Quantum-Degenerate Mixture of Fermionic Lithium and Bosonic Rubidium Gases

Abstract

We report on the observation of sympathetic cooling of a cloud of fermionic 6 Li atoms which are thermally coupled to evaporatively cooled bosonic 87 Rb. Using this technique we obtain a mixture of quantum-degenerate gases, where the Rb cloud is colder than the critical temperature for BoseEinstein condensation and the Li cloud colder than the Fermi temperature. From measurements of the thermalization velocity we estimate the interspecies s-wave triplet scattering length |as| = 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.PACS numbers: 05.30. Fk, 05.30.Jp, 32.80.Pj, The recent realization of degenerate gases of homonuclear diatomic molecules is an important achievement for the physics of ultracold dilute quantum gases [1,2]. The key to this spectacular success was the use of Feshbach resonances which allow for an adiabatic transformation from free pairs to molecules in the least bound state close to the dissociation energy [3]. The large internal energy makes the gas unstable against collisions between the molecules, which transform the energy into kinetic energy of the collision partners [4]. The very long lifetimes of up to several 10 s observed for dimers made of fermionic 6 Li [2] are explained by the fact that the atoms are only loosely bound with a binding energy of 1 µK such that they still behave very much like individual atoms [5]. Collisions are thus suppressed due to Pauli-blocking. Fermionic lithium is now a very promising candidate for exploring the cross-over regime between the Bardeen-Cooper-Schrieffer (BCS) model for superconductivity based on Cooper pairs and the regime of a Bose-Einstein condensate of composite bosons [6].In contrast to homonuclear dimers molecules made of two different atomic species exhibit a permanent electric dipole moment. The LiRb dimer has a very large permanent electric dipole moment of up to 4.2 Debye [7]. The intermolecular interaction in such a heteronuclear gas has a pronounced long-range character, which fundamentally alters its behavior at low temperatures [8] and brings global properties of the gas into play. The geometric shape of the cloud for instance will influence the total interaction energy and thus the stability of the gas [9].A possible approach to generate a polar molecular gas would use a Feshbach resonance in close analogy to the homonuclear experiments [1,2]. Only recently first heteronuclear Feshbach resonances have been observed [10]. Weakly bound heteronuclear molecules are expected to be relatively stable against collisions if one of the two atoms is a fermion [5]. For such molecules Pauli-blocking is at least partially effective, even though reactive collisions may reduce their lifetime [11]. As an alternative to Feshbach resonances, photoassociation is used to transform atomic pairs into molecules. Photoassociation has been extensively investigated for the homonuclear case [12], and there are already examples of successf...

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

“…The 40 K-87 Rb inter-species scattering cross section is relatively large [16] and we successfully cooled 40 K into quantum degeneracy using a 29 s long MW-evaporation ramp for rubidium. However, sympathetic cooling of 6 Li by 87 Rb is more challenging because thermalization is considerably slower due to the small 6 Li-87 Rb inter-species scattering cross section [11], which is roughly two orders of magnitude smaller than the 40 K-87 Rb cross section (in the low-temperature limit). In addition, the larger mass difference results in both a lower energy transfer per elastic collision, which is only partially compensated by a higher mean thermal relative velocity, and a reduced density overlap of the two clouds.…”

Dual-species Bose-Einstein condensate ofRb87andCs133McCarron1, Cho2, Jenkin3 et al. 2011Phys. Rev. A243122680View full textAdd to dashboardCite

“…Quantum degenerate mixtures [1][2][3][4][5][6][7][8][9][10][11][12] of atomic gases are currently the subject of intensive research, exhibiting rich physics inaccessible in single-species experiments. Phase separation of the constituent gases is a particularly dramatic example [2,[8][9][10].…”

Spontaneous Quantum Hall Effect in an Atomic Spinor Bose-Fermi Mixture

“…Changing the laser frequency can further enhance this capability to reverse the relative amplitude of interspecies lattice potential [35]. As in experiments only the interspecies s-wave triplet scattering length (∼ 20 a B ) between 87 Rb and 6 Li has been measured [33], the parameters in our model, U bf and J bf , cannot be determined. Whether Bose-Fermi density interaction are strong enough to induce DWs requires future experimental developments.…”