Tunable dual-species Bose-Einstein condensates of K 39 and Rb 87

We present the production of dual-species Bose-Einstein condensates (BECs) of K39 and Rb87. Preparation of both species in the F=1,mF=-1 state enabled us to exploit a total of three Feshbach resonances, which allows for simultaneous Feshbach tuning of the K39 intraspecies and the K39-Rb87 interspecies scattering length. Thus dual-species Bose-Einstein condensates were produced by sympathetic cooling of K39 with Rb87. A dark spontaneous force optical trap was used for Rb87 to reduce the losses in K39 due to light-assisted collisions in the optical trapping phase, which can be of benefit for other dual-species experiments. The tunability of the scattering length was used to perform precision spectroscopy of the interspecies Feshbach resonance located at 117.56(2) G and to determine the width of the resonance to 1.21(5) G by rethermalization measurements. The transition region from miscible to immiscible dual-species condensates was investigated and the interspecies background scattering length was determined to 28.5a0 using an empirical model. This paves the way for dual-species experiments with K39 and Rb87 BECs ranging from molecular physics to precision metrology.