Fermion Mediated Pairing in the Ruderman-Kittel-Kasuya-Yosida to Efimov Transition Regime

The Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction and Efimov physics are two distinct quantum phenomena in condensed matter and nuclear physics, respectively. The RKKY interaction describes correlations between impurities mediated by an electron gas, while Efimov physics describes universal bound states of three particles with resonant interactions. Recently, both effects have been observed in Bose-Fermi mixtures in the weak and resonant interaction regimes, respectively. Intriguing conjectures exist to elucidate how the two phenomena meet in the transition regime where the mixture is strongly interacting. In this Letter, we explore the RKKY-Efimov transition in a mixture of bosonic ^{133}Cs and fermionic ^{6}Li near a tunable interspecies Feshbach resonance. From dispersion and relaxation measurements, we find that the transition is highlighted by a fermion-mediated scattering resonance between Cs atoms and a weaker resonance on Li atoms. These resonances represent reactive scattering of Cs and Li atoms in the many-body regime, which reduce to an Efimov resonance in the thermal gas regime. Our observation demonstrates the intriguing interplay of two-, three-, and many-body physics in a Bose-Fermi mixture that connects condensed matter physics, nuclear physics, and quantum many-body chemistry.