Quantum Degenerate Two-Species Fermi-Fermi Mixture Coexisting with a Bose-Einstein Condensate

We report on the generation of a quantum degenerate Fermi-Fermi mixture of two different atomic species. The quantum degenerate mixture is realized employing sympathetic cooling of fermionic ${}^6\mathrm{Li}$ and ${}^{40}\mathrm{K}$ gases by an evaporatively cooled bosonic ${}^{87}\mathrm{Rb}$ gas. We describe the combination of trapping and cooling methods that proved crucial to successfully cool the mixture. In particular, we study the last part of the cooling process and show that the efficiency of sympathetic cooling of the ${}^6\mathrm{Li}$ gas by ${}^{87}\mathrm{Rb}$ is increased by the presence of ${}^{40}\mathrm{K}$ through catalytic cooling. Because of the differing physical properties of the two components, the quantum degenerate ${}^6\mathrm{Li}\mathrm{\text{−}}{}^{40}\mathrm{K}$ Fermi-Fermi mixture is an excellent candidate for a stable, heteronuclear system allowing the study of several so far unexplored types of quantum matter.

Figure 1

Time of flight absorption images of the quantum degenerate Fermi-Fermi-Bose mixture. Shown are the optical column densities (left) and projections of the optical column densities (right) along with corresponding fits (lines). The projections are taken along the symmetry axis of the trap ($x$ axis) for the fermionic species and along the radial axis ($y$ axis) for rubidium. Expansion times are 15 ms, 4 ms, and 20 ms for potassium, lithium, and rubidium, respectively.

Figure 2

Schematic of the apparatus in top view. The two fermionic and the bosonic species are magneto-optically trapped in a first vacuum chamber. They are then magnetically transferred into an UHV glass cell, where they are simultaneously cooled into quantum degeneracy.

Figure 3

Temperatures (a) and atom numbers (b) during the very last part of simultaneous trapping and cooling of three species (filled symbols). ${}^{87}\mathrm{Rb}$ is evaporatively cooled, ${}^6\mathrm{Li}$ and ${}^{40}\mathrm{K}$ are sympathetically cooled by thermal contact with the rubidium cloud. Corresponding data for lithium in a two-species ${}^6\mathrm{Li}\mathrm{\text{−}}{}^{87}\mathrm{Rb}$ mixture are also shown (empty rectangles). The horizontal lines in (a) indicate the critical temperature $T_c$ for rubidium and half the Fermi temperature $T_F$ for the fermions, respectively. For our trap parameters and typical atom numbers of $1.5\times {10}^5$ for rubidium and ${10}^5$ for the fermionic species, the critical temperature is $T_c^{\mathrm{Rb}}=190\mathrm{nK}$ and the Fermi temperatures are $T_F^{\mathrm{Li}}=1.2\mu \mathrm{K}$ and $T_F^{\mathrm{K}}=470\mathrm{nK}$. The error bars represent the rms deviation for at least three consecutive repetitions.