Collisional Properties of Ultracold K-Rb Mixtures

We determine the interspecies s-wave triplet scattering length a₃ for all K-Rb isotopic mixtures by measuring the elastic cross section for collisions between ⁴¹K and ⁸⁷Rb in different temperature regimes. The positive value a₃=+163_-12⁺⁵⁷a₀ ensures the stability against collapse of binary ⁴¹K-⁸⁷Rb Bose-Einstein condensates. For the fermion-boson mixture ⁴⁰K-⁸⁷Rb, we obtain a large and negative scattering length which implies an efficient sympathetic cooling of the fermionic species down to the degenerate regime.