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Chinese Physics B

Experimental investigation of evaporative cooling mixture of bosonic 87 Rb and fermionic 40 K atoms with microwave and radio frequency radiation

Pengjun Wang, Dezhi Xiong, Zhengkun Fu, Jing Zhang

Published Jan 1, 2011• 5 citations
Publisher

Abstract

We investigate sympathetic cooling fermions 40 K by evaporatively cooling bosonic 87 Rb atoms in a magnetic trap with microwave and radio frequency induced evaporations in detail. The mixture of bosonic and fermionic atoms is prepared in their polarized spin states | F = 9/2, m F = 9/2〉 for 40 K and | F = 2, m F = 2〉 for 87 Rb, which is trapped in Quadrupole—Ioffe—Configuration trap. Comparing microwave with radio frequency evaporatively cooling bosonic 87 Rb atoms with sympathetically cooling Fermi gas 40 K, we find that the presence of rubidium atoms in the |2, 1〉 Zeeman states, which are generated in the evaporative process, gives rise to a significant loss of 40 K due to inelastic collisions. Thus, the rubidium atoms populated in the |2, 1〉 Zeeman states should be removed in order to effectively perform sympathetically cooling 40 K with the evaporatively cooled 87 Rb atoms.

DOI
10.1088/1674-1056/20/1/016701

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We investigate sympathetic cooling fermions 40 K by evaporatively cooling bosonic 87 Rb atoms in a magnetic trap with microwave and radio frequency induced evaporations in detail. The mixture of bosonic and fermionic atoms is prepared in their polarized spin states | F = 9/2, m F = 9/2〉 for 40 K and | F = 2, m F = 2〉 for 87 Rb, which is trapped in Quadrupole—Ioffe—Configuration trap. Comparing microwave with radio frequency evaporatively cooling bosonic 87 Rb atoms with sympathetically cooling Fermi gas 40 K, we find that the presence of rubidium atoms in the |2, 1〉 Zeeman states, which are generated in the evaporative process, gives rise to a significant loss of 40 K due to inelastic collisions. Thus, the rubidium atoms populated in the |2, 1〉 Zeeman states should be removed in order to effectively perform sympathetically cooling 40 K with the evaporatively cooled 87 Rb atoms.
We report the experimental achievement of 87 Rb Bose-Einstein condensation in a magnetic trap with microwave and radio frequency (RF) induced evaporation.Evaporative cooling is realized by using 6.8 GHz microwave radiation driving the 87 Rb atoms to transit from the ground-state hyperfine state |F = 2, mF = 2 to |F = 1, mF = 1 .Compared with RF-induced evaporation, 87 Rb atoms are hardly to achieve pure condensate by microwave evaporation cooling due to the effect of atoms in the |F = 1, mF = 1 state being pumped back into the trapping |F = 2, mF = 1 state.
We report the experimental achievement of 87Rb Bose-Einstein condensation in a magnetic trap with microwave and radio frequency (RF) induced evaporation. Evaporative cooling is realized by using 6.8 GHz microwave radiation driving the 87Rb atoms to transit from the ground-state hyperfine state |F = 2,mF =2> to |F = 1,mF = 1>. Compared with RF-induced evaporation, 87Rb atoms are hardly to achieve pure condensate by microwave evaporation cooling due to the effect of atoms in the |F = 1,mF = 1> state being pumped back into the trapping |F = 2,mF = 1> state.
We report on the observation of sympathetic cooling of a cloud of fermionic 6Li atoms which are thermally coupled to evaporatively cooled bosonic 87Rb. Using this technique we obtain a mixture of quantum-degenerate gases, where the Rb cloud is colder than the critical temperature for Bose-Einstein condensation and the Li cloud is colder than the Fermi temperature. From measurements of the thermalization velocity we estimate the interspecies s-wave triplet scattering length |amx|=20(+9)(-6)aB. We found that the presence of residual rubidium atoms in the |2, 1> and the |1, -1> Zeeman substates gives rise to important losses due to inelastic collisions.
Progress on the realization of an experimental setup for the simultaneous creation of quantum degenerate samples of fermionic /sup 40/K and bosonic /sup 87/Rb is presented. The apparatus is based on a double MOT design with a 2D source MOT and a 3D collection MOT both acting on both species. After laser cooling selective forced microwave evaporative cooling of /sup 87/Rb on the hyperfine transition is planned in a magnetic Ioffe-Pritchard trap. The fermionic /sup 40/K component will be sympathetically cooled by the rubidium atoms without additional loss of particles until quantum degeneracy is reached. A high number of fermions is expected by using isotope enriched /sup 40/K dispensers for feeding the 2D source MOT.
Sympathetic cooling of two atomic isotopes is experimentally investigated. Using forced evaporation of a bosonic 7Li{}^{7}\mathrm{Li}7Li gas in a magnetic trap, a sample of $1.3\ifmmode\times\else\texttimes\fi{}{10}^{5}$ 6Li{}^{6}\mathrm{Li}6Li fermions has been sympathetically cooled to $9(3) \ensuremath{\mu}\mathrm{K},$ corresponding to 2.2(0.8)2.2(0.8)2.2(0.8) times the Fermi temperature. The measured rate constant for two-body inelastic collisions of the 7Li{}^{7}\mathrm{Li}7Li ∣2,2〉|2,2〉∣2,2〉 state at low magnetic field is ${1.0}_{\ensuremath{-}0.5}^{+0.8}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14} {\mathrm{cm}}^{3}{\mathrm{s}}^{\ensuremath{-}1}.$