1,310 research outputs found
Momentum transferred to a trapped Bose-Einstein condensate by stimulated light scattering
The response of a trapped Bose-Einstein condensed gas to a density
perturbation generated by a two-photon Bragg pulse is investigated by solving
the time-dependent Gross-Pitaevskii equation. We calculate the total momentum
imparted to the condensate as a function of both the time duration of the pulse
and the frequency difference of the two laser beams. The role of the dynamic
response function in characterizing the time evolution of the system is pointed
out, with special emphasis to the phonon regime. Numerical simulations are
compared with the predictions of local density approximation. The relevance of
our results for the interpretation of current experiments is also discussed.Comment: 7 pages, 3 postscript figure
Physical properties, starspot activity, orbital obliquity, and transmission spectrum of the Qatar-2 planetary system from multi-colour photometry
We present seventeen high-precision light curves of five transits of the
planet Qatar-2b, obtained from four defocussed 2m-class telescopes. Three of
the transits were observed simultaneously in the SDSS griz passbands using the
seven-beam GROND imager on the MPG/ESO 2.2-m telescope. A fourth was observed
simultaneously in Gunn grz using the CAHA 2.2-m telescope with BUSCA, and in r
using the Cassini 1.52-m telescope. Every light curve shows small anomalies due
to the passage of the planetary shadow over a cool spot on the surface of the
host star. We fit the light curves with the prism+gemc model to obtain the
photometric parameters of the system and the position, size and contrast of
each spot. We use these photometric parameters and published spectroscopic
measurements to obtain the physical properties of the system to high precision,
finding a larger radius and lower density for both star and planet than
previously thought. By tracking the change in position of one starspot between
two transit observations we measure the orbital obliquity of Qatar-2 b to be
4.3 \pm 4.5 degree, strongly indicating an alignment of the stellar spin with
the orbit of the planet. We calculate the rotation period and velocity of the
cool host star to be 11.4 \pm 0.5 d and 3.28 \pm 0.13 km/s at a colatitude of
74 degree. We assemble the planet's transmission spectrum over the 386-976 nm
wavelength range and search for variations of the measured radius of Qatar-2 b
as a function of wavelength. Our analysis highlights a possible H2/He Rayleigh
scattering in the blue.Comment: 20 pages, 14 figures, to appear in Monthly Notices of the Royal
Astronomical Societ
Computer-assisted placement technique in hip resurfacing arthroplasty: improvement in accuracy?
Freehand positioning of the femoral drill guide is difficult during hip resurfacing and the surgeon is often unsure of the implant position achieved peroperatively. The purpose of this study was to find out whether, by using a navigation system, acetabular and femoral component positioning could be made easier and more precise. Eighteen patients operated on by the same surgeon were matched by sex, age, BMI, diagnosis and ASA score (nine patients with computer assistance, nine with the regular ancillary). Pre-operative planning was done on standard AP and axial radiographs with CT scan views for the computer-assisted operations. The final position of implants was evaluated by the same radiographs for all patients. The follow-up was at least 1year. No difference between both groups in terms of femoral component position was observed (p > 0.05). There was also no difference in femoral notching. A trend for a better cup position was observed for the navigated hips, especially for cup anteversion. There was no additional operating time for the navigated hips. Hip navigation for resurfacing surgery may allow improved visualisation and hip implant positioning, but its advantage probably will be more obvious with mini-incisions than with regular incision surger
Kinetic theory and dynamic structure factor of a condensate in the random phase approximation
We present the microscopic kinetic theory of a homogeneous dilute Bose
condensed gas in the generalized random phase approximation (GRPA), which
satisfies the following requirements: 1) the mass, momentum and energy
conservation laws; 2) the H-theorem; 3) the superfluidity property and 4) the
recovery of the Bogoliubov theory at zero temperature \cite{condenson}. In this
approach, the condensate influences the binary collisional process between the
two normal atoms, in the sense that their interaction force results from the
mediation of a Bogoliubov collective excitation traveling throughout the
condensate. Furthermore, as long as the Bose gas is stable, no collision
happens between condensed and normal atoms. In this paper, we show how the
kinetic theory in the GRPA allows to calculate the dynamic structure factor at
finite temperature and when the normal and superfluid are in a relative motion.
The obtained spectrum for this factor provides a prediction which, compared to
the experimental results, allows to validate the GRPA.
PACS numbers:03.75.Hh, 03.75.Kk, 05.30.-dComment: 6 pages, 1 figures, QFS2004 conferenc
Multi-minicore Disease Revisited.
Multi-minicore disease (MmD) is an infrequent congenital myopathy, defined by structural changes in optic and electron microscopy, namely, multiple small areas lacking oxidative enzyme activity and focal disorganization of contractile proteins involving at most a few sarcomeres. The classical form of the disease manifests as more or less severe hypotonia and generalized weakness with predominance in axial and proximal limb muscles. Clinical variants also exist. Usually MmD is inherited as an autosomal recessive trait. Genetic heterogeneity is recognized and up to now mutations in the genes of RYR1 and SEPN1 have been detected. We record three unrelated cases of MmD. Case 1, with the classical benign form, was followed-up for 15 years. Case 2, presenting pharyngolaryngeal involvement and severe delay of head control, improved gradually, until independent gait was acquired at age of six years. A moderate restriction of daily life activities remains. Case 3, of antenatal-onset, was expressed by arthrogryposis of hands, predominance of scapular girdle deficit and a stable course after ten years on physiotherapy. All cases were selected by the characteristic morphological abnormalities in biceps brachii samples, including electron microscopy. Emphasis is given to case 2 due to type 1 fiber uniformity and mild endomysial fibrosis, posing a difficult differential diagnosis with congenital muscular dystrophy were it not for the significant number of multi-minicores.62935-
Quantized vortices and collective oscillations of a trapped Bose condensed gas
Using a sum rule approach we calculate the frequency shifts of the quadrupole
oscillations of a harmonically trapped Bose gas due to the presence of a
quantized vortex. Analytic results are obtained for positive scattering lengths
and large N where the shift relative to excitations of opposite angular
momentum is found to be proportional to the quantum circulation of the vortex
and to decrease as N^{-2/5}. Results are also given for smaller values of N
covering the transition between the ideal gas and the Thomas-Fermi limit. For
negative scattering lengths we predict a macroscopic instability of the vortex.
The splitting of the collective frequencies in toroidal configurations is also
discussed.Comment: Rextex, 4 pages, 1 postscript figur
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