Prospects for Mixed-Isotope Bose-Einstein Condensates in Rubidium

We calculate binary collision parameters for mixtures of ultracold gases of ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$ 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 ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$ 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.