13,175 research outputs found
Predicting spinor condensate dynamics from simple principles
We study the spin dynamics of quasi-one-dimensional F=1 condensates both at
zero and finite temperatures for arbitrary initial spin configurations. The
rich dynamical evolution exhibited by these non-linear systems is explained by
surprisingly simple principles: minimization of energy at zero temperature, and
maximization of entropy at high temperature. Our analytical results for the
homogeneous case are corroborated by numerical simulations for confined
condensates in a wide variety of initial conditions. These predictions compare
qualitatively well with recent experimental observations and can, therefore,
serve as a guidance for on-going experiments.Comment: 4 pages, 2 figures. v3: matches version appeared in PR
An ab initio study of magneto-electric coupling of
The present paper proposes the direct calculation of the microscopic
contributions to the magneto-electric coupling, using ab initio methods. The
electrostrictive and the Dzyaloshinskii-Moriya contributions were evaluated
individually. For this purpose a specific method was designed, combining DFT
calculations and embedded fragments, explicitely correlated, quantum chemical
calculations. This method allowed us to calculate the evolution of the magnetic
couplings as a function of an applied electric field. We found that in the Dzyaloshinskii-Moriya contribution to the magneto-electric effect
is three orders of magnitude weaker than the electrostrictive contribution.
Strictive effects are thus dominant in the magnetic exchange evolution under an
applied electric field, and by extension on the magneto-electric effect. These
effects remain however quite small and the modifications of the magnetic
excitations under an applied electric field will be difficult to observe
experimentally. Another important conclusion is that the amplitude of the
magneto-electric effect is very small. Indeed, it can be shown that the linear
magneto-electric tensor is null due to the inter-layer symmetry operations.Comment: J. Phys. Cond. Matter 201
Spin mixing in colliding spinor condensates: formation of an effective barrier
The dynamics of F=1 spinor condensates initially prepared in a double-well
potential is studied in the mean field approach. It is shown that a small seed
of atoms on a system with initially well separated m=1 and m=-1
condensates has a dramatic effect on their mixing dynamics, acting as an
effective barrier for a remarkably long time. We show that this effect is due
to the spinor character of the system, and provides an observable example of
the interplay between the internal spin dynamics and the macroscopic evolution
of the magnetization in a spinor Bose-Einstein condensate.Comment: Accepted for publication at the Europhysics Letter
The different origins of magnetic fields and activity in the Hertzsprung gap stars, OU Andromedae and 31 Comae
Context: When crossing the Hertzsprung gap, intermediate-mass stars develop a
convective envelope. Fast rotators on the main sequence, or Ap star
descendants, are expected to become magnetic active subgiants during this
evolutionary phase. Aims: We compare the surface magnetic fields and activity
indicators of two active, fast rotating red giants with similar masses and
spectral class but diferent rotation rates - OU And (Prot=24.2 d) and 31 Com
(Prot=6.8 d) - to address the question of the origin of their magnetism and
high activity.
Methods: Observations were carried out with the Narval spectropolarimeter in
2008 and 2013.We used the least squares deconvolution technique to extract
Stokes V and I profiles to detect Zeeman signatures of the magnetic field of
the stars. We provide Zeeman-Doppler imaging, activity indicator monitoring,
and a precise estimation of stellar parameters. We use stellar evolutionary
models to infer the evolutionary status and the initial rotation velocity on
the main sequence.
Results: The detected magnetic field of OU And is a strong one. Its
longitudinal component Bl reaches 40 G and presents an about sinusoidal
variation with reversal of the polarity. The magnetic topology of OU And is
dominated by large scale elements and is mainly poloidal with an important
dipole component, and a significant toroidal component. The detected magnetic
field of 31 Com is weaker, with a magnetic map showing a more complex field
geometry, and poloidal and toroidal components of equal contributions. The
evolutionary models show that the progenitors of OU And and 31 Com must have
been rotat
Conclusions: OU And appears to be the probable descendant of a magnetic Ap
star, and 31 Com the descendant of a relatively fast rotator on the main
sequence.Comment: 16 pages, 12 figure
Intestinal absorption of macromolecules during viral enteritis: an experimental study on rotavirus-infected conventional and germ-free mice.
Epithelial transport and degradation of horseradish peroxidase (HRP), a macromolecular tracer, was studied in conventional and germ-free suckling mice following an experimental infection with rotavirus. Conventional and germ-free mice developed diarrhea from days 2 to 8 postinfection (pi), with growth failure. In mucosal homogenates, infectious virus detected by immunofluorescence on MA 104 cells was present from day 2 through day 8 pi in germ-free mice, but persisted longer (day 13 pi) in conventional mice. Only mild histological lesions were observed during diarrhea, but obvious macrovacuolation of epithelial cells and increased cellular density occurred during the convalescence period (days 9 to 13 pi). Intact and degraded HRP fluxes from mucosa to serosa were measured in vitro on segments of jejunum mounted in Ussing chambers. Both groups of mice developed increased HRP permeability during the experimental period, but at different times after inoculation: during the diarrheal period (days 2 and 3 pi) conventional mouse epithelium absorbed five times more HRP than noninfected controls and during the convalescence period (days 9 to 13 pi) HRP absorption in germ-free mice rose 10-fold as compared to its level before infection. In both cases, this increase in HRP permeability was entirely due to an increase in intact HRP absorption, probably via a transcellular route, and occurred without any alteration in degraded HRP transport. These results indicate that in mice, rotavirus infection causes a transient rise in gut permeability to undegraded proteins. The intestinal microflora seems to affect the timing, magnitude, and duration of this increased permeability
Artificially induced positronium oscillations in a two-sheeted spacetime: consequences on the observed decay processes
Following recent theoretical results, it is suggested that positronium (Ps)
might undergo spontaneous oscillations between two 4D spacetime sheets whenever
subjected to constant irrotational magnetic vector potentials. We show that
these oscillations that would come together with o-Ps/p-Ps oscillations should
have important consequences on Ps decay rates. Experimental setup and
conditions are also suggested for demonstrating in non accelerator experiments
this new invisible decay mode.Comment: 9 pages, 2 figures. Minor form correction. Accepted for publication
in Int. J. of Modern Physics
Conductance fluctuations in metallic nanogaps made by electromigration
We report on low temperature conductance measurements of gold nanogaps
fabricated by controlled electromigration. Fluctuations of the conductance due
to quantum interferences and depending both on bias voltage and magnetic field
are observed. By analyzing the voltage and magnetoconductance correlation
functions we determine the type of electron trajectories generating the
observed quantum interferences and the effective characteristic time of phase
coherence in our device.Comment: 5 pages, 4 figures, to appear in J. Appl. Phy
A polarity reversal in the large-scale magnetic field of the rapidly rotating Sun HD 190771
Aims. We investigate the long-term evolution of the large-scale photospheric
magnetic field geometry of the solar-type star HD 190771. With fundamental
parameters very close to those of the Sun except for a shorter rotation period
of 8.8 d, HD 190771 provides us with a first insight into the specific impact
of the rotation rate in the dynamo generation of magnetic fields in 1
stars.
Methods. We use circularly polarized, high-resolution spectra obtained with
the NARVAL spectropolarimeter (Observatoire du Pic du Midi, France) and compute
cross-correlation line profiles with high signal-to-noise ratio to detect
polarized Zeeman signatures. From three phase-resolved data sets collected
during the summers of 2007, 2008, and 2009, we model the large-scale
photospheric magnetic field of the star by means of Zeeman-Doppler imaging and
follow its temporal evolution.
Results. The comparison of the magnetic maps shows that a polarity reversal
of the axisymmetric component of the large-scale magnetic field occurred
between 2007 and 2008, this evolution being observed in both the poloidal and
toroidal magnetic components. Between 2008 and 2009, another type of global
evolution occured, characterized by a sharp decrease of the fraction of
magnetic energy stored in the toroidal component. These changes were not
accompanied by significant evolution in the total photospheric magnetic energy.
Using our spectra to perform radial velocity measurements, we also detect a
very low-mass stellar companion to HD 190771.Comment: Accepted by Astronomy and Astrophysics (Letter to the Editor
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