104 research outputs found

    Feshbach resonances in ultracold gases

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    In this chapter, we describe scattering resonance phenomena in general, and focus on the mechanism of Feshbach resonances, for which a multi-channel treatment is required. We derive the dependence of the scattering phase shift on magnetic field and collision energy. From this, the scattering length and effective range coefficient can be extracted, expressions which are particularly useful for ultracold gases.Comment: 23 pages, 3 figures. This article will be published as Chapter 4 in "Quantum gas experiments - exploring many-body states", edited by P. T\"orm\"a and K. Sengstock, Imperial College Press, London, to be published 201

    Ramsey fringes in a Bose-Einstein condensate between atoms and molecules

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    In a recent experiment, a Feshbach scattering resonance was exploited to observe Ramsey fringes in a 85^{85}Rb Bose-Einstein condensate. The oscillation frequency corresponded to the binding energy of the molecular state. We show that the observations are remarkably consistent with predictions of a resonance field theory in which the fringes arise from oscillations between atoms and molecules.Comment: 5 pages, 5 figure

    BCS-BEC crossover in a strongly correlated Fermi gas

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    We study the BCS-BEC crossover in the strongly correlated regime of an ultra-cold rotating two component Fermi gas. Strong correlations are shown to generate an additional long-range interaction which results in a modified crossover region compared to the non-rotating situation. The two-particle correlation function reveals a smooth crossover between the s-wave paired fermionic fractional quantum Hall state and the bosonic Laughlin state.Comment: 4 pages, 3 figure

    Feshbach resonances and collapsing Bose-Einstein condensates

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    We investigate the quantum state of burst atoms seen in the recent Rb-85 experiments at JILA. We show that the presence of a resonance scattering state can lead to a pairing instability generating an outflow of atoms with energy comparable to that observed. A resonance effective field theory is used to study this dynamical process in an inhomogeneous system with spherical symmetry

    Feshbach resonances with large background scattering length: interplay with open-channel resonances

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    Feshbach resonances are commonly described by a single-resonance Feshbach model, and open-channel resonances are not taken into account explicitly. However, an open-channel resonance near threshold limits the range of validity of this model. Such a situation exists when the background scattering length is much larger than the range of the interatomic potential. The open-channel resonance introduces strong threshold effects not included in the single-resonance description. We derive an easy-to-use analytical model that takes into account both the Feshbach resonance and the open-channel resonance. We apply our model to 85^{85}Rb, which has a large background scattering length, and show that the agreement with coupled-channels calculations is excellent. The model can be readily applied to other atomic systems with a large background scattering length, such as 6^6Li and 133^{133}Cs. Our approach provides full insight into the underlying physics of the interplay between open-channel (or potential) resonances and Feshbach resonances.Comment: 16 pages, 12 figures, accepted for publication in Phys. Rev. A; v2: added reference

    Scattering hypervolume for ultracold bosons from weak to strong interactions

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    The elastic scattering properties of three bosons at low energy enter the many-body description of ultracold Bose gases via the three-body scattering hypervolume DD. We study this quantity for identical bosons that interact via a pairwise finite-range potential. Our calculations cover the regime from strongly repulsive potentials towards attractive potentials supporting multiple two-body bound states and are consistent with the few existing predictions for DD. We present the first numerical confirmation of the universal predictions for DD in the strongly interacting regime, where Efimov physics dominates, for a local nonzero-range potential. Our findings highlight how DD is influenced by three-body quasibound states with strong dd-wave or gg-wave characteristics in the weakly interacting regime.Comment: 13 pages, 8 figure

    Production of Long-Lived Ultracold Li2 Molecules from a Fermi gas

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    We create weakly-bound Li2 molecules from a degenerate two component Fermi gas by sweeping a magnetic field across a Feshbach resonance. The atom-molecule transfer efficiency can reach 85% and is studied as a function of magnetic field and initial temperature. The bosonic molecules remain trapped for 0.5 s and their temperature is within a factor of 2 from the Bose-Einstein condensation temperature. A thermodynamical model reproduces qualitatively the experimental findings
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