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Journal of Modern Optics

Degenerate K–Rb Fermi–Bose gas mixtures with large particle numbers

Silke Ospelkaus, C. Ospelkaus, R. Dinter, Jacob Fuchs, M. Nakat, K. Sengstock, Kai Bongs

Published Aug 23, 2006• 5 citations
Publisher

Abstract

We present experimental investigations on the mutual influence of fermionic 40K and bosonic 87Rb atoms during the various trapping and cooling stages needed to reach quantum degeneracy of the sample. In particular we present particle number optimization strategies, which have led to large 40K–87Rb mixtures in the quantum degenerate regime. In fact the achievable particle numbers are only limited by a collapse due to an attractive overall mean-field interaction for the mixture. We discuss the influence of dynamical effects on the collapse parameters and find our critical particle numbers consistent with predictions based on recent Feshbach resonance measurements. We also investigate loss mechanisms of the mixture where we have identified three-body K–Rb–Rb losses as the dominant loss process and have measured the loss coefficient to be .

DOI
10.1080/09500340600777763

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We present experimental investigations on the mutual influence of fermionic 40K and bosonic 87Rb atoms during the various trapping and cooling stages needed to reach quantum degeneracy of the sample. In particular we present particle number optimization strategies, which have led to large 40K–87Rb mixtures in the quantum degenerate regime. In fact the achievable particle numbers are only limited by a collapse due to an attractive overall mean-field interaction for the mixture. We discuss the influence of dynamical effects on the collapse parameters and find our critical particle numbers consistent with predictions based on recent Feshbach resonance measurements. We also investigate loss mechanisms of the mixture where we have identified three-body K–Rb–Rb losses as the dominant loss process and have measured the loss coefficient to be .
We have studied effects of interspecies attraction in a Fermi-Bose mixture over a large regime of particle numbers in the $^{40}\mathrm{K}\mathrm{\text{\ensuremath{-}}}^{87}\mathrm{Rb}$ system. We report on the observation of a mean-field driven collapse at critical particle numbers of $1.2\ifmmode\times\else\texttimes\fi{}{10}^{6}$ 87Rb^{87}\mathrm{Rb}87Rb atoms in the condensate and $7.5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ 40K^{40}\mathrm{K}40K atoms consistent with mean-field theory for a scattering length of ${a}_{\mathrm{FB}}=\ensuremath{-}284{a}_{0}$. For large overcritical particle numbers, we see evidence for revivals of the collapse. Part of our detailed study of the decay dynamics and mechanisms is a measurement of the ($^{87}\mathrm{Rb}\mathrm{\text{\ensuremath{-}}}^{87}\mathrm{Rb}\mathrm{\text{\ensuremath{-}}}^{40}\mathrm{K}$) three-body loss coefficient ${K}_{3}=(2.8\ifmmode\pm\else\textpm\fi{}1.1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}28}\text{ }\text{ }{\mathrm{cm}}^{6}/\mathrm{s}$, which is an important parameter for dynamical studies of the system.
We demonstrate tuning of interactions between fermionic 40K and bosonic 87Rb atoms by Feshbach resonances and access the complete phase diagram of the harmonically trapped mixture from phase separation to collapse. On the attractive side of the resonance, we observe a strongly enhanced mean-field energy of the condensate due to the mutual mean-field confinement, predicted by a Thomas-Fermi model. As we increase heteronuclear interactions beyond a threshold, we observe an induced collapse of the mixture. On the repulsive side of the resonance, we observe vertical phase separation of the mixture in the presence of the gravitational force, thus entering a completely unexplored part of the phase diagram of the mixture. In addition, we identify the 515 G resonance as p wave by its characteristic doublet structure.
We report on the optimal production of the Bose and Fermi mixtures with 87Rb and 40K in a crossed optical dipole trap (ODT). We measure the atomic number and lifetime of the mixtures in combination of the spin state |F = 9/2, m F = 9/2〉 of 40K and |1, 1〉 of 87Rb in the ODT, which is larger and longer compared with the combination of the spin state |9/2, 9/2〉 of 40K and |2, 2〉 of 87Rb in the ODT. We observe the atomic numbers of 87Rb and 40K 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 obtain the large atomic number of 40K (∼6 × 106) or the mixtures of atoms with an equal number (∼1.6 × 106) at the end of evaporative cooling in the ODT. We experimentally investigate the evaporative cooling in an enlarged volume of the ODT via adding a third laser beam to the crossed ODT and found that more atoms (8 × 106) 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 87Rb40K.
Recent experiments performed with fermionic /sup 6/Li and /sup 40/K atoms, which possess both Feshbach resonances at convenient magnetic fields are discussed. When /sup 40/K is mixed with a Bose-Einstein condensate of /sup 87/Rb, for sufficient atom numbers of both species, a collapse of the Fermi gas is observed. This is the result of the strong interspecies attractive interaction which overcomes the Fermi pressure in the quantum degenerate regime. As for the case of phonon-induced interaction between electrons in superconductors, it is predicted that the boson-fermion attractive interaction favors Cooper pairing of fermions when prepared in a mixture of two states.
We report a compact experimental setup for producing a quantum degenerate mixture of Bose 23Na and Fermi 40K gases. The atoms are collected in dual dark magneto–optical traps (MOT) with species timesharing loading to reduce the light-induced loss, and then further cooled using the gray molasses technique on the D 2 line for 23Na and D 1 line for 40K. The microwave evaporation cooling is used to cool 23Na in | F = 2,mF = 2〉 in an optically plugged magnetic trap, meanwhile, 40K in | F = 9/2,mF = 9/2〉 is sympathetically cooled. Then the mixture is loaded into a large volume optical dipole trap where 23Na atoms are immediately transferred to |1,1〉 for further effective cooling to avoid the strong three-body loss between 23Na atoms in |2,2〉 and 40K atoms in |9/2,9/2〉. At the end of the evaporation in optical trap, a degenerate Fermi gas of 40K with 1.9 × 105 atoms at T/TF = 0.5 in the |9/2,9/2〉 hyperfine state coexists with a Bose–Einstein condensate (BEC) of 23Na with 8 × 104 atoms in the |1,1〉 hyperfine state at 300 nK. We also can produce the two species mixture with the tunable population imbalance by adjusting the 23Na magneto–optical trap loading time.