Cross-dimensional relaxation of ${}^7$Li-${}^{87}$Rb atomic gas mixtures in a spherical-quadrupole magnetic trap

We measure the interspecies interaction strength between ${}^7$Li and ${}^{87}$Rb atoms through cross-dimensional relaxation of two-element gas mixtures trapped in a spherical quadrupole magnetic trap. We record the relaxation of an initial momentum-space anisotropy in a lithium gas when co-trapped with rubidium atoms, with both species in the $\mathrm{\mid }F=1,m_F=-1⟩$ hyperfine state. Our measurements are calibrated by observing cross-dimensional relaxation of a ${}^{87}$Rb-only trapped gas. Through Monte Carlo simulations, we compare the observed relaxation to that expected given the theoretically predicted energy-dependent differential cross section for ${}^7$Li-${}^{87}$Rb collisions. The experimentally observed relaxation occurs significantly faster than predicted theoretically, a deviation that appears incompatible with other experimental data characterising the ${}^7$Li-${}^{87}$Rb molecular potential.