Observation of a Strong Atom-Dimer Attraction in a Mass-Imbalanced Fermi-Fermi Mixture

We investigate a mixture of ultracold fermionic K-40 atoms and weakly bound (LiK)-Li-6-K-40 dimers on the repulsive side of a heteronuclear atomic Feshbach resonance. By radio-frequency spectroscopy we demonstrate that the normally repulsive atom-dimer interaction is turned into a strong attraction. The phenomenon can be understood as a three-body effect in which two heavy K-40 fermions exchange the light Li-6 atom, leading to attraction in odd partial-wave channels (mainly p wave). Our observations show that mass imbalance in a fermionic system can profoundly change the character of interactions as compared to the well-established mass-balanced case.