39K Bose-Einstein condensate with tunable interactions

We produce a Bose-Einstein condensate of K-39 atoms. Condensation of this species with a naturally small and negative scattering length is achieved by a combination of sympathetic cooling with Rb-87 and direct evaporation, exploiting the magnetic tuning of both inter- and intraspecies interactions at Feshbach resonances. We explore the tunability of the self-interactions by studying the expansion and the stability of the condensate. We find that a K-39 condensate is interesting for future experiments requiring a weakly-interacting Bose gas.