Comparing Contact and Dipolar Interactions in a Bose-Einstein Condensate

We have measured the relative strength "dd of the magnetic dipole-dipole interaction compared with thecontact interaction in a dipolar chromium Bose-Einstein condensate. We analyze the asymptotic velocitiesof expansion of the condensate with different orientations of the atomic magnetic moments. By comparingthe experimental results with numerical solutions of the hydrodynamic equations for dipolar condensates,we obtain edd= 0,159 +- 0,034.We use this result to determine the s-wave scattering length a = 5,08 +-1,06 x 10^-9 m = 96 +- 20 a0 of 52Cr. This is fully consistent with our previous measurements on thebasis of Feshbach resonances and therefore confirms the validity of the theoretical approach used todescribe the dipolar Bose-Einstein condensate.