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 Li and 40 K gases by an evaporatively cooled bosonic 87 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 Li gas by 87 Rb is increased by the presence of 40 K through catalytic cooling. Due to the differing physical properties of the two components, the quantum degenerate 6 Li-40 K Fermi-Fermi mixture is an excellent candidate for a stable, heteronuclear system allowing to study several so far unexplored types of quantum matter. [3], which allowed to study the crossover regime between a molecular Bose-Einstein condensate (BEC) and a BardeenCooper-Schrieffer (BCS) like gas of paired fermions [4]. Current research aims at simulating correlated manybody quantum systems with ultracold gases. A particularly intriguing goal is the realization of a fermionic quantum gas with two different atomic species, which is a well controllable system and is predicted to be stable [5]. Due to the mass difference, it offers a variety of analogies to other many-body systems, in particular to a spatially inhomogeneous superfluid phase predicted to occur in certain types of high temperature superconductors [6]. Further, a transition to a cristalline phase in the bulk gas [7] and the possibility to simulate baryonic phases of QCD [8] have been theoretically proposed. Moreover, the mixture bears the prospect to create heteronuclear ground state molecules [9], in this way realizing a quantum gas with a particularly large dipolar interaction [10]. Finally, a two-species mixture offers the additional possibility to tune interactions and to conveniently apply componentselective methods. The main result reported in this letter is the first production of such a quantum degenerate twospecies Fermi-Fermi mixture opening the door to aforementioned unexplored types of quantum matter. This goal was attained by achieving efficient sympathetic cooling of fermionic 6 Li and 40 K by an evaporatively cooled bosonic 87 Rb gas. Moreover, we have also realized the first triple quantum degenerate mixture (see fig.1), and therefore will be able to compare quantum properties of Fermi-Fermi and Bose-Fermi mixtures directly.The basic idea of our experimental strategy is to sym- pathetically cool the fermions by a large rubidium cloud. In this way, the atom numbers of the fermions are in principle not reduced by evaporation and the initial fermion clouds can be loaded with reduced experimental effort. However, the challenge is to combine the different constraints which the individual atomic species enforce on the set of trapping and cooling parameters. Especially, arXiv:0710.2779v1 [cond-mat.other]
We report on the first creation of ultracold bosonic heteronuclear molecules of two fermionic species, 6 Li and 40 K, by a magnetic field sweep across an interspecies s-wave Feshbach resonance. This allows us to associate up to 4 × 10 4 molecules with high efficiencies of up to 50%. Using direct imaging of the molecules, we measure increased lifetimes of the molecules close to resonance of more than 100 ms in the molecule-atom mixture stored in a harmonic trap.PACS numbers: 03.75. Ss, 37.10.Pq Two-component mixtures of fermionic quantum gases have attracted much interest over the past years. In these systems, long-lived, weakly bound molecules were produced made up of two atoms of the same species in different internal states [1]. The long lifetimes observed for these molecular gases are a consequence of the Pauli principle, which suppresses three-body collisions, and hence vibrational quenching, in a system of not more than two distinguishable components [2]. More recently, the interest has shifted towards ultracold heteronuclear diatomic molecules, which can have a large electric dipole moment [3]. So far, Bose-Bose [4] and Bose-Fermi [5] dimers have been produced. However, among the ultracold heteronuclear dimers the Fermi-Fermi molecules are of special interest since they are expected to exhibit long lifetimes for the same reasons as in the homonuclear case [6]. Long-lived polar molecules open the door to the creation of a molecular Bose-Einstein condensate (BEC) [7] with anisotropic, electric dipolar interaction and show potential for precision measurements [8] and novel quantum information experiments [9]. Furthermore, the two-species Fermi-Fermi mixture may allow the realization of novel quantum phases [10,11,12] and offers the possibility to tune interactions and to conveniently apply componentselective experimental methods.In this Letter, we present the first production of ultracold diatomic molecules composed of two different fermionic atomic species. We study the creation process of the molecules, their lifetime in a molecule-atom mixture and give an upper bound for their magnetic moment.We initially create an ultracold two-species FermiFermi mixture by sympathetic cooling of the fermionic species 6 Li and 40 K with an evaporatively cooled bosonic species, 87 Rb, as described previously [13,14]. During the cooling process, the three species are confined in a magnetic trap in their most strongly confined and collisional stable states 87 Rb |F = 2, m F = 2 , 40 K |9/2, 9/2 , and 6 Li |3/2, 3/2 . For the exploitation and study of Feshbach resonances (FR), the apparatus was extended by an optical dipole trap (ODT) and a setup that allows us to apply a stable homogeneous magnetic field (FB field) of up to 1 kG in the horizontal plane. The ODT is realized by two perpendicular laser beams with the two foci coinciding at the center of the magnetic trap. The first (second) beam points along the horizontal (vertical) axis and has a 1/e 2 -radius of 55 µm (50 µm). The two beams originate from a single-mode, s...
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