479 research outputs found
Three-dimensionality in quasi-two dimensional flows: recirculations and barrel effects
A scenario is put forward for the appearance of three-dimensionality both in
quasi-2D rotating flows and quasi-2D magnetohydrodynamic (MHD) flows. We show
that 3D recirculating flows and currents originate in wall boundary layers and
that, unlike in ordinary hydrodynamic flows, they cannot be ignited by
confinement alone. They also induce a second form of three-dimensionality with
quadratic variations of velocities and current across the channel. This
scenario explains both the common tendency of these flows to two-dimensionality
and the mechanisms of the recirculations through a single formal analogy
covering a wide class of flow including rotating and MHD flows. These
trans-disciplinary effects are thus active in atmospheres, oceans or the
cooling blankets of nuclear fusion reactors.Comment: 6 pages, 1 Figur
Vortex nucleation through edge states in finite Bose-Einstein condensates
We study the vortex nucleation in a finite Bose-Einstein condensate. Using a
set of non-local and chiral boundary conditions to solve the
Schrdinger equation of non-interacting bosons in a rotating trap, we
obtain a quantitative expression for the characteristic angular velocity for
vortex nucleation in a condensate which is found to be 35% of the transverse
harmonic trapping frequency.Comment: 24 pages, 8 figures. Both figures and the text have been revise
Mesoscopic scattering of spin s particles
Quantum effects in weakly disordered systems are governed by the properties
of the elementary interaction between propagating particles and impurities.
Long range mesoscopic effects due to multiple scattering are derived by
iterating the single scattering vertex, which has to be appropriately
diagonalized. In the present contribution, we present a systematic and detailed
diagonalisation of the diffuson and cooperon vertices responsible for weak
localisation effects. We obtain general expressions for eigenvalues and
projectors onto eigenmodes, for any spin and arbitrary elementary interaction
with impurities. This description provides a common frame for a unified theory
of mesoscopic spin physics for electrons, photons, and other quantum particles.
We treat in detail the case of spin-flip scattering of electrons by freely
orientable magnetic impurities and briefly review the case of photon scattering
from degenerate dipole transitions in cold atomic gases.Comment: published version, with a new figure and new section
Hanbury Brown and Twiss Correlations of Anderson Localized Waves
When light waves propagate through disordered photonic lattices, they can
eventually become localized due to multiple scattering effects. Here we show
experimentally that while the evolution and localization of the photon density
distribution is similar in the two cases of diagonal and off-diagonal disorder,
the density-density correlation carries a distinct signature of the type of
disorder. We show that these differences reflect a symmetry in the spectrum and
eigenmodes that exists in off-diagonally disordered lattices but is absent in
lattices with diagonal disorder.Comment: 4 pages, 3 figures, comments welcom
Localization of Matter Waves in 2D-Disordered Optical Potentials
We consider ultracold atoms in 2D-disordered optical potentials and calculate
microscopic quantities characterizing matter wave quantum transport in the
non-interacting regime. We derive the diffusion constant as function of all
relevant microscopic parameters and show that coherent multiple scattering
induces significant weak localization effects. In particular, we find that even
the strong localization regime is accessible with current experimental
techniques and calculate the corresponding localization length.Comment: 4 pages, 3 figures, figures changed, references update
Coherent Backscattering of light in a magnetic field
This paper describes how coherent backscattering is altered by an external
magnetic field. In the theory presented, magneto-optical effects occur inside
Mie scatterers embedded in a non-magnetic medium. Unlike previous theories
based on point-like scatterers, the decrease of coherent backscattering is
obtained in leading order of the magnetic field using rigorous Mie theory. This
decrease is strongly enhanced in the proximity of resonances, which cause the
path length of the wave inside a scatterer to be increased. Also presented is a
novel analysis of the shape of the backscattering cone in a magnetic field.Comment: 27 pages, 5 figures, Revtex, to appear in Phys. Rev.
Multiple scattering of photons by atomic hyperfine multiplets
Mesoscopic interference effects in multiple scattering of photons depend
crucially on the internal structure of the scatterers. In the present article,
we develop the analytical theory of multiple photon scattering by cold atoms
with arbitrary internal hyperfine multiplets. For a specific application, we
calculate the enhancement factor of elastic coherent backscattering as a
function of detuning from an entire hyperfine multiplet of neighboring
resonances that cannot be considered isolated. Our theory permits to understand
why atoms behave differently from classical Rayleigh point-dipole scatterers,
and how the classical description is recovered for larger but still microscopic
objects like molecules or clusters.Comment: minor changes, published versio
Universal conductance fluctuations in epitaxial GaMnAs ferromagnets: structural and spin disorder
Mesoscopic transport measurements reveal a large effective phase coherence
length in epitaxial GaMnAs ferromagnets, contrary to usual 3d-metal
ferromagnets. Universal conductance fluctuations of single nanowires are
compared for epilayers with a tailored anisotropy. At large magnetic fields,
quantum interferences are due to structural disorder only, and an unusual
behavior related to hole-induced ferromagnetism is evidenced, for both quantum
interferences and decoherence. At small fields, phase coherence is shown to
persist down to zero field, even in presence of magnons, and an additional spin
disorder contribution to quantum interferences is observed under domain walls
nucleation.Comment: 15 pages, 4 figure
Anomalous Coherent Backscattering of Light from Opal Photonic Crystals
We studied coherent backscattering (CBS) of light from opal photonic crystals
in air at different incident inclination angles, wavelengths and along various
[hkl] directions inside the opals. Similar to previously obtained CBS cones
from various random media, we found that when Bragg condition with the incident
light beam is not met then the CBS cones from opals show a triangular line
shape in excellent agreement with light diffusion theory. At Bragg condition,
however, we observed a dramatic broadening of the opal CBS cones that depends
on the incident angle and [hkl] direction. This broadening is explained as due
to the light intensity decay in course of propagation along the Bragg direction
{\em before the first} and {\em after the last} scattering events. We modified
the CBS theory to incorporate the attenuation that results from the photonic
band structure of the medium. Using the modified theory we extract from our CBS
data the light mean free path and Bragg attenuation length at different [hkl].
Our study shows that CBS measurements are a unique experimental technique to
explore photonic crystals with disorder, when other spectroscopical methods
become ambiguous due to disorder-induced broadening.Comment: 10 pages, 5 figure
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