We report the observation of a broad magnetic Feshbach resonance with a large background scattering length in an ultracold fermionic mixture of ${}^{23}\mathrm{Na}{}^{40}\mathrm{K}$ molecules and ${}^{40}\mathrm{K}$ 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.

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• Cold and ultracold molecules
• Fermi gases
• Fermionic condensates
• Mixtures of atomic and/or molecular quantum gases
• Ultracold collisions
• Feshbach resonance