Fermi-Bose Quantum Degenerate ${}^{\mathrm{40}}\mathrm{K}\mathrm{\text{−}}{}^{\mathrm{87}}\mathrm{R}\mathrm{b}$ Mixture with Attractive Interaction

We report on the achievement of simultaneous quantum degeneracy in a mixed gas of fermionic ${}^{\mathrm{40}}\mathrm{K}$ and bosonic ${}^{\mathrm{87}}\mathrm{R}\mathrm{b}$. Potassium is cooled to 0.3 times the Fermi temperature by means of an efficient thermalization with evaporatively cooled rubidium. Direct measurement of the collisional cross-section confirms a large interspecies attraction. This interaction is shown to affect the expansion of the Bose-Einstein condensate released from the magnetic trap, where it is immersed in the Fermi sea.

Figure 1

Simultaneous onset of Fermi degeneracy for ${}^{\mathrm{40}}\mathrm{K}$ (left) and of Bose-Einstein condensation for ${}^{\mathrm{87}}\mathrm{R}\mathrm{b}$ (right). The absorption images are taken for three decreasing temperatures, after 4.5 ms of expansion for K and 17.5 ms for Rb, and the sections show the profile of the momentum distributions. The bosons provide a thermometry of the system: in the coldest sample, the Rb BEC has a 40% thermal component, and the temperature is $T=0.74T_c=80\mathrm{n}\mathrm{K}$, which corresponds to $T=0.3T_F$ for K. The Thomas-Fermi radius of the K profile is $R=52\mu \mathrm{m}$, consistent with the radius expected for a degenerate Fermi gas.

Figure 2

Gaussian $1/e$ radius of the radial distribution of K atoms versus the reduced temperature. The temperature is given by the Rb sample and $T_F=250\mathrm{n}\mathrm{K}$. The solid line is the theoretical prediction for an ideal Fermi gas, while the dotted line is the classical behavior.

Figure 3

Thermal exchange between the two degenerate gases. The Gaussian width of ${}^{\mathrm{40}}\mathrm{K}$ (circles) increases only when ${}^{\mathrm{87}}\mathrm{R}\mathrm{b}$ atoms (triangles) are almost completely evaporated from the trap, as explained in the text. Each data point is the average of three or four measurements, and the solid lines are guides to the eye.

Figure 4

Modification of the expansion of a Rb BEC due to interaction with the K Fermi gas. The radial-to-axial aspect ratio increases more rapidly with time for condensates created with K (solid circles) than for pure condensates (open circles). The two curves are the theoretical predictions for the expansion of a pure BEC for the trap parameters of the present experiment (solid line) and for trap frequencies 10% larger (dashed line).