Strongly Enhanced Inelastic Collisions in a Bose-Einstein Condensate near Feshbach Resonances

The properties of Bose-Einstein condensed gases can be strongly altered by tuning the external magnetic field near a Feshbach resonance. Feshbach resonances affect elastic collisions and lead to the observed modification of the scattering length. However, as we report here, the observed rate of inelastic collisions was strongly enhanced in a sodium Bose-Einstein condensate when the scattering length was tuned to both larger or smaller values than the off-resonant value. These strong losses impose severe limitations for using Feshbach resonances to tune the properties of Bose-Einstein condensates. [S0031-9007(99)08767-0] Most of the properties of Bose-Einstein condensates in dilute alkali gases are dominated by two-body collisions, which can be characterized by the s-wave scattering length a. The sign and the absolute value of the scattering length determine, e.g., stability, internal energy, formation rate, size, and collective excitations of a condensate. Near a Feshbach resonance the scattering length varies dispersively [1,2] covering the whole range of positive and negative values. Thus it should be possible to study strongly interacting, weakly or noninteracting, or collapsing condensates [3], all with the same alkali species and experimental setup. A Feshbach resonance occurs when the energy of a molecular (quasi-) bound state is tuned to the energy of two colliding atoms by applying an external magnetic field. Such resonances have been observed in a BoseEinstein condensate of Na sF › 1, mF › 11) atoms at 853 and 907 G [4,5], and in two experiments with cold clouds of 85 Rb (F › 2, mF › 22) atoms at 164 G [6]. In the sodium experiment, the scattering length a was observed to vary dispersively as a function of the magnetic field B, in agreement with the theoretical prediction [2]: