1,498 research outputs found
Giant Vortex Lattice Deformations in Rapidly Rotating Bose-Einstein Condensates
We have performed numerical simulations of giant vortex structures in rapidly
rotating Bose-Einstein condensates within the Gross-Pitaevskii formalism. We
reproduce the qualitative features, such as oscillation of the giant vortex
core area, formation of toroidal density hole, and the precession of giant
vortices, observed in the recent experiment [Engels \emph{et.al.}, Phys. Rev.
Lett. {\bf 90}, 170405 (2003)]. We provide a mechanism which quantitatively
explains the observed core oscillation phenomenon. We demonstrate the clear
distinction between the mechanism of atom removal and a repulsive pinning
potential in creating giant vortices. In addition, we have been able to
simulate the transverse Tkachenko vortex lattice vibrations.Comment: 5 pages, 6 figures; revised description of core oscillation, new
subfigur
Real-time ultrasound elastography: Does it improve B-mode ultrasound characterization of solid breast lesions?
AbstractIntroductionElastography is a non-invasive medical imaging technique that detects tumors based on their stiffness (elasticity). Strain images display the relative stiffness of lesions compared with the stiffness of surrounding tissue as cancerous tumors tend to be many times stiffer than the normal tissue, which “gives” under compression. An image in which different degrees of stiffness show as different shades of light and dark is called an elastogram.PurposeTo prospectively evaluate the sensitivity and specificity of the real-time sonoelastography as compared with B-mode US for distinguishing between benign and malignant solid breast masses. The density of the glandular breast tissue was taken in consideration in addition to the Breast Imaging Reporting and Data System (BI-RADS) categories of the lesions, with biopsy results as the reference standard.MethodsA total of 216 candidate solid lesions (123 benign and 93 malignant) in 188 patients were examined with 2-dimensional ultrasonography, elastosonography and mammography (for 147 patients). The lesions were classified according to the density of the glandular breast tissue into low density group (D1) and a high density group (D2) and were categorized with the BIRADS score. Elastographic images were assigned an elasticity score of 1 to 5 (1–3, benign; 4 and 5, malignant) according to the Multi-Center Team of Study and the strain ratios of the lesions were measured. Concordance between the imaging findings and histopathologic results was documented. Statistical analysis was performed and sensitivity, specificity and positive and negative predictive values for both elastography and conventional sonography were calculated.ResultsElastography showed less sensitivity but higher specificity than conventional sonography in the differentiation of benign from malignant solid lesions: B-mode sonography had sensitivity of 85.1%, specificity of 93.9%, a positive predictive value of 92.5% and a negative predictive value of 87.8%, compared with the sensitivity of 80.1%, specificity of 97.1%, a positive predictive value of 96.8% and a negative predictive value of 82.1% for elastography. Elastography was superior to B-mode US in diagnosing solid lesions in the low density group (D1) (96.6% vs. 92.4% specificity) and less in the dense glandular tissue (97.8% vs. 95.9% specificity).ConclusionsReal-time sonoelastography is an useful technique for the characterization of benign and malignant solid lesions as it increases the diagnostic specificity comparable to B-mode ultrasound, particularly in both ACR 1 and 2, thus reducing the false-positive rate
Systèmes de production et pratiques à risque en agriculture urbaine: Cas du maraîchage dans la ville de Yamoussoukro en Côte d’Ivoire
La question de l’impact de l’agriculture urbaine sur l’environnement revient, fréquemment, au coeur des débats dans la gestion des villes africaines, comme Yamoussoukro. Les inquiétudes liées à la préservation de l’environnement urbain, eu égard à l’usage de pesticides et à la qualité des eaux utilisées, constituent une préoccupation prise en compte dans le cadre de cette étude. Celle-ci porte, aussi bien sur l’évaluation du risque sur l’environnement consécutif à de mauvaises pratiques agricoles que sur l’évaluation du risque sanitaire consécutif à l’usage des eaux usées. Pour atteindre cet objectif, la méthodologie utilisée a été d’une part, la collecte de données, à travers une enquête et d’autre part, des analyses au laboratoire. Il ressort que le maraîchage urbain à Yamoussoukro est une activité informelle. Les cultures pratiquées sont dominées par la laitue, l’oignon vert et la carotte. Le recours aux intrants, notamment les pesticides, est très répandu. Les dispositifs règlementaires sur la distribution des produits agrochimiques ne sont pas appliqués. Ainsi, 74% des pesticides recensés ne sont pas homologués pour les cultures maraîchères, 12% sont très toxiques (classe Ib) et 57% nocifs (classe II). Les risques encourus sont accentués par un traitement chimique qui n’est pas suffisamment maîtrisé. Les analyses physico-chimiques et microbiologiques ont mis en évidence, d’une part, des eaux d’irrigation aux caractéristiques très hétérogènes et d’autre part, une grande diversité microbienne et une contamination de ces eaux, suggérant des risques sanitaires avérés.Mots clés: cultures maraîchères, environnement, risque sanitaire, pesticides, eau
Response of an atomic Bose-Einstein condensate to a rotating elliptical trap
We investigate numerically the response of an atomic Bose-Einstein condensate
to a weakly-elliptical rotating trap over a large range of rotation
frequencies. We analyse the quadrupolar shape oscillation excited by rotation,
and discriminate between its stable and unstable regimes. In the latter case,
where a vortex lattice forms, we compare with experimental observations and
find good agreement. By examining the role of thermal atoms in the process, we
infer that the process is temperature-independent, and show how terminating the
rotation gives control over the number of vortices in the lattice. We also
study the case of critical rotation at the trap frequency, and observe large
centre-of-mass oscillations of the condensate.Comment: 14 pages, 8 figure
Anomalous hydrodynamics and "normal" fluids in rapidly rotating BECs
In rapidly rotating bose systems we show that there is a region of anomalous
hydrodynamics whilst the system is still condensed, which coincides with the
mean field quantum Hall regime. An immediate consequence is the absence of a
normal fluid in any conventional sense. However, even the superfluid
hydrodynamics is not described by conventional Bernoulli and continuity
equations. We show there are kinematic constraints which connect spatial
variations of density and phase, that the positions of vortices are not the
simplest description of the dynamics of such a fluid (despite their utility in
describing the instantaneous state of the condensate) and that the most compact
description allows solution of some illuminating examples of motion. We
demonstrate, inter alia, a very simple relation between vortices and surface
waves. We show the surface waves can form a "normal fluid" which absorbs energy
and angular momentum from vortex motion in the trap. The time scale of this
process is sensitive to the initial configuration of the vortices, which can
lead to long-lived vortex patches - perhaps related to those observed at JILA.Comment: 4 pages; 1 sentence and references modifie
Vortex states of rapidly rotating dilute Bose-Einstein condensates
We show that, in the Thomas-Fermi regime, the cores of vortices in rotating
dilute Bose-Einstein condensates adjust in radius as the rotation velocity,
, grows, thus precluding a phase transition associated with core
overlap at high vortex density. In both a harmonic trap and a rotating
hard-walled bucket, the core size approaches a limiting fraction of the
intervortex spacing. At large rotation speeds, a system confined in a bucket
develops, within Thomas-Fermi, a hole along the rotation axis, and eventually
makes a transition to a giant vortex state with all the vorticity contained in
the hole.Comment: 4 pages, 2 figures, RevTex4. Version as published; discussion
extended, some references added and update
High-Contrast Interference in a Thermal Cloud of Atoms
The coherence properties of a gas of bosonic atoms above the BEC transition
temperature were studied. Bragg diffraction was used to create two spatially
separated wave packets, which interfere during expansion. Given sufficient
expansion time, high fringe contrast could be observed in a cloud of arbitrary
temperature. Fringe visibility greater than 90% was observed, which decreased
with increasing temperature, in agreement with a simple model. When the sample
was "filtered" in momentum space using long, velocity-selective Bragg pulses,
the contrast was significantly enhanced in contrast to predictions
Fifty-fold improvement in the number of quantum degenerate fermionic atoms
We have produced a quantum degenerate Li-6 Fermi gas with up to 7 x 10^7
atoms, an improvement by a factor of fifty over all previous experiments with
degenerate Fermi gases. This was achieved by sympathetic cooling with bosonic
Na-23 in the F=2, upper hyperfine ground state. We have also achieved
Bose-Einstein condensation of F=2 sodium atoms by direct evaporation
Stellar parameters of Be stars observed with X-shooter
Aims. The X-shooter archive of several thousand telluric star spectra was
skimmed for Be and Be-shell stars to derive the stellar fundamental parameters
and statistical properties, in particular for the less investigated late type
Be stars, and the extension of the Be phenomenon into early A stars. Methods.
An adapted version of the BCD method is used, utilizing the Balmer
discontinuity parameters to determine effective temperature and surface
gravity. This method is optimally suited for late B stars. The projected
rotational velocity was obtained by profile fitting to the Mg ii lines of the
targets, and the spectra were inspected visually for the presence of peculiar
features such as the infrared Ca ii triplet or the presence of a double Balmer
discontinuity. The Balmer line equivalent widths were measured, but due to
uncertainties in determining the photospheric contribution are useful only in a
subsample of Be stars for determining the pure emission contribution. Results.
A total of 78 Be stars, mostly late type ones, were identified in the X-shooter
telluric standard star archive, out of which 48 had not been reported before.
The general trend of late type Be stars having more tenuous disks and being
less variable than early type ones is confirmed. The relatively large number
(48) of relatively bright (V > 8.5) additional Be stars casts some doubt on the
statistics of late type Be stars; they are more common than currently thought:
The Be/B star fraction may not strongly depend on spectral subtype.Comment: Accepted for publication in A&
Analysis of a vacuum-based photovoltaic thermal collector
In this study, a new design of photovoltaic thermal (PV/T) collector is proposed. This design uses a vacuum layer above the silicon wafer and not exists in the traditional PV/T collector. This layer is used to decrease the heat loss from the top surface of the PV/T collector. The analysis is conducted using a 3D thermal modeling. The new collector design with the vacuum layer achieved a 26.6% increase in the thermal power while keeping the electrical the same at Reynolds number of 50 and solar radiation of 1000 W/m2. In addition, the degradation of the vacuum pressure slightly influence the thermal performance while increasing the vacuum pressure from 0.01 Pa to 10 Pa. While further increase in the vacuum pressure from 10 Pa to 1.013×105 Pa substantially decreases the gained thermal power with insignificant increase in the electrical power
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