7,680 research outputs found
On the Shape of the Tail of a Two Dimensional Sand Pile
We study the shape of the tail of a heap of granular material. A simple
theoretical argument shows that the tail adds a logarithmic correction to the
slope given by the angle of repose. This expression is in good agreement with
experiments. We present a cellular automaton that contains gravity, dissipation
and surface roughness and its simulation also gives the predicted shape.Comment: LaTeX file 4 pages, 4 PS figures, also available at
http://pmmh.espci.fr
Infrared spectroscopy of diatomic molecules - a fractional calculus approach
The eigenvalue spectrum of the fractional quantum harmonic oscillator is
calculated numerically solving the fractional Schr\"odinger equation based on
the Riemann and Caputo definition of a fractional derivative. The fractional
approach allows a smooth transition between vibrational and rotational type
spectra, which is shown to be an appropriate tool to analyze IR spectra of
diatomic molecules.Comment: revised + extended version, 9 pages, 6 figure
Homoclinic standing waves in focussing DNLS equations --Variational approach via constrained optimization
We study focussing discrete nonlinear Schr\"{o}dinger equations and present a
new variational existence proof for homoclinic standing waves (bright
solitons). Our approach relies on the constrained maximization of an energy
functional and provides the existence of two one-parameter families of waves
with unimodal and even profile function for a wide class of nonlinearities.
Finally, we illustrate our results by numerical simulations.Comment: new version with revised introduction and improved condition (A3); 16
pages, several figure
Polarization entangled photon-pair source based on a type-II PPLN waveguide emitting at a telecom wavelength
We report the realization of a fiber coupled polarization entangled
photon-pair source at 1310 nm based on a birefringent titanium in-diffused
waveguide integrated on periodically poled lithium niobate. By taking advantage
of a dedicated and high-performance setup, we characterized the quantum
properties of the pairs by measuring two-photon interference in both
Hong-Ou-Mandel and standard Bell inequality configurations. We obtained, for
the two sets of measurements, interference net visibilities reaching nearly
100%, which represent important and competitive results compared to similar
waveguide-based configurations already reported. These results prove the
relevance of our approach as an enabling technology for long-distance quantum
communication.Comment: 13 pages, 4 figures, to appear in New Journal of Physic
Magnetic separation-based blood purification: a promising new approach for the removal of disease-causing compounds?
Bursts in a fiber bundle model with continuous damage
We study the constitutive behaviour, the damage process, and the properties
of bursts in the continuous damage fiber bundle model introduced recently.
Depending on its two parameters, the model provides various types of
constitutive behaviours including also macroscopic plasticity. Analytic results
are obtained to characterize the damage process along the plastic plateau under
strain controlled loading, furthermore, for stress controlled experiments we
develop a simulation technique and explore numerically the distribution of
bursts of fiber breaks assuming infinite range of interaction. Simulations
revealed that under certain conditions power law distribution of bursts arises
with an exponent significantly different from the mean field exponent 5/2. A
phase diagram of the model characterizing the possible burst distributions is
constructed.Comment: 9 pages, 11 figures, APS style, submitted for publicatio
Theory of continuum percolation III. Low density expansion
We use a previously introduced mapping between the continuum percolation
model and the Potts fluid (a system of interacting s-states spins which are
free to move in the continuum) to derive the low density expansion of the pair
connectedness and the mean cluster size. We prove that given an adequate
identification of functions, the result is equivalent to the density expansion
derived from a completely different point of view by Coniglio et al. [J. Phys A
10, 1123 (1977)] to describe physical clustering in a gas. We then apply our
expansion to a system of hypercubes with a hard core interaction. The
calculated critical density is within approximately 5% of the results of
simulations, and is thus much more precise than previous theoretical results
which were based on integral equations. We suggest that this is because
integral equations smooth out overly the partition function (i.e., they
describe predominantly its analytical part), while our method targets instead
the part which describes the phase transition (i.e., the singular part).Comment: 42 pages, Revtex, includes 5 EncapsulatedPostscript figures,
submitted to Phys Rev
Dune formation on the present Mars
We apply a model for sand dunes to calculate formation of dunes on Mars under
the present Martian atmospheric conditions. We find that different dune shapes
as those imaged by Mars Global Surveyor could have been formed by the action of
sand-moving winds occuring on today's Mars. Our calculations show, however,
that Martian dunes could be only formed due to the higher efficiency of Martian
winds in carrying grains into saltation. The model equations are solved to
study saltation transport under different atmospheric conditions valid for
Mars. We obtain an estimate for the wind speed and migration velocity of
barchan dunes at different places on Mars. From comparison with the shape of
bimodal sand dunes, we find an estimate for the timescale of the changes in
Martian wind regimes.Comment: 16 pages, 12 figure
Saltation transport on Mars
We present the first calculation of saltation transport and dune formation on
Mars and compare it to real dunes. We find that the rate at which grains are
entrained into saltation on Mars is one order of magnitude higher than on
Earth. With this fundamental novel ingredient, we reproduce the size and
different shapes of Mars dunes, and give an estimate for the wind velocity on
Mars.Comment: 4 pages, 3 figure
Field transformations and simple models illustrating the impossibility of measuring off-shell effects
In the context of simple models illustrating field transformations in
Lagrangian field theories we discuss the impossibility of measuring off-shell
effects in nucleon-nucleon bremsstrahlung, Compton scattering, and related
processes. To that end we introduce a simple phenomenological Lagrangian
describing nucleon-nucleon bremsstrahlung and perform an appropriate change of
variables leading to different off-shell behavior in the nucleon-nucleon
amplitude as well as the photon-nucleon vertex. As a result we obtain a class
of equivalent Lagrangians, generating identical S-matrix elements, of which the
original Lagrangian is but one representative. We make use of this property in
order to show that what appears as an off-shell effect in an S-matrix element
for one Lagrangian may originate in a contact term from an equivalent
Lagrangian. By explicit calculation we demonstrate for the case of
nucleon-nucleon bremsstrahlung as well as nucleon Compton scattering the
equivalence of observables from which we conclude that off-shell effects cannot
in any unambiguous way be extracted from an S-matrix element. Finally, we also
discuss some implications of introducing off-shell effects on a
phenomenological basis, resulting from the requirement that the description of
one process be consistent with that of other processes described by the same
Lagrangian.Comment: 19 pages, Latex, using RevTe
- …