Measurement of the interaction strength in a Bose-Fermi mixture with ⁸⁷Rb and ⁴⁰K

A quantum degenerate, dilute gas mixture of bosonic and fermionic atoms was produced using ⁸⁷Rb and ⁴⁰K. The onset of degeneracy was confirmed by observing the spatial distribution of the gases after time-of-flight expansion. Furthermore, the magnitude of the interspecies scattering length between the doubly spin-polarized states of ⁸⁷Rb and ⁴⁰K, ∣a_RbK∣, was determined from cross-dimensional thermal relaxation. The uncertainty in this collision measurement was greatly reduced by taking the ratio of interspecies and intraspecies relaxation rates, yielding ∣a_RbK∣=250±30 a₀, which is a lower value than what was reported in [ M. Modugno et al. Phys. Rev. A 68 043626 (2003)]. Using the value for ∣a_RbK∣ reported here, current T=0 theory would predict a threshold for mechanical instability that is inconsistent with the experimentally observed onset for sudden loss of fermions in [ G. Modugno et al. Science 297 2240 (2002)].