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arXiv:0710.2479 · Physical Review Letters

Collisional Stability of Fermionic Feshbach Molecules

J. J. Zirbel, Kang-Kuen Ni, Silke Ospelkaus, José Paulo D'Incao, Carl Wieman, Jun Ye, D. S. Jin

Published Apr 8, 2008• 131 citations
arXivHTMLPDF

Abstract

Using a Feshbach resonance, we create ultracold fermionic molecules starting from a Bose-Fermi atom gas mixture. The resulting mixture of atoms and weakly bound molecules provides a rich system for studying few-body collisions because of the variety of atomic collision partners for molecules; either bosonic, fermionic, or distinguishable atoms. Inelastic loss of the molecules near the Feshbach resonance is dramatically affected by the quantum statistics of the colliding particles and the scattering length. In particular, we observe a molecule lifetime as long as 100 ms near the Feshbach resonance.

DOI
10.1103/physrevlett.100.143201

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We study ultracold three-body collisions of bosons and fermions when the interatomic interaction is tuned near a narrow Feshbach resonance. We show that the width of the resonance has a substantial impact on the collisional properties of ultracold gases in the strongly interacting regime. From our numerical and analytical analyses, we identify universal features dependent on the resonance width. Remarkably, we find that all inelastic processes near narrow resonances leading to deeply bound states in bosonic systems are suppressed, while those for fermionic systems are enhanced. As a result, narrow resonances present a scenario that is the reverse of that found for broad resonances, opening up the possibility of creating stable samples of ultracold bosonic gases with large scattering lengths.
Elastic and inelastic properties of weakly bound sss- and ppp-wave molecules of fermionic atoms that collide with a third atom are investigated. Analysis of calculated collisional properties of sss-wave dimers of fermions in different spin states permit us to compare and highlight the physical mechanisms that determine the stability of sss-wave and ppp-wave molecules. In contrast to sss-wave molecules, the collisional properties of ppp-wave molecules are found to be largely insensitive to variations of the ppp-wave scattering length, and these collisions will usually result in short molecular lifetimes. We also discuss the importance of this result for both theories and experiments involving degenerate Fermi gases.
Ultracold fermionic Feshbach molecules are promising candidates for exploring quantum matter with strong ppp-wave interactions; however, their lifetimes were measured to be short. Here we characterize the ppp-wave collisions of ultracold fermionic 23Na40K^{23}\mathrm{Na}^{40}\mathrm{K}23Na40K Feshbach molecules for different scattering lengths and temperatures. By increasing the binding energy of the molecules, the two-body loss coefficient reduces by three orders of magnitude, leading to a second-long lifetime 20 times longer than that of ground-state NaK molecules. We exploit the scaling of elastic and inelastic collisions with the scattering length and temperature to identify a regime where the elastic collisions dominate over the inelastic ones, allowing the molecular sample to thermalize. Our results provide a benchmark for four-body calculations of molecular collisions and pave the way for investigating quantum many-body phenomena with fermionic Feshbach molecules.
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This work describes experiments with quantum-degenerate atomic mixtures at ultracold temperatures, where quantum statistics determine macroscopic system properties. The first heteronuclear molecules at ultracold temperatures are formed in a quantum degenerate two-species Fermi-Fermi mixture on the repulsive side of a narrow s-wave Feshbach resonance. Elastic collisions in this mixture are investigated with the method of cross-dimensional relaxation. Long-lived two-body bound states on the atomic side of the resonance are detected due to a many-body effect at the crossover of the narrow Feshbach resonance. In addition, atom scattering with fermionic 40K on a light field grating in the Bragg and Kapitza-Dirac regimes is realized for the first time. The versatile experimental platform, where the investigations are done, offers the possibility to perform studies on mixtures involving the bosonic species 87Rb and the two fermionic species 6Li and 40K. Within this work, mainly interactions between the two fermionic species are considered. A quantum-degenerate mixture of 6Li and 40K can be used to create heteronuclear bosonic molecules close to an interspecies s-wave Feshbach resonance. By an adiabatic magnetic field sweep, up to 4 × 10^4 molecules are produced with conversion efficiencies close to 50%. A direct and sensitive molecule detection method is developed to probe molecule properties. The lifetime of the molecules in an atom-molecule mixture exhibits a strong magnetic field dependence. Close to resonance, lifetimes of more than 100ms are observed what offers excellent starting conditions for further investigation and manipulation of the molecular cloud. The interspecies Feshbach resonance, which serves for the production of molecules, is further characterized. The method of cross-dimensional relaxation is applied for the first time to a Fermi-Fermi mixture. For this method, a non-equilibrium state is created, which rethermalizes by pure interspecies collisions due to the fermionic nature of the two species. The lighter atomic species, 6Li, relaxes faster in the mixture than the heavier one, 40K. This is verified by an analytical model, Monte-Carlo simulations, and measurements. With this technique, elastic scattering cross sections are measured over a wide range of magnetic field strengths across the Feshbach resonance. The position (B0 = 154.71(5)G) and the magnetic field width of the Feshbach resonance (Delta = 1.02(7)G) are determined. By comparison of the several measurements, long-lived bound states exist on the atomic side of the resonance due to a many-body effect in the crossover regime of the resonance. In addition, atomic scattering with ultracold 40K on a light field crystal is studied for the first time. The light grating is generated by two counter-propagating laser beams. Suitable pulse parameters for the realization of atom scattering in the Bragg and Kapitza-Dirac regime are found. The momentum spread of the cloud determines the efficiency of the scattering process, which is increased by lowering the temperature of the system.