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

Abstract

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]

Time-Dependent Impurity in Ultracold Fermions: Orthogonality Catastrophe and Beyond

“…The basic ingredients required for the experimental implementation of our proposal are the following: (a) a quantum-degenerate Bose-Fermi [29][30][31][32][33][34][35][36] or Fermi-Fermi [37][38][39][40][41][42][43] mixture, (b) the ability to trap one type of atom by a strong optical-lattice potential, (c) the control of the interaction strength between the impurity and host atom, which is achieved by changing the impurity hyperfine state, and (d) the ability to achieve temperatures that are low enough to observe the OC.…”

Thermodynamics of Ultracold Fermi Gases

“…The mass di erence degree of freedom is expected to enrich the phase diagram with new states of matter [248,249,250]. Recent experiments exploring the physics of 6 Li and 40 K mixtures could address this subject in a near future [251,252]. As an illustration of our method, we determined the equation of state of a weakly-interacting Bose gas.…”

Heteropairing and component-dependent pseudogap phenomena in an ultracold Fermi gas with different species with different masses

“…Although the superfluid phase transition of this Fermi-Fermi mixture has not been reported yet, the Fermi degenerate regime has been achieved [20,24]. In addition, since a tunable interaction associated with a Feshbach resonance between 6 Li and 40 K atoms [19,22,23], as well as the formation of 6 Li-40 K hetero molecules [21], has been realized, the observation of superfluid behaviors seems imminent.…”