Observation of Feshbach Resonances in an Ultracold Gas of ${}^{52}\mathrm{Cr}$

We have observed Feshbach resonances in collisions between ultracold ${}^{52}\mathrm{Cr}$ atoms. This is the first observation of collisional Feshbach resonances in an atomic species with more than one valence electron. The zero nuclear spin of ${}^{52}\mathrm{Cr}$ and thus the absence of a Fermi-contact interaction leads to regularly spaced resonance sequences. By comparing resonance positions with multichannel scattering calculations we determine the $s$-wave scattering length of the lowest ${}^{2S+1}\Sigma _g^{+}$ potentials to be $112(14)a_0$, $58(6)a_0$, and $-7(20)a_0$ for $S=6$, 4, and 2, respectively, where $a_0=0.0529\mathrm{nm}$.

Figure 1

Inelastic loss measurement of the Feshbach resonances at 28.66 and 29.03 mT. The dashed lines are Gaussian fits to the data and determine the position and width of the loss features. The experimental data points are averages over many experimental measurements. Each measurement was separately normalized to the offset of a Gaussian fit to the data.

Figure 2

Calculated scattering length of two $m_s=-3$ ${}^{52}\mathrm{Cr}$ atoms versus magnetic field, for model parameters $a_6=111.56a_0$, $a_4=57.61a_0$, $a_2=-7.26a_0$, $C_6=733\mathrm{a}.\mathrm{u}.$, and $C_8=75\times {10}^3\mathrm{a}.\mathrm{u}.$ The feature near 29 mT is a pair of nearly degenerate Feshbach resonances (see Fig. 1 for the corresponding experimental data).