Prospects for mixed-isotope Bose-Einstein condensates in rubidium

We calculate binary collision parameters for mixtures of ultracold gases of Rb-85 and Rb-87 atoms. We predict a large positive triplet scattering length for mixed-isotope collisions, which implies that a stable mixed-isotope double condensate can be formed by sympathetic evaporative cooling. Moreover. collisions between Rb-85 and Rb-87 atoms in the lowest hyperfine manifold exhibit magnetic-field-induced Feshbach resonances which should allow partial control of the interaction between condensates. These considerations make Rb an ideal system for the study of interpenetrating bosonic superfluids.