Experimental Study of the BEC-BCS Crossover Region in Lithium 6

We report Bose-Einstein condensation of weakly bound ${}^6\mathrm{L}\mathrm{i}_2$ molecules in a crossed optical trap near a Feshbach resonance. We measure a molecule-molecule scattering length of ${170}_{-60}^{+100}\mathrm{n}\mathrm{m}$ at $770\mathrm{G}$, in good agreement with theory. We study the 2D expansion of the cloud and show deviation from hydrodynamic behavior in the BEC-BCS crossover region.

Figure 1

Onset of Bose-Einstein condensation in a cloud of $2\times {10}^4$ ${}^6\mathrm{L}\mathrm{i}$ dimers at 770 G (a) and of $2\times {10}^4$ ${}^7\mathrm{L}\mathrm{i}$ atoms at 610 G (b) in the same optical trap. (a) 1.2 ms expansion profiles along the weak direction $x$ of confinement. (b) 1.4 ms expansion. The different sizes of the condensates reflect the large difference in scattering length $a_{\mathrm{m}}=170\mathrm{n}\mathrm{m}$ for ${}^6\mathrm{L}\mathrm{i}$ dimers and $a_7=0.55\mathrm{n}\mathrm{m}$ for ${}^7\mathrm{L}\mathrm{i}$. Solid line: Gaussian$+$Thomas-Fermi fit. Dashed line: Gaussian component. Condensate fractions are both $28\%$. ${\omega }_x/2\pi =0.59\left(4\right)\mathrm{k}\mathrm{H}\mathrm{z}$, ${\omega }_y/2\pi =1.6\left(1\right)\mathrm{k}\mathrm{H}\mathrm{z}$, and ${\omega }_z/2\pi =1.7\left(1\right)\mathrm{k}\mathrm{H}\mathrm{z}$ in (a). ${\omega }_x/2\pi =0.55\left(4\right)\mathrm{k}\mathrm{H}\mathrm{z}$, ${\omega }_y/2\pi =1.5\left(1\right)\mathrm{k}\mathrm{H}\mathrm{z}$, and ${\omega }_z/2\pi =1.6\left(1\right)\mathrm{k}\mathrm{H}\mathrm{z}$ in (b).

Figure 2

Integrated density profiles across the BEC-BCS crossover region. 1.4 ms time of flight expansion in the axial (a) and radial (b) direction. The magnetic field is varied over the whole region of the Feshbach resonance from $a>0$ ($B<822\mathrm{G}$) to $a<0$ ($B>822\mathrm{G}$). ●: Feshbach resonance peak.

Figure 3

(a) scattering length between the $|1/2,1/2\rangle$ and $|1/2,-1/2\rangle$ ${}^6\mathrm{L}\mathrm{i}$ states. The Feshbach resonance peak is located at 822 G (dotted line). (b): anisotropy of the cloud, (c) release energy across the BEC-BCS crossover region. In (c), the dot-dashed line corresponds to a $T=0$ ideal Fermi gas. The dashed curve is the release energy from a pure condensate in the Thomas-Fermi limit. The solid curve corresponds to a finite temperature mean field model described in the text with $T=0.6T_{\mathrm{C}}^0$. Arrow: $k_{\mathrm{F}}\left|a\right|=3$.