Observation of fermion-mediated interactions between bosonic atoms

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 potential(1,2), and phonon exchange between electrons gives rise to Cooper pairing in superconductors(3). 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 mechanism(4), results from a coherent three-body scattering process. Such mediated interactions are expected to form new magnetic phases(5) 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.