Expansion of a Fermi Gas Interacting with a Bose-Einstein Condensate

We study the expansion of an atomic Fermi gas interacting attractively with a Bose-Einstein condensate. We observe a slower evolution of the radial-to-axial aspect ratio which reveals the importance of the mutual attraction between the two samples during the first phase of the expansion. For large atom numbers, we also observe a bimodal momentum distribution of the Fermi gas, which reflects the spatial distribution of the mixture in trap. This effect allows us to extract important information on the overlap of the two species across the collapse.

Figure 1

Modification of the expansion of a K Fermi gas due to the interaction with a Rb BEC. The radial-to-axial aspect ratio of a cloud of $4\times {10}^4$ fermions evolves more slowly in the presence of $3.5\times {10}^4$ condensed bosons (solid circles) than in the presence of a similar number of uncondensed bosons (open circles). Each data point is the average of five or six measurements. The dashed line is the calculated expansion of a pure Fermi gas, while the solid line is the prediction for an interacting Fermi gas [10].

Figure 2

Aspect ratio of the Fermi gas at $8\mathrm{ms}$ of expansion varying the atom numbers in the mixture. The interspecies overlap region is enhanced either (a) decreasing $N_F$, for $N_B\simeq 6\times {10}^4$ or (b) increasing $N_B$ with $N_F\simeq 2.4\times {10}^4$. Above $N_B=6\times {10}^4$, we observe the appearance of a bimodal radial [see Fig. 3b] . The aspect ratio of the central component (solid triangles) is obtained fitting both the radial and axial distribution with a two-peak Gaussian function. The dotted lines indicate the nominal value of the aspect ratio for a pure Fermi gas.

Figure 3

Radial momentum distribution of the Fermi gas, detected after $12\mathrm{ms}$ of expansion, for different atom numbers in the mixture. For $N_F=2\times {10}^4$, (a) we observe that below $N_B=6\times {10}^4$ the distribution is slightly affected by interspecies potential, while (b) above $N_B=6\times {10}^4$ a bimodal structure appears. (c) After the collapse, the remaining $1.2\times {10}^4$ fermions coexisting with $7.4\times {10}^4$ bosons exhibit a narrower distribution than the noninteracting gas (dotted line).

Figure 4

Evolution of the aspect ratio of the Fermi gas (solid triangles) and of the number of bosons (open circles) and fermions (solid circles) at the collapse [9]. During the final stage of the bosons evaporation, the aspect ratio decreases, and suddenly drops to a much lower value after the collapse (highlighted region). The aspect ratio is measured at $5\mathrm{ms}$ of expansion and the error bars are the standard deviation of different measurements.