Feshbach-molecule formation in a Bose-Fermi mixture (original) (raw)
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Physical Review Letters, 2004
We explain why the experimental efficiency observed in the conversion of ultracold Fermi gases of 40 K and 6 Li atoms into diatomic Bose gases is limited to 0.5 when the Feshbach resonance sweep rate is sufficiently slow to pass adiabatically through the Landau-Zener transition but faster than ''the collision rate'' in the gas, and increases beyond 0.5 when it is slower. The 0.5 efficiency limit is due to the preparation of a statistical mixture of two spin states, required to enable s-wave scattering. By constructing the many-body state of the system we show that this preparation yields a mixture of even and odd parity pair states, where only even parity can produce molecules. The odd parity spinsymmetric states must decorrelate before the constituent atoms can further Feshbach scatter, thereby increasing the conversion efficiency; ''the collision rate'' is the pair decorrelation rate.
Collective Molecule Formation in a Degenerate Fermi Gas via a Feshbach Resonance
Physical Review Letters, 2004
We model collisionless collective conversion of a degenerate Fermi gas into bosonic molecules via a Feshbach resonance, treating the bosonic molecules as a classical field and seeding the pairing amplitudes with random phases. A dynamical instability of the Fermi sea against association into molecules initiates the conversion. The model qualitatively reproduces several experimental observations [Regal et al., Nature 424, 47 (2003)]. We predict that the initial temperature of the Fermi gas sets the limit for the efficiency of atom-molecule conversion.
Molecular production at a broad Feshbach resonance in a Fermi gas of cooled atoms
EPL (Europhysics Letters), 2008
The problem of molecular production from degenerate gas of fermions at a wide Feshbach resonance, in a single-mode approximation, is reduced to the linear Landau-Zener problem for operators. The strong interaction leads to significant renormalization of the gap between adiabatic levels. In contrast to static problem the close vicinity of exact resonance does not play substantial role. Two main physical results of our theory is the high sensitivity of molecular production to the initial value of magnetic field and generation of a large BCS condensate distributed over a broad range of momenta in inverse process of the molecule dissociation.
Condensation of Pairs of Fermionic Atoms near a Feshbach Resonance
Physical Review Letters, 2004
We have observed Bose-Einstein condensation of pairs of fermionic atoms in an ultracold 6 Li gas at magnetic fields above a Feshbach resonance, where no stable 6 Li2 molecules would exist in vacuum. We accurately determined the position of the resonance to be 822±3 G. Molecular Bose-Einstein condensates were detected after a fast magnetic field ramp, which transferred pairs of atoms at close distances into bound molecules. Condensate fractions as high as 80% were obtained. The large condensate fractions are interpreted in terms of pre-existing molecules which are quasi-stable even above the two-body Feshbach resonance due to the presence of the degenerate Fermi gas.
The nature of Feshbach molecules in Bose-Einstein condensates
2003
Recent experiments at JILA found evidence for molecular Bose-Einstein condensation by probing coherence properties of the assembly of atoms plus molecules. We discuss the special long range nature of the molecules produced. The properties of these molecules depend on the full two-body Hamiltonian and not just on the states of the system in the absence of interchannel couplings. The very
Optimal conversion of Bose-Einstein-condensed atoms into molecules via a Feshbach resonance
Physical Review A, 2006
In many experiments involving conversion of quantum degenerate atomic gases into molecular dimers via a Feshbach resonance, an external magnetic field is linearly swept from above the resonance to below resonance. In the adiabatic limit, the fraction of atoms converted into molecules is independent of the functional form of the sweep and is predicted to be 100%. However, for nonadiabatic sweeps through resonance, Landau-Zener theory predicts that a linear sweep will result in a negligible production of molecules. Here we employ a genetic algorithm to determine the functional time dependence of the magnetic field that produces the maximum number of molecules for sweep times that are comparable to the period of resonant atom-molecule oscillations, 2πΩ −1
Dynamics of a molecular Bose-Einstein condensate near a Feshbach resonance
2005
We consider the dissociation of a molecular Bose-Einstein condensate during a magnetic-field sweep through a Feshbach resonance that starts on the molecular side of the resonance and ends on the atomic side. In particular, we determine the energy distribution of the atoms produced after the sweep. We find that the shape of the energy distribution strongly depends on the rate of the magnetic-field sweep, in a manner that is in good agreement with recent experiments.
Optimal conversion of Bose condensed atoms into molecules via a Feshbach resonance
2006
In many experiments involving conversion of quantum degenerate atomic gases into molecular dimers via a Feshbach resonance, an external magnetic field is linearly swept from above the resonance to below resonance. In the adiabatic limit, the fraction of atoms converted into molecules is independent of the functional form of the sweep and is predicted to be 100%. However, for nonadiabatic sweeps through resonance, Landau-Zener theory predicts that a linear sweep will result in a negligible production of molecules. Here we employ a genetic algorithm to determine the functional time dependence of the magnetic field that produces the maximum number of molecules for sweep times that are comparable to the period of resonant atom-molecule oscillations, 2πΩ −1
Coherent dimer formation near Feshbach resonances in Bose-Einstein condensates
2003
The results of a recent experiment with 85 Rb Bose-Einstein condensates are analyzed within the mean-field approximation including dissipation due to three-body recombination. The intensity of the dissipative term is chosen from the three-body theory for large positive scattering lengths. The remaining number of condensed atoms in the experiment, obtained with applied magnetic field pulses, were used to adjust the intensity of the dissipative term. We found that the three-body recombination parameter depends on the pulse rise time; i.e., for longer rise times the values found become consistent with the three-body theory, while for shorter pulses this coefficient is found to be much larger. We interpret this finding as an indication of a coherent formation of dimers.
Physical Review A, 2007
We predict the resonance enhanced magnetic field dependence of atom-dimer relaxation and three-body recombination rates in a 87 Rb Bose-Einstein condensate (BEC) close to 1007 G. Our exact treatments of threeparticle scattering explicitly include the dependence of the interactions on the atomic Zeeman levels. The Feshbach resonance distorts the entire diatomic energy spectrum causing interferences in both loss phenomena. Our two independent experiments confirm the predicted recombination loss over a range of rate constants that spans four orders of magnitude.