506 research outputs found
Critical velocity for a toroidal Bose-Einstein condensate flowing through a barrier
We consider the setup employed in a recent experiment (Ramanathan et al 2011
Phys. Rev. Lett. 106 130401) devoted to the study of the instability of the
superfluid flow of a toroidal Bose-Einstein condensate in presence of a
repulsive optical barrier. Using the Gross-Pitaevskii mean-field equation, we
observe, consistently with what we found in Piazza et al (2009 Phys. Rev. A 80
021601), that the superflow with one unit of angular momentum becomes unstable
at a critical strength of the barrier, and decays through the mechanism of
phase slippage performed by pairs of vortex-antivortex lines annihilating.
While this picture qualitatively agrees with the experimental findings, the
measured critical barrier height is not very well reproduced by the
Gross-Pitaevskii equation, indicating that thermal fluctuations can play an
important role (Mathey et al 2012 arXiv:1207.0501). As an alternative
explanation of the discrepancy, we consider the effect of the finite resolution
of the imaging system. At the critical point, the superfluid velocity in the
vicinity of the obstacle is always of the order of the sound speed in that
region, . In particular, in the hydrodynamic regime
(not reached in the above experiment), the critical point is determined by
applying the Landau criterion inside the barrier region. On the other hand, the
Feynman critical velocity is much lower than the observed critical
velocity. We argue that this is a general feature of the Gross-Pitaevskii
equation, where we have with being a
small parameter of the model. Given these observations, the question still
remains open about the nature of the superfluid instability.Comment: Extended versio
Mathematical formulation of a dynamical system with dry friction subjected to external forces
We consider the response of a one-dimensional system with friction. S.W. Shaw
(Journal of Sound and Vibration, 1986) introduced the set up of different
coefficients for the static and dynamic phases (also called stick and slip
phases). He constructs a step by step solution, corresponding to an harmonic
forcing. In this paper, we show that the theory of variational inequalities
provides an elegant and synthetic approach to obtain the existence and
uniqueness of the solution, avoiding the step by step construction. We then
apply the theory to a real structure with real data and show that the model is
quite accurate. In our case, the forcing motion comes from dilatation, due to
temperature
Noise Correlations in low-dimensional systems of ultracold atoms
We derive relations between standard order parameter correlations and the
noise correlations in time of flight images, which are valid for systems with
long range order as well as low dimensional systems with algebraic decay of
correlations. Both Bosonic and Fermionic systems are considered. For one
dimensional Fermi systems we show that the noise correlations are equally
sensitive to spin, charge and pairing correlations and may be used to
distinguish between fluctuations in the different channels. This is in contrast
to linear response experiments, such as Bragg spectroscopy, which are only
sensitive to fluctuations in the particle-hole channel (spin or charge). For
Bosonic systems we find a sharp peak in the noise correlation at opposite
momenta that signals pairing correlations in the depletion cloud. In a
condensate with true long range order, this peak is a delta function and we can
use Bogoliubov theory to study its temperature dependence. Interestingly we
find that it is enhanced with temperature in the low temperature limit. In one
dimensional condensates with only quasi-long range (i.e. power-law) order the
peak in the noise correlations also broadens to a power-law singularity.Comment: 14 pages, 7 figure
Phase fluctuations in anisotropic Bose condensates: from cigars to rings
We study the phase-fluctuating condensate regime of ultra-cold atoms trapped
in a ring-shaped trap geometry, which has been realized in recent experiments.
We first consider a simplified box geometry, in which we identify the
conditions to create a state that is dominated by thermal phase-fluctuations,
and then explore the experimental ring geometry. In both cases we demonstrate
that the requirement for strong phase fluctuations can be expressed in terms of
the total number of atoms and the geometric length scales of the trap only. For
the ring-shaped trap we discuss the zero temperature limit in which a
condensate is realized where the phase is fluctuating due to interactions and
quantum fluctuations. We also address possible ways of detecting the phase
fluctuating regime in ring condensates.Comment: 10 pages, 5 figures, minor edit
Intrinsic Photoconductivity of Ultracold Fermions in Optical Lattices
We report on the experimental observation of an analog to a persistent
alternating photocurrent in an ultracold gas of fermionic atoms in an optical
lattice. The dynamics is induced and sustained by an external harmonic
confinement. While particles in the excited band exhibit long-lived
oscillations with a momentum dependent frequency a strikingly different
behavior is observed for holes in the lowest band. An initial fast collapse is
followed by subsequent periodic revivals. Both observations are fully explained
by mapping the system onto a nonlinear pendulum.Comment: 5+7 pages, 4+4 figure
Maternal neurofascin-specific autoantibodies bind to structures of the fetal nervous system during pregnancy, but have no long term effect on development in the rat
Neurofascin was recently reported as a target for axopathic autoantibodies in patients with multiple sclerosis (MS), a response that will exacerbate axonal pathology and disease severity in an animal model of multiple sclerosis. As transplacental transfer of maternal autoantibodies can permanently damage the developing nervous system we investigated whether intrauterine exposure to this neurofascin-specific response had any detrimental effect on white matter tract development. To address this question we intravenously injected pregnant rats with either a pathogenic anti-neurofascin monoclonal antibody or an appropriate isotype control on days 15 and 18 of pregnancy, respectively, to mimic the physiological concentration of maternal antibodies in the circulation of the fetus towards the end of pregnancy. Pups were monitored daily with respect to litter size, birth weight, growth and motor development. Histological studies were performed on E20 embryos and pups sacrificed on days 2, 10, 21, 32 and 45 days post partum. Results: Immunohistochemistry for light and confocal microscopy confirmed passively transferred anti-neurofascin antibody had crossed the placenta to bind to distinct structures in the developing cortex and cerebellum. However, this did not result in any significant differences in litter size, birth weight, or general physical development between litters from control mothers or those treated with the neurofascin-specific antibody. Histological analysis also failed to identify any neuronal or white matter tract abnormalities induced by the neurofascin-specific antibody. Conclusions: We show that transplacental transfer of circulating anti-neurofascin antibodies can occur and targets specific structures in the CNS of the developing fetus. However, this did not result in any pre- or post-natal abnormalities in the offspring of the treated mothers. These results assure that even if anti-neurofascin responses are detected in pregnant women with multiple sclerosis these are unlikely to have a negative effect on their children
Review: ‘Gimme five’: future challenges in multiple sclerosis. ECTRIMS Lecture 2009
This article is based on the ECTRIMS lecture given at the 25th ECTRIMS meeting which was held in Düsseldorf, Germany, from 9 to 12 September 2009. Five challenges have been identified: (1) safeguarding the principles of medical ethics; (2) optimizing the risk/benefit ratio; (3) bridging the gap between multiple sclerosis and experimental autoimmune encephalitis; (4) promoting neuroprotection and repair; and (5) tailoring multiple sclerosis therapy to the individual patient. Each of these challenges will be discussed and placed in the context of current research into the pathogenesis and treatment of multiple sclerosis
Intramyocardial hemorrhage and microvascular obstruction after primary percutaneous coronary intervention
Reperfusion may cause intramyocardial hemorrhage (IMH) by extravasation of erythrocytes through severely damaged endothelial walls. The purpose of the study was to evaluate the clinical significance of IMH in relation to infarct size, microvascular obstruction (MVO) and function in patients after primary percutaneous intervention. Forty-five patients underwent cardiovascular MR imaging (CMR) 1 week and 4 months after primary stenting for a first acute myocardial infarction. T2-weighted spin-echo imaging (T2W) was used to assess infarct related edema and IMH, and delayed enhancement (DE) was used to assess infarct size and MVO. Cine CMR was used to assess left ventricular volumes and function at baseline and at 4 months follow-up. In 22 (49%) patients, IMH was detected as areas of attenuated signal in the core of the high signal intensity region on T2W images. Patients with IMH had larger infarcts, higher left ventricular volumes and lower ejection fraction. Contrast-to-noise ratio (CNR) between hyperintense periphery and the hypo-intense core of the T2W ischemic area correlated to peak CKMB, total infarct size and MVO size. Using univariable analysis, CNR predicted ejection fraction at baseline (β = −0.62, P = 0.003) and follow-up (β = −0.84, P < 0.001). However, after multivariable analysis, baseline ejection fraction and presence of MVO were the only parameters that predicted functional changes at follow-up. IMH was found in the majority of patients with MVO after reperfused myocardial infarction. It was closely related to markers of infarct size, MVO and function, but did not have prognostic significance beyond MVO
Benzofuran-fused Phosphole: Synthesis, Electronic, and Electroluminescence Properties
International audienceA synthetic route to novel benzofuran-fused phosphole derivatives 3-5 is described. These compounds showed optical and electrochemical properties that differ from their benzothiophene analog. Preliminary results show that 4 can be used as an emitter in OLEDs, illustrating the potential of these new compounds for opto-electronic applications
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