A double species 23Na and 87Rb Bose-Einstein condensate with tunable miscibility via an interspecies Feshbach resonance

We have realized a dual-species Bose-Einstein condensate (BEC) of Na-23 and Rb-87 atoms and observed its immiscibility. Because of the favorable background intra-and inter-species scattering lengths, stable condensates can be obtained via efficient evaporative cooling and sympathetic cooling without the need for fine tuning of the interactions. Our system thus provides a clean platform for studying inter-species interactions-driven effects in superfluid mixtures. With a Feshbach resonance, we have successfully created double BECs with largely tunable inter-species interactions and studied the miscible-immiscible phase transition.