93 research outputs found
Three-body recombination of ultra-cold atoms to a weakly bound level
We discuss three-body recombination of ultra-cold atoms to a weakly bound
level. In this case, characterized by large and positive scattering length
for pair interaction, we find a repulsive effective potential for three-body
collisions, which strongly reduces the recombination probability and makes
simple Jastrow-like approaches absolutely inadequate. In the zero temperature
limit we obtain a universal relation, independent of the detailed shape of the
interaction potential, for the (event) rate constant of three-body
recombination: , where is the atom mass.Comment: 10 pages, 3 Postscript figure
Three-body recombination in Bose gases with large scattering length
An effective field theory for the three-body system with large scattering
length is applied to three-body recombination to a weakly-bound s-wave state in
a Bose gas. Our model independent analysis demonstrates that the three-body
recombination constant alpha is not universal, but can take any value between
zero and 67.9 \hbar a^4/m, where a is the scattering length. Other low-energy
three-body observables can be predicted in terms of a and alpha. Near a
Feshbach resonance, alpha should oscillate between those limits as the magnetic
field B approaches the point where a -> infinity. In any interval of B over
which a increases by a factor of 22.7, alpha should have a zero.Comment: 8 pages, RevTex, 3 postscript figures, uses epsf.sty, rotate.sty,
references added, discussion improve
Prospects for measurement and control of the scattering length of metastable helium using photoassociation techniques
A numerical investigation of two-laser photoassociation (PA) spectroscopy on
spin-polarized metastable helium (He*) atoms is presented within the context of
experimental observation of the least-bound energy level in the scattering
potential and subsequent determination of the s-wave scattering length.
Starting out from the model developed by Bohn and Julienne [Phys. Rev. A
\textbf{60}, (1999) 414], PA rate coefficients are obtained as a function of
the parameters of the two lasers. The rate coefficients are used to simulate
one- and two-laser PA spectra. The results demonstrate the feasibility of a
spectroscopic determination of the binding energy of the least-bound level. The
simulated spectra may be used as a guideline when designing such an experiment,
whereas the model may also be employed for fitting experimentally obtained PA
spectra. In addition, the prospects for substantial modification of the He*
scattering length by means of optical Feshbach resonances are considered.
Several experimental issues relating to the numerical investigation presented
here are discussed.Comment: 9 pages, 7 figure
Zero-temperature phase diagram of binary boson-fermion mixtures
We calculate the phase diagram for dilute mixtures of bosons and fermions at
zero temperature. The linear stability conditions are derived and related to
the effective boson-induced interaction between the fermions. We show that in
equilibrium there are three possibilities: a) a single uniform phase, b) a
purely fermionic phase coexisting with a purely bosonic one and c) a purely
fermionic phase coexisting with a mixed phase.Comment: 8 pages, revtex, 3 postscript figures; NORDITA-1999/71 C
Kripke Semantics for Martin-L\"of's Extensional Type Theory
It is well-known that simple type theory is complete with respect to
non-standard set-valued models. Completeness for standard models only holds
with respect to certain extended classes of models, e.g., the class of
cartesian closed categories. Similarly, dependent type theory is complete for
locally cartesian closed categories. However, it is usually difficult to
establish the coherence of interpretations of dependent type theory, i.e., to
show that the interpretations of equal expressions are indeed equal. Several
classes of models have been used to remedy this problem. We contribute to this
investigation by giving a semantics that is standard, coherent, and
sufficiently general for completeness while remaining relatively easy to
compute with. Our models interpret types of Martin-L\"of's extensional
dependent type theory as sets indexed over posets or, equivalently, as
fibrations over posets. This semantics can be seen as a generalization to
dependent type theory of the interpretation of intuitionistic first-order logic
in Kripke models. This yields a simple coherent model theory, with respect to
which simple and dependent type theory are sound and complete
Structure and stability of bosonic clouds: alkali atoms with negative scattering length
We investigate the form and stability of a cloud of atoms confined in a
harmonic trap when the scattering length is negative. We find that, besides the
known low density metastable solution, a new branch of Bose condensate appears
at higher density when non locality effects in the attractive part are taken
into account. The transition between the two classes of solutions as a function
of the number of atoms can be either sharp or smooth according to the
strength and range of the attractive interaction. Use of tight traps is
favorable for investigating the evolution of the system as the strength of the
effective interaction increases with .Comment: 11 pages, Latex, 2 figures, to be published in Phys. Rev.
Low energy atomic collision with dipole interactions
We apply quantum defect theory to study low energy ground state atomic
collisions including aligned dipole interactions such as those induced by an
electric field. Our results show that coupled even () relative orbital
angular momentum partial wave channels exhibit shape resonance structures while
odd () channels do not. We analyze and interpret these resonances within the
framework of multichannel quantum defect theory (MQDT).Comment: 27 pages, 17 figures, an inadvertent typo correcte
Inter-isotope determination of ultracold rubidium interactions from three high-precision experiments
Combining the measured binding energies of four of the most weakly bound
rovibrational levels of the Rb molecule with the results of two
other recent high-precision rubidium experiments, we obtain exceptionally
strong constraints on the atomic interaction parameters in a highly model
independent analysis. The comparison of Rb and Rb data, where the
two isotopes are related by a mass scaling procedure, plays a crucial role.
Using the consistent picture of the interactions that thus arises we are led to
predictions for scattering lengths, clock shifts, Feshbach resonance fields and
widths with an unprecedented level of accuracy. To demonstrate this, we predict
two Feshbach resonances in mixed-spin scattering channels at easily accessible
magnetic field strengths, which we expect to play a role in the damping of
coherent spin oscillations
Superfluidity of spin-polarized 6Li
We study the prospects for observing superfluidity in a spin-polarized atomic
gas of Li atoms, using state-of-the-art interatomic potentials. We
determine the spinodal line and show that a BCS transition to the superfluid
state can indeed occur in the (meta)stable region of the phase diagram if the
densities are sufficiently low. Moreover, for a total density of
, which still fulfills this requirement, we find a critical
temperature of only . We also discuss the stability of the gas due to
exchange and dipolar relaxation and conclude that the prospects for observing
superfluidity in a magnetically trapped atomic Li gas are particularly
promising for magnetic bias fields larger than .Comment: 4 pages of ReVTeX and 2 uuencoded figures. Submitted for publication
in Physical Review Letter
Stimulated Raman adiabatic passage from an atomic to a molecular Bose-Einstein condensate
The process of stimulated Raman adiabatic passage (STIRAP) provides a
possible route for the generation of a coherent molecular Bose-Einstein
condensate (BEC) from an atomic BEC. We analyze this process in a
three-dimensional mean-field theory, including atom-atom interactions and
non-resonant intermediate levels. We find that the process is feasible, but at
larger Rabi frequencies than anticipated from a crude single-mode lossless
analysis, due to two-photon dephasing caused by the atomic interactions. We
then identify optimal strategies in STIRAP allowing one to maintain high
conversion efficiencies with smaller Rabi frequencies and under experimentally
less demanding conditions.Comment: Final published versio
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