Resonant Magnetic Field Control of Elastic Scattering in ColdR85b

Abstract

A magnetic field dependent Feshbach resonance has been observed in the elastic scattering collision rate between atoms in the F = 2, M = −2 state of 85 Rb. Changing the magnetic field by several Gauss caused the collision rate to vary by a factor of 10 4 , and the sign of the scattering length could be reversed. The resonance peak is at 155.2(4) G and its width is 11.6(5) G. From these results we extract much improved values for the three quantities that characterize the interaction potential: the van der Waals coefficient C6, the singlet scattering length aS, and the triplet scattering length aT .Very low-temperature collision phenomena can be quite sensitive to applied electromagnetic fields. Several groups have altered inelastic collision rates in optical traps by applying laser fields [1]. There have also been numerous proposals [2-6] for using laser and static electric or magnetic fields to influence the S-wave scattering length (a), and equivalently, the elastic collision cross sections (σ = 8πa 2 ) between cold atoms. Particularly notable is the prediction by Verhaar and co-workers [2] that as a function of magnetic field there should be Feshbach resonances in collisions between cold (∼ µK) alkali atoms. These resonances were predicted to cause dramatic changes in the cross section, and to even allow the sign of the scattering length to be changed. Such resonances are very interesting collision physics, but they also offer a means to manipulate Bose-Einstein condensates (BEC), the properties of which are primarily determined by the scattering length. Some time ago we searched for such resonances in 133 Cs and 87 Rb, without success [7,8]. This was not surprising because there was enormous uncertainty as to the positions and widths of the predicted resonances. However, recent photoassociation spectroscopy has greatly improved the knowledge of the alkali interatomic interaction potentials, allowing these resonance parameters to be predicted with much less uncertainty [9]. As we show below, the fact that the widths and positions of these resonances are so sensitive to the interatomic potentials makes their measurement a good way to determine these potentials.The improved predictions for the resonance positions facilitated their observation. In the past few months Feshbach resonances have been observed in both 23 Na [10] and 85 Rb [11]. In the sodium work, two magnetic field dependent processes were observed: 1) a change in the expansion rate of BEC due to a change in the scattering length, and 2) an enormous, and as yet unexplained, increase in the loss rate. This loss rate precluded the study of collisions in the interesting field regime near the center of the resonance where the scattering length changes sign; the width of the resonance was not measured. In the rubidium work [11] the resonance was detected as a magnetic field dependence of the photoassociation spectra. The resulting resonance width was measured to be far larger than originally predicted.Here we report the study of a Feshbach resonance in t...

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Nonlinear waves in Bose–Einstein condensates: physical relevance and mathematical techniques

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Feshbach spectroscopy and analysis of the interaction potentials of ultracold sodium

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