In high energy and condensed matter physics, particle exchange plays an essential role in the understanding of long-range interactions. For example, the exchange of massive bosons leads to the Yukawa potential [1,2]. Phonon exchange between electrons gives rise to Cooper pairing in superconductors [3]. When a Bose-Einstein condensate (BEC) of Cs is embedded in a degenerate Fermi gas of Li, we show that interspecies interactions can give rise to an effective trapping potential, damping, and attractive boson-boson interactions mediated by fermions. The latter, related to the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism [4], results from a coherent three-body scattering process. Such mediated interactions are expected to form novel magnetic phases [5] and supersolids [6]. We show that for suitable conditions, the mediated interactions can convert a stable BEC into a train of "Bose-Fermi solitons" [7,8].

Efimov physics is a universal phenomenon in quantum threebody systems. For systems with resonant two-body interactions, Efimov predicted an infinite series of three-body bound states with geometric scaling symmetry 1 . These Efimov states, first observed in cold caesium atoms 2 The first Efimov resonance position is crucial to understanding Efimov physics, as from it one may derive the absolute scale of parameters for subsequent Efimov resonances. Previous experiments in homonuclear systems yield an interesting observation: the first Efimov resonance position a − appears to follow a universal formula a − ≈ −9 r vdW (ref. 14), where r vdW is the van der Waals length of the molecular potential 15 . Calculations from universal theory based on a single-channel model confirm this 'van der Waals universality' for Feshbach resonances with s res > 1 (refs 16-20), where the resonance strength parameter s res characterizes the strength of the coupling between open and closed channels of the Feshbach resonance 15 . Similar theories also give universal but modified predictions for heteronuclear systems 21,22 . Further calculations predict significant deviations from the universal prediction for resonances with s res 1 (refs 18,23-25). Experiments reaching down to s res = 0.11, however, have shown little or no dependence on s res (ref. 9). Experimental results, including this work, are summarized in Fig. 1.In this Letter, we compare Efimov spectra in a 6 Li-133 Cs mixture near one broad (s res = 0.66) and one very narrow (s res = 0.05) interspecies Feshbach resonance at B 0 = 889 and 893 G, respectively. These two Feshbach resonances, shown in Fig. 2a, differ significantly in resonance strength, but only slightly in Cs-Cs scattering length (a CsCs = 200 and 260a 0 , where a 0 is the Bohr radius). Therefore, this pair of resonances is an excellent case to assess the influence of the Feshbach resonance strength on Efimov physics while keeping other parameters nearly identical. We observe a significant difference in Efimov resonance position between the two Feshbach resonances. Our measurements also show that the resonance associated with the first Efimov state is suppressed near both Feshbach resonances. To resolve Efimov features near the narrow Feshbach resonance at 893 G, we require milligauss level control of our magnetic field. For this, we improve our magnetic field control from our previous work 12 . In addition, we develop a scheme to precisely monitor the magnetic field, achieving a precision below 3 mG in each measurement (see Methods). We also incorporate a new dual colour optical trap to eliminate relative gravitational sag of the atoms, which prevented mixing of the two species at temperatures below 200 nK in our previous work 12 (see Methods).With these improvements to the apparatus, we prepare spinpolarized Li and Cs samples at temperatures as low as 50 nK with tunable overlap. We vary magnetic field to tune the interspecies scattering length. In each scan, we randomly order the magnetic fields to eliminate sys...

We report on the formation of a stable quantum degenerate mixture of fermionic 6 Li and bosonic 133 Cs in an optical trap by sympathetic cooling near an interspecies Feshbach resonance. New regimes of the quantum degenerate mixtures are identified. With moderate attractive interspecies interactions, we show that a degenerate Fermi gas of Li can be fully confined in the Cs condensate without external potentials. For stronger attraction where mean-field collapse is expected, no such instability is observed. In this case, we suggest the stability is a result of dynamic equilibrium, where the interspecies three-body loss prevents the collapse. Our picture is supported by a rate equation model, and the crossover between the thermalization rate and the observed inelastic loss rate in the regime where the mean-field collapse is expected to occur.