Ultracold Thermalization of $^7$Li and $^{87}$Rb

TL;DR

This study measures the ultracold thermalization between $^7$Li and $^{87}$Rb atoms, deriving the scattering length and proposing future experiments to determine the interaction sign via condensate behavior.

Contribution

It provides the first measurement of the cross-species scattering length between $^7$Li and $^{87}$Rb in the ultracold regime and suggests a method to determine its sign through condensate phase separation.

Findings

Measured the pure triplet s-wave scattering length as |a_{7,87}|=(59±19) a_B.

Proposed numerical simulations indicating phase separation and mean-field stabilization in the condensate.

Discussed the potential to determine the sign of a_{7,87} through future experiments.

Abstract

We report on measurements of cross-species thermalization inside a magnetically trapped spin-polarized mixture of $^{87}$Rb and $^7$Li atoms with both atoms in their respective low field seeking magnetic substates $|F=2,m_F=2\right>$. Measurement of the thermalization velocity in the ultracold regime below $10 \mu$K allows for the derivation of the absolute value of the pure triplet s-wave scattering length governing the interaction. We find $|a_{7,87}|=(59\pm19) a_{\rm B}$. 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 $|2,2\right>$ clouds occurs along with a mean-field stabilization allowing for larger atom numbers of condensed $^7$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.