538 research outputs found
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
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
Atomic dynamics in evaporative cooling of trapped alkali atoms in strong magnetic fields
We investigate how the nonlinearity of the Zeeman shift for strong magnetic
fields affects the dynamics of rf field induced evaporative cooling in magnetic
traps. We demonstrate for the 87-Rb and 23-Na F=2 trapping states with wave
packet simulations how the cooling stops when the rf field frequency goes below
a certain limit (for the 85-Rb F=2 trapping state the problem does not appear).
We examine the applicability of semiclassical models for the strong field case
as an extension of our previous work [Phys. Rev. A 58, 3983 (1998)]. Our
results verify many of the aspects observed in a recent Rb experiment
[Phys. Rev. A 60, R1759 (1999)].Comment: 9 pages, RevTex, eps figures embedde
Efficient Two-dimensional Subrecoil Raman Cooling of Atoms in a Tripod Configuration
We present an efficient method for subrecoil cooling of neutral atoms by
applying Raman cooling in 2D to a four-level tripod-system. The atoms can be
cooled simultaneously in two directions using only three laser beams. We
describe the cooling process with a simple model showing that the momentum
distribution can be rapidly narrowed to velocity spread down to
, corresponding to effective temperature equal to
. This method opens new possibilities for cooling of neutral
atoms.Comment: 6 pages, 3 figure
Nonlinear level crossing models
We examine the effect of nonlinearity at a level crossing on the probability
for nonadiabatic transitions . By using the Dykhne-Davis-Pechukas formula,
we derive simple analytic estimates for for two types of nonlinear
crossings. In the first type, the nonlinearity in the detuning appears as a
{\it perturbative} correction to the dominant linear time dependence. Then
appreciable deviations from the Landau-Zener probability are found to
appear for large couplings only, when is very small; this explains why the
Landau-Zener model is often seen to provide more accurate results than
expected. In the second type of nonlinearity, called {\it essential}
nonlinearity, the detuning is proportional to an odd power of time. Then the
nonadiabatic probability is qualitatively and quantitatively different from
because on the one hand, it vanishes in an oscillatory manner as the
coupling increases, and on the other, it is much larger than . We
suggest an experimental situation when this deviation can be observed.Comment: 9 pages final postscript file, two-column revtex style, 5 figure
- …