Control of the interaction in a Fermi-Bose mixture

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 ${}^{40}\mathrm{K}$ and bosonic ${}^{87}\mathrm{Rb}$. 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.

Figure 1

(Color online) (a) K atom number left in the trap after $50\mathrm{ms}$ hold time at a fixed magnetic field $B$ in nondegenerate mixture $(T=1.1\mu \mathrm{K})$; (b) efficiency of sympathetic cooling of fermions around the zero-crossing position; (c) theoretical expectation for the fermion-boson scattering length $a_{FB}$ at the Feshbach resonance; and (d) width of the Rb distribution after ballistic expansion from the trap. Here the degenerate mixture is prepared at $B=543.4\mathrm{G}$ (circles) or $B=551\mathrm{G}$ (triangles) and adiabatically brought to the final field $B$. The highlighted region marks the transition from a stable to an unstable regime.

Figure 2

(Color online) Profiles of the Bose gas in the mixture for various final values of the magnetic field ramp starting at $B=543.4\mathrm{G}$. The condensate survives despite the large atom losses as long as $B<B_0$ (continous lines). For $B>B_0$ the condensate is destroyed by collapse (dotted lines).

Figure 3

(Color online) (a) Evolution of the atom number in the bosonic component (circles) and in the fermionic component (triangles) after a rapid change of the magnetic field from $B_f=551\mathrm{G}$ to $B_f=547.5\mathrm{G}$. (b) Evolution of the width and vertical position (inset) of the bosonic component.