Quantum degenerate Bose-Fermi mixture of chemically different atomic species with widely tunable interactions

We have created a quantum degenerate Bose-Fermi mixture of Na-23 and K-40 with widely tunable interactions via broad interspecies Feshbach resonances. Over 30 Feshbach resonances between Na-23 and K-40 were identified, including p-wave multiplet resonances. The large and negative triplet background scattering length between Na-23 and K-40 causes a sharp enhancement of the fermion density in the presence of a Bose condensate. As explained via the asymptotic bound-state model, this strong background scattering leads to wide Feshbach resonances observed at low magnetic fields. Our work opens up the prospect to create chemically stable, fermionic ground-state molecules of Na-23-K-40, where strong, long-range dipolar interactions would set the dominant energy scale.