Observation of interspecies Feshbach resonances in an ultracold Rb-Cs mixture

We report on the observation of interspecies Feshbach resonances in an ultracold, optically trapped mixture of Rb and Cs atoms. In a magnetic-field range up to 300 G we find 23 interspecies Feshbach resonances in the lowest spin channel and two resonances in a higher channel of the mixture. The extraordinarily rich Feshbach spectrum suggests the importance of different partial waves in both the open and closed channels of the scattering problem along with higher-order coupling mechanisms. Our results, on one hand, provide fundamental experimental input to characterize the Rb-Cs scattering properties and, on the other hand, identify possible starting points for the association of ultracold heteronuclear RbCs molecules.