In high-energy and condensed-matter physics, particle exchange has an essential role in the understanding of long-range interactions and correlations. For example, the exchange of massive bosons leads to the Yukawa potential1,2, and phonon exchange between electrons gives rise to Cooper pairing in superconductors3. Here we show that, when a Bose–Einstein condensate of caesium atoms is embedded in a degenerate Fermi gas of lithium atoms, interspecies interactions can give rise to an effective trapping potential, damping, and attractive boson–boson interactions mediated by fermions. The latter, which is related to the Ruderman–Kittel–Kasuya–Yosida mechanism4, results from a coherent three-body scattering process. Such mediated interactions are expected to form new magnetic phases5 and supersolids6. We show that under suitable conditions, the mediated interactions can convert a stable Bose–Einstein condensate into a train of ‘Bose–Fermi solitons’7,8. The predicted long-range nature of the mediated interactions opens up the possibility of correlating distant atoms and preparing new quantum phases.
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The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.
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We thank M. Gajda for discussions and M. McDonald for a careful reading of the manuscript. This work was supported by National Science Foundation (NSF) grant no. PHY-1511696 and the University of Chicago Materials Research Science and Engineering Center, which is funded by the NSF under grant no. DMR-1420709.
Nature thanks Georg Bruun and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
Extended Data Fig. 1 Caesium–caesium and lithium–caesium scattering lengths over the studied magnetic field range.
The caesium–caesium scattering length aBB is shown with a blue line, and the lithium–caesium scattering length aBF with a red line. The interspecies Feshbach resonance near B = 892 G is used for sample preparation and the effective trapping frequency measurements, indicated by the light green shaded area. The caesium–caesium zero crossing near B = 880 G is used for the effective scattering length measurements, indicated by the blue shaded area. In this region, the lithium–caesium scattering length is nearly constant at aBF = −60a0 (ref. 22).
Top, Measured radius of the caesium BEC with (red circles) and without (blue circles) lithium. Bottom, Measured BEC number with (red circles) and without (blue circles) lithium. At every field, the size of the BEC with lithium present is smaller than the corresponding measurement without lithium. However, overall experimental drift makes absolute comparison difficult. Calibrating each measurement point-by-point as described enables us to extract the difference in scattering length cleanly, as can be seen in Fig. 3b. Error bars are calculated as the standard error of the mean from multiple measurements.