  We control the interspecies interaction in a two-species atomic quantum
mixture by tuning the magnetic field at a Feshbach resonance. The mixture is
composed by fermionic 40K and bosonic 87Rb. We observe effects of the large
attractive and repulsive interaction energy across the resonance, such as
collapse or a reduced spatial overlap of the mixture, and we accurately locate
the resonance position and width. Understanding and controlling instabilities
in this mixture opens the way to a variety of applications, including formation
of heteronuclear molecular quantum gases.
