423 research outputs found
Kicked-rotor quantum resonances in position space: Application to situations of experimental interest
In this work we apply the formalism developed in [M. Lepers \emph{et al}.,
Phys. Rev. A \textbf{77}, 043628 (2008)] to different initial conditions
corresponding to systems usually met in real-life experiments, and calculate
the observable quantities that can be used to characterize the dynamics of the
system. The position space point of view allows highly intuitive pictures of
the physics at play.Comment: accepted in Eur. Phys. J.
Collective modes of trapped Fermi gases with in-medium interaction
Due to Pauli blocking of intermediate states, the scattering matrix (or
matrix) of two fermionic atoms in a Fermi gas becomes different from that of
two atoms in free space. This effect becomes particularly important near a
Feshbach resonance, where the interaction in free space is very strong but
becomes effectively suppressed in the medium. We calculate the in-medium
matrix in ladder approximation and study its effects on the properties of
collective modes of a trapped gas in the normal-fluid phase. We introduce the
in-medium interaction on both sides of the Boltzmann equation, namely in the
calculation of the mean field and in the calculation of the collision rate.
This allows us to explain the observed upward shift of the frequency of the
quadrupole mode in the collisionless regime. By including the mean field, we
also improve considerably the agreement with the measured temperature
dependence of frequency and damping rate of the scissors mode, whereas the use
of the in-medium cross section deteriorates the description, in agreement with
previous work.Comment: 17 page
Long-range interactions between polar bialkali ground-state molecules in arbitrary vibrational levels
We have calculated the isotropic coefficients characterizing the
long-range van der Waals interaction between two identical heteronuclear
alkali-metal diatomic molecules in the same arbitrary vibrational level of
their ground electronic state . We consider the ten species made
up of Li, Na, K, Rb and Cs. Following our
previous work [M.~Lepers \textit{et.~al.}, Phys.~Rev.~A \textbf{88}, 032709
(2013)] we use the sum-over-state formula inherent to the second-order
perturbation theory, composed of the contributions from the transitions within
the ground state levels, from the transition between ground-state and excited
state levels, and from a crossed term. These calculations involve a combination
of experimental and quantum-chemical data for potential energy curves and
transition dipole moments. We also investigate the case where the two molecules
are in different vibrational levels and we show that the Moelwyn-Hughes
approximation is valid provided that it is applied for each of the three
contributions to the sum-over-state formula. Our results are particularly
relevant in the context of inelastic and reactive collisions between ultracold
bialkali molecules, in deeply bound or in Feshbach levels
Photoassociation of a cold atom-molecule pair: long-range quadrupole-quadrupole interactions
The general formalism of the multipolar expansion of electrostatic
interactions is applied to the calculation the potential energy between an
excited atom (without fine structure) and a ground state diatomic molecule at
large separations. Both partners exhibit a permanent quadrupole moment, so that
their mutual quadrupole-quadrupole long-range interaction is attractive enough
to bind trimers. Numerical results are given for an excited Cs(6P) atom and a
ground state Cs2 molecule. The prospects for achieving photoassociation of a
cold atom/dimer pair is thus discussed and found promising. The formalism can
be easily generalized to the long-range interaction between molecules to
investigate the formation of cold tetramers.Comment: 5 figure
Numerical solution of the Boltzmann equation for the collective modes of trapped Fermi gases
We numerically solve the Boltzmann equation for trapped fermions in the
normal phase using the test-particle method. After discussing a couple of tests
in order to estimate the reliability of the method, we apply it to the
description of collective modes in a spherical harmonic trap. The numerical
results are compared with those obtained previously by taking moments of the
Boltzmann equation. We find that the general shape of the response function is
very similar in both methods, but the relaxation time obtained from the
simulation is significantly longer than that predicted by the method of
moments. It is shown that the result of the method of moments can be corrected
by including fourth-order moments in addition to the usual second-order ones
and that this method agrees very well with our numerical simulations.Comment: 13 pages, 8 figures, accepted for publication in Phys. Rev.
The expanding roles of the Sda/Cad carbohydrate antigen and its cognate glycosyltransferase B4GALNT2
Background The histo-blood group antigens are carbohydrate structures present in tissues and body fluids, which contribute to the definition of the individual immunophenotype. One of these, the Sda antigen, is expressed on the surface of erythrocytes and in secretions of the vast majority of the Caucasians and other ethnic groups. Scope of review We describe the multiple and unsuspected aspects of the biology of the Sda antigen and its biosynthetic enzyme \u3b21,4-N-acetylgalactosaminyltransferase 2 (B4GALNT2) in various physiological and pathological settings. Major conclusions The immunodominant sugar of the Sda antigen is a \u3b21,4-linked N-acetylgalactosamine (GalNAc). Its cognate glycosyltransferase B4GALNT2 displays a restricted pattern of tissue expression, is regulated by unknown mechanisms - including promoter methylation, and encodes at least two different proteins, one of which with an unconventionally long cytoplasmic portion. In different settings, the Sda antigen plays multiple and unsuspected roles. 1) In colon cancer, its dramatic down-regulation plays a potential role in the overexpression of sialyl Lewis antigens, increasing metastasis formation. 2) It is involved in the lytic function of murine cytotoxic T lymphocytes. 3) It prevents the development of muscular dystrophy in various dystrophic murine models, when overexpressed in muscular fibers. 4) It regulates the circulating half-life of the von Willebrand factor (vWf), determining the onset of a bleeding disorder in a murine model. General significance The expression of the Sda antigen has a wide impact on the physiology and the pathology of different biological systems
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