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 {sup 87}Rb and {sup 40}K. 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 {sup 87}Rb and {sup 40}K, a{sub 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{sub RbK}=250{+-}30 a{sub 0}, 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{sub 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)].