139 research outputs found
Formation of molecules in an expanding Bose-Einstein condensate
A mean field theory of expanding hybrid atom-molecule Bose-Einstein
condensates is applied to the recent MPI experiments on Rb that
demonstrated the formation of ultracold molecules due to Feshbach resonance.
The subsequent dissociation of the molecules is treated using a non-mean-field
parametric approximation. The latter method is also used in determining optimal
conditions for the formation of molecular BECComment: 5 pages with 5 figure
Formation of molecules from a Cs Bose-Einstein condensate
Conversion of an expanding Bose-Einstein condensate of Cs atoms to a
molecular one with an efficiency of more than 30% was observed recently in
experiments by M. Mark et al., Europhys. Lett. 69, 706 (2005). The theory
presented here describes the experimental results. Values of resonance strength
of 8 mG and rate coefficients for atom-molecule deactivation of cms and molecule-molecule one of
cms are estimated by a fit of the theoretical results to the
experimental data. Near the resonance, where the highest conversion efficiency
was observed, the results demonstrate strong sensitivity to the magnetic field
ripple and inhomogeneity. A conversion efficiency of about 60% is predicted by
non-mean-field calculations for the densities and sweep rates lower than the
ones used in the experiments.Comment: 9 pages, 10 figure
One-dimensional Bose chemistry: effects of non-integrability
Three-body collisions of ultracold identical Bose atoms under tight
cylindrical confinement are analyzed. A Feshbach resonance in two-body
collisions is described by a two-channel zero-range interaction. Elimination of
the closed channel in the three-body problem reduces the interaction to a
one-channel zero-range one with an energy dependent strength. The related
problem with an energy independent strength (the Lieb-Liniger-McGuire model)
has an exact solution and forbids all chemical processes, such as three-atom
association and diatom dissociation, as well as reflection in atom-diatom
collisions. The resonant case is analyzed by a numerical solution of the
Faddeev-Lovelace equations. The results demonstrate that as the internal
symmetry of the Lieb-Liniger-McGuire model is lifted, the reflection and
chemical reactions become allowed and may be observed in experiments.Comment: 5 pages, 4 figure
Scale-Free Networks are Ultrasmall
We study the diameter, or the mean distance between sites, in a scale-free
network, having N sites and degree distribution p(k) ~ k^-a, i.e. the
probability of having k links outgoing from a site. In contrast to the diameter
of regular random networks or small world networks which is known to be d ~
lnN, we show, using analytical arguments, that scale free networks with 2<a<3
have a much smaller diameter, behaving as d ~ lnlnN. For a=3, our analysis
yields d ~ lnN/lnlnN, as obtained by Bollobas and Riordan, while for a>3, d ~
lnN. We also show that, for any a>2, one can construct a deterministic scale
free network with d ~ lnlnN, and this construction yields the lowest possible
diameter.Comment: Latex, 4 pages, 2 eps figures, small corrections, added explanation
Atom loss from Bose-Einstein condensates due to Feshbach resonance
In recent experiments on Na Bose-Einstein condensates [S. Inouye et al,
Nature 392, 151 (1998); J. Stenger et al, Phys. Rev. Lett. 82, 2422 (1999)],
large loss rates were observed when a time-varying magnetic field was used to
tune a molecular Feshbach resonance state near the state of pairs of atoms
belonging to the condensate many-body wavefunction. A mechanism is offered here
to account for the observed losses, based on the deactivation of the resonant
molecular state by interaction with a third condensate atom.Comment: LaTeX, 4 pages, 4 PostScript figures, uses REVTeX and psfig,
submitted to Physical Review A, Rapid Communication
Counterintuitive transitions in multistate curve crossing involving linear potentials
Two problems incorporating a set of horizontal linear potentials crossed by a
sloped linear potential are analytically solved and compared with numerical
results: (a) the case where boundary conditions are specified at the ends of a
finite interval, and (b) the case where the sloped linear potential is replaced
by a piecewise-linear sloped potential and the boundary conditions are
specified at infinity. In the approximation of small gaps between the
horizontal potentials, an approach similar to the one used for the degenerate
problem (Yurovsky V A and Ben-Reuven A 1998 J. Phys. B 31,1) is applicable for
both problems. The resulting scattering matrix has a form different from the
semiclassical result obtained by taking the product of Landau-Zener amplitudes.
Counterintuitive transitions involving a pair of successive crossings, in which
the second crossing precedes the first one along the direction of motion, are
allowed in both models considered here.Comment: LaTeX 2.09 using ioplppt.sty and psfig.sty, 16 pages with 5 figures.
Submitted to J. Phys.
Resilience of the Internet to random breakdowns
A common property of many large networks, including the Internet, is that the
connectivity of the various nodes follows a scale-free power-law distribution,
P(k)=ck^-a. We study the stability of such networks with respect to crashes,
such as random removal of sites. Our approach, based on percolation theory,
leads to a general condition for the critical fraction of nodes, p_c, that need
to be removed before the network disintegrates. We show that for a<=3 the
transition never takes place, unless the network is finite. In the special case
of the Internet (a=2.5), we find that it is impressively robust, where p_c is
approximately 0.99.Comment: latex, 3 pages, 1 figure (eps), explanations added, Phys. Rev. Lett.,
in pres
Tree-body loss of of trapped ultracold Rb atoms due to a Feshbach resonance
The loss of ultracold trapped atoms in the vicinity of a Feshbach resonance
is treated as a two-stage reaction, using the Breit-Wigner theory. The first
stage is the formation of a resonant diatomic molecule, and the second one is
its deactivation by inelastic collisions with other atoms. This model is
applied to the analysis of recent experiments on Rb, leading to an
estimated value of cms for the deactivation rate
coefficient.Comment: LaTeX, 4 pages with 1 figures, uses REVTeX4, uses improved
experimental dat
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