Collisional properties of ultracold K-Rb mixtures

We determine the interspecies s-wave triplet scattering length a(3) for all K-Rb isotopic mixtures by measuring the elastic cross section for collisions between K-41 and Rb-87 in different temperature regimes. The positive value a(3)=+163(-12)(+57)a(0) ensures the stability against collapse of binary K-41- Rb-87 Bose-Einstein condensates. For the fermion-boson mixture K-40- Rb-87, we obtain a large and negative scattering length which implies an efficient sympathetic cooling of the fermionic species down to the degenerate regime.