7,022 research outputs found
Theory of Combined Photoassociation and Feshbach Resonances in a Bose-Einstein Condensate
We model combined photoassociation and Feshbach resonances in a Bose-Einstein
condensate, where the shared dissociation continuum allows for quantum
interference in losses from the condensate, as well as a dispersive-like shift
of resonance. A simple analytical model, based on the limit of weakly bound
molecules, agrees well with numerical experiments that explicitly include
dissociation to noncondensate modes. For a resonant laser and an off-resonant
magnetic field, constructive interference enables saturation of the
photoassociation rate at user-friendly intensities, at a value set by the
interparticle distance. This rate limit is larger for smaller condensate
densities and, near the Feshbach resonance, approaches the rate limit for
magnetoassociation alone. Also, we find agreement with the unitary limit--set
by the condensate size--only for a limited range of near-resonant magnetic
fields. Finally, for a resonant magnetic field and an off-resonant laser,
magnetoassociation displays similar quantum interference and a dispersive-like
shift. Unlike photoassociation, interference and the fieldshift in resonant
magnetoassociation is tunable with both laser intensity and detuning. Also, the
dispersive-like shift of the Feshbach resonance depends on the size of the
Feshbach molecule, and is a signature of non-universal physics in a strongly
interacting system.Comment: 10 pages, 5 figures, 82 reference
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In vivo and in vitro models of demyelinating diseases. V. Comparison of the assembly of mouse hepatitis virus, strain JHM, in two murine cell lines.
The developmental sequence of a neurotropic strain (JHM) of mouse hepatitis virus was examined by transmission electron microscopy and immunocytology. The nucleoprotein core of this coronavirus, which contains RNA of positive polarity and is helical in configuration, becomes incorporated into enveloped particles in the same manner as the nucleocapsids of the orthomyxo- and paramyxoviruses. However, JHM virus is assembled intracellularly by budding at surfaces of smooth membranous vacuoles. A comparison of JHM virus replication in L2 and 17Cl-1 cell lines revealed that L2 cells undergo more rapid cytopathology and cease virus production much sooner than 17Cl-l cells. In L2 cells the accumulation of core material appears to continue after the abrupt cessation of virus assembly. This is evident by the massive cytoplasmic accumulation of structure resembling nucleocapsids, which react with hybridoma antibody to the nucleocapsid antigen as demonstrated by the immunoperoxidase procedure. The current findings are consistent with our previously published demonstration, using cells of neural and other deviation, of the fundamental role of the host cell type in regulating the replication and expression of coronaviruses
Shortcut to a Fermi-Degenerate Gas of Molecules via Cooperative Association
We theoretically examine the creation of a Fermi-degenerate gas of molecules
via a photoassociation or Feshbach resonance applied to a degenerate Bose-Fermi
mixture of atoms. This problem raises an interest because, unlike bosons,
fermions in general do not behave cooperatively, so that the collective
conversion of, say, two million atoms into one million molecules is not to be
expected. Nevertheless, we find that the coupled Fermi system displays
collective Rabi-like oscillations and adiabatic passage between atoms and
molecules, thereby mimicking Bose-Einstein statistics. Cooperative association
of a degenerate mixture of Bose and Fermi gases could therefore serve as a
shortcut to a degenerate gas of Fermi molecules.Comment: 4 pages, 2 figures, submitted to PRL; v2: expanded intro, added
discussion on neglect of collisions and when mimicking should occu
Mean-Field Theory of Feshbach-Resonant Interactions in 85Rb Condensates
Recent Feshbach-resonance experiments with 85Rb Bose-Einstein condensates
have led to a host of unexplained results: dramatic losses of condensate atoms
for an across-resonance sweep of the magnetic field, a collapsing condensate
with a burst of atoms emanating from the remnant condensate, increased losses
for decreasing interaction times-- until short times are reached, and seemingly
coherent oscillations between remnant and burst atoms. Using a simple yet
realistic mean-field model, we find that rogue dissociation, molecular
dissociation to noncondensate atom pairs, is strongly implicated as the
physical mechanism responsible for these observations.Comment: v2: numbers changed, not conclusions; 5 pages, 3 figures, submitted
to PR
Atom-molecule coherence in a one-dimensional system
We study a model of one-dimensional fermionic atoms that can bind in pairs to
form bosonic molecules. We show that at low energy, a coherence develops
between the molecule and fermion Luttinger liquids. At the same time, a gap
opens in the spin excitation spectrum. The coherence implies that the order
parameters for the molecular Bose-Einstein Condensation and the atomic BCS
pairing become identical. Moreover, both bosonic and fermionic charge density
wave correlations decay exponentially, in contrast with a usual Luttinger
liquid. We exhibit a Luther-Emery point where the systems can be described in
terms of noninteracting pseudofermions. At this point, we provide closed form
expressions for the density-density response functions.Comment: 5 pages, no figures, Revtex 4; (v2) added a reference to
cond-mat/0505681 where related results are reported; (v3) Expression of
correlation functions given in terms of generalized hypergeometric function
Linear numeral systems
We investigate numeral systems in the lambda calculus; specifically in the linear lambda calculus where terms cannot be copied or erased. Our interest is threefold: representing numbers in the linear calculus, finding constant time arithmetic operations when possible for successor, addition and predecessor, and finally, efficiently encoding subtraction—an operation that is problematic in many numeral systems. This paper defines systems that address these points, and in addition provides a characterisation of linear numeral systems
Hormone replacement therapy after surgery for stage 1 or 2 cutaneous melanoma
A total of 206 women were followed for a minimum of 5 years after primary melanoma surgery to establish if hormone replacement therapy (HRT) adversely affected prognosis. In all, 123 had no HRT and 22 have died of melanoma; 83 had HRT for varying periods and one has died of melanoma. After controlling for known prognostic factors, we conclude that HRT after melanoma does not adversely affect prognosis
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