Attractive atom-dimer interaction in a mass-imbalanced Fermi-Fermi mixture*

Mass imbalance in strongly interacting mixtures of ultracold fermions is predicted to lead to new pairing phenomena and quantum phases. We report on a striking phenomenon that appears as a unique consequence of mass imbalance, having no counterpart in a mass-balanced case. We investigate a $^{6}$Li-$^{40}$K Fermi-Fermi mixture in the regime of strong interactions on the repulsive side of an interspecies Feshbach resonance. Using radio-frequency spectroscopy we find that, for a sufficiently strong repulsive {\it s}-wave interaction, the $^{40}$K atoms and the $^{6}$Li$^{40}$K dimers interact attractively, which is in strong contrast to the mass-balanced case. This surprising behavior is related to the existence of a $\uparrow \uparrow \downarrow$ trimer state in $\uparrow \downarrow$ Fermi-Fermi mixtures with a mass ratio m$_{\uparrow}$/m$_{\downarrow} > 8.2 $. For our mass ratio of m$_{\rm K}$/m$_{\rm Li} = 6.64$, this trimer state turns into a {\it p}-wave atom-dimer scattering resonance, giving rise to an attractive interaction.