational condition for the controlling potential. Then, the above class of
localized solutions are constructed as the product of the solutions of the
transverse and longitudinal equations. On the basis of these exact 3D
analytical solutions, a stability analysis is carried out, focusing our
attention on the physical conditions for having collapsing or non-collapsing
solutions.Comment: 21 pages, 14 figure
Measurement of W Polarisation at LEP
The three different helicity states of W bosons produced in the reaction e+
e- -> W+ W- -> l nu q q~ at LEP are studied using leptonic and hadronic W
decays. Data at centre-of-mass energies \sqrt s = 183-209 GeV are used to
measure the polarisation of W bosons, and its dependence on the W boson
production angle. The fraction of longitudinally polarised W bosons is measured
to be 0.218 \pm 0.027 \pm 0.016 where the first uncertainty is statistical and
the second systematic, in agreement with the Standard Model expectation
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