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J. Stuhler, A. Griesmaier, T. Koch, S. Götz, M. Fattori, T. Pfau
I will report on two experiments with ultra-cold chromium atoms. In the first one, we have investigated the expansion of a Bose-Einstein condensate of strongly magnetic Cr atoms. The long-range and anisotropic magnetic dipole-dipole interaction leads to a magnetostriction-like anisotropic deformation of the expanding Cr condensate which depends on the orientation of the atomic dipole moments. Our measurements are consistent with the theory of dipolar quantum gases and show that a Cr condensate is an excellent model system to study dipolar interactions in such gases. The expansion data allow for a determination of εdd, the ratio between dipole-dipole and contact interaction. Taking this εdd, we were able to extract a value of the s-wave scattering length of Cr which is in agreement with the results obtained by the analysis of our observed Feshbach resonances in ultra-cold Cr. In the second experiment, we have demonstrated that spin-changing collisions driven by the dipole-dipole interaction (dipolar relaxation) can be used to cool a dipolar gas with efficiency well beyond evaporative cooling using a continuous demagnetization cooling technique. |
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