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arXiv:cond-mat/0107442 · Physical Review Letters

Quasipure Bose-Einstein Condensate Immersed in a Fermi Sea

Florian Schreck, Lev Khaykovich, Kristan L. Corwin, G. Ferrari, Thomas Bourdel, J. Cubizolles, Ch Salomon

Published Aug 7, 2001• 866 citations
arXivHTMLPDF

Abstract

We report the observation of coexisting Bose-Einstein condensate (BEC) and Fermi gas in a magnetic trap. With a very small fraction of thermal atoms, the ${}^{7}\mathrm{Li}$ condensate is quasipure and in thermal contact with a ${}^{6}\mathrm{Li}$ Fermi gas. The lowest common temperature is $0.28\ensuremath{\mu}\mathrm{K}\ensuremath{\simeq}0.2(1){T}_{\mathrm{C}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.2(1){T}_{\mathrm{F}}$ where ${T}_{\mathrm{C}}$ is the BEC critical temperature and ${T}_{\mathrm{F}}$ the Fermi temperature. The ${}^{7}\mathrm{Li}$ condensate has a one-dimensional character.

DOI
10.1103/physrevlett.87.080403

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This thesis presents studies of quantum degenerate atomic gases of fermionic 6Li and bosonic 7Li. Degeneracy is reached by evaporative cooling of 7Li in a strongly confining magnetic trap. Since at low temperatures direct evaporative cooling is not possible for a polarized fermionic gas, 6Li is sympathetically cooled by thermal contact with 7Li. In a first series of experiments both isotopes are trapped in their low-field seeking higher hyperfine states. A Fermi degeneracy of is achieved for 105 fermions. For more than ~300 atoms, the 7Li condensate collapses, due to the attractive interatomic interaction in this state. This limits the degeneracy reached for both species. To overcome this limit, in a second series of experiments 7Li and 6Li atoms are transferred to their low field seeking lower hyperfine states, where the boson-boson interaction is repulsive but weak. The inter-isotope collisions are used to thermalize the mixture. A 7Li Bose-Einstein condensate (BEC) of 104 atoms immersed in a Fermi sea is produced. The BEC is quasi-one-dimensional and the thermal fraction can be negligible. The measured degeneracies are . The temperature is measured using the bosonic thermal fraction, which vanishes at the lowest temperatures, limiting our measurement sensitivity. In a third series of experiments, the bosons are transferred into an optical trap and their internal state is changed to , the lowest energy state. A Feshbach resonance is detected and used to produce a BEC with tunable atomic interactions. When the effective interaction between atoms is tuned to be small and attractive, we observe the formation of a matter-wave bright soliton. Propagation of the soliton without spreading over a macroscopic distance of 1.1~mm is observed.
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We theoretically study the static and dynamic behavior of a BEC immersed in a large Fermi sea of ultracold atoms under conditions of tunable interspecies interaction. The degenerate Bose-Fermi mixture is kept in an elongated trap, typical for a single-beam optical dipole trap. We focus on the case of repulsive Bose-Fermi interaction and develop mean-field models to simulate the system over a wide range of repulsion strength. We further get analytical solutions in the regimes of phase separation and weak interaction. We obtain static density profiles and the frequency of the radial breathing mode, which is an elementary dynamic phenomenon of the mixture. Our results unveil the structure of the Bose-Fermi interface and describe the origin of the frequency shift of the breathing mode when the components become phase-separated at strong repulsion. We show that the mediated interaction between bosons induced by the Fermi sea can be understood as an adiabatic second-order mean-field effect, which is valid also beyond the weak-interaction regime for relevant experimental conditions. These results are consistent with our recent observations [Lous et al., Phys. Rev. Lett. 120, 243403 (2018); Huang et al., Phys. Rev. A 99, 041602(R) (2019)] in a mixture of 41K^{41}\mathrm{K}41K and 6Li^{6}\mathrm{Li}6Li.
We report on the attainment of a spin-polarized Fermi sea of 87Sr^{87}\mathrm{Sr}87Sr in thermal contact with a Bose-Einstein condensate (BEC) of 84Sr^{84}\mathrm{Sr}84Sr. Interisotope collisions thermalize the fermions with the bosons during evaporative cooling. A degeneracy with T/TF=0.30(5)T/{T}_{F}=0.30(5)T/TF​=0.30(5) is reached for a 87Sr^{87}\mathrm{Sr}87Sr Fermi sea of $2\ifmmode\times\else\texttimes\fi{}{10}^{4}$ atoms together with an almost pure 84Sr^{84}\mathrm{Sr}84Sr BEC of 105{10}^{5}105 atoms.