We report on measurements of cross-species thermalization inside a magnetically trapped spin-polarized mixture of ${}^{87}\mathrm{Rb}$ and ${}^7\mathrm{Li}$ atoms with both atoms in their respective low-field-seeking magnetic substates $\mid F=2,m_F=2⟩$. Measurement of the thermalization velocity in the ultracold regime below $10\mathrm{\mu }\mathrm{K}$ allows for the derivation of the absolute value of the pure triplet $s$-wave scattering length governing the interaction. We find $\mid a_{7,87}\mid =(59\pm 19)a_{\mathrm{B}}$. We propose to study both species in the condensed regime to derive the sign of $a_{7,87}$. In this context, we present numerical solutions to the coupled Gross-Pitaevskii equation based on the hypothesis of a positive sign. According to the simulations, phase separation of the Li and Rb $\mid 2,2⟩$ clouds occurs along with a mean-field stabilization allowing for larger atom numbers of condensed ${}^7\mathrm{Li}$ atoms before collapse sets in. Observation of this mean-field stabilization would directly fix the sign of $a_{7,87}$. We discuss our results in the light of this proposal.

• Received 18 May 2007

DOI:https://doi.org/10.1103/PhysRevA.76.052704