Broad Feshbach Resonance with a Large Background Scattering Length in a Fermionic Atom-Molecule Mixture

We report the observation of a broad magnetic Feshbach resonance with a large background scattering length in an ultracold fermionic mixture of ^{23}Na^{40}K molecules and ^{40}K atoms, with both species prepared in their lowest hyperfine states. The Feshbach resonance is characterized by measuring resonantly enhanced loss rates and elastic scattering cross sections via cross-species thermalization. The large background scattering length can drive the atom-molecule mixture into the hydrodynamic regime when the magnetic field is far from the resonance. We observe that the center-of-mass motions of the atoms and molecules are phase locked and oscillate with a common frequency due to hydrodynamic drag effects. This broad atom-molecule Feshbach resonance with its large background scattering length opens up a new avenue toward studying strongly interacting fermionic gases with mass imbalance.