Observation of Feshbach resonances in an ultracold gas of chromium atoms

We have observed Feshbach resonances (FRs) in collisions between ultracold $^{52}$Cr atoms by monitoring atom loss from a crossed optical dipole trap as a function of applied magnetic offset fields (0-60 mT). This is the first observation of FRs in an atomic species with more than one valence electron. Chromium has the electronic configuration [Ar]3d$^{5}$4s$^{1}$, which gives rise to its large magnetic moment of 6 Bohr magnetons. The zero nuclear spin of $^{52}$Cr leads to regularly-spaced resonance sequences. Comparing the experimental FR positions with multi-channel scattering calculations of A. Simoni and E. Tiesinga (NIST Gaithersburg, USA), we are able to assign all eleven FR that are expected up to second order in the dipole-dipole coupling for the S=6, MS=-6 s-wave entrance channel and to determine the s-wave scattering lengths of the lowest $^{2S+1}\Sigma ^{+}_{g}$ potentials to be 112(14) a0, 58(6) a0 and -7(20) a0 (a0 = 0.0529 nm) for S = 6, 4, and 2, respectively. Exploiting one of the FRs to tune the isotropic contact interaction in our recently realized Bose-Einstein condensate (BEC) of $^{52}$Cr should allow us to create a dipolar BEC in which the anisotropic (magnetic) dipole-dipole interaction is dominant.