Role of Interactions in {sup 87}Rb-{sup 40}K Bose-Fermi Mixtures in a 3D Optical Lattice

We investigate the effect of interspecies interaction on a degenerate mixture of bosonic {sup 87}Rb and fermionic {sup 40}K atoms in a three-dimensional optical lattice potential. Using a Feshbach resonance, the {sup 87}Rb-{sup 40}K interaction is tuned over a wide range. Through an analysis of the {sup 87}Rb momentum distribution, we find a pronounced asymmetry between strong repulsion and strong attraction. In the latter case, we observe a marked shift in the superfluid to Mott insulator transition, which we attribute to a renormalization of the Bose-Hubbard parameters due to self-trapping.