Optimal preparation of Bose and Fermi atomic gas mixtures of ⁸⁷Rb and ⁴⁰K in a crossed optical dipole trap

Abstract

We report on the optimal production of the Bose and Fermi mixtures with $^{87}$Rb and $^{40}$K in a crossed optical dipole trap (ODT). We measure the atomic number and lifetime of the mixtures at the combination of the spin state $|F=9/2, m_{F}=9/2\rangle$ of $^{40}$K and $|1, 1\rangle$ of $^{87}$Rb in the ODT, which is larger and longer compared with the combination of the spin state $|9/2, 9/2\rangle$ of $^{40}$K and $|2, 2\rangle$ of $^{87}$Rb in the ODT. We observe the atomic numbers of $^{87}$Rb and $^{40}$K shown in each stage of the sympathetic cooling process while gradually reducing the depth of the optical trap. By optimizing the relative loading time of atomic mixtures in the MOT, we can obtain the large atomic number of $^{40}$K ($\sim$6$\times$10$^{6}$) or the mixtures of atoms with an equal number ($\sim$1.6$\times$10$^{6}$) at the end of evaporative cooling in ODT. We have experimentally investigated the evaporative cooling in an enlarged volume of ODT via adding a third laser beam to the crossed ODT and found that more atoms (8$\times$10$^{6}$) and higher degeneracy ($T/T_F$=0.25) of Fermi gases are obtained. The ultracold atomic gas mixtures pave the way to explore phenomena such as few-body collisions and the Bose-Fermi Hubbard model, as well as for creating ground-state molecules of $^{87}$Rb$^{40}$K.