Production of Long-Lived Ultracold ${\mathrm{L}\mathrm{i}}_2$ Molecules from a Fermi Gas

We create weakly bound ${\mathrm{L}\mathrm{i}}_2$ molecules from a degenerate two component Fermi gas by sweeping a magnetic field across a Feshbach resonance. The atom-molecule transfer efficiency can reach $85\%$ and is studied as a function of magnetic field and initial temperature. The bosonic molecules remain trapped for 0.5 s and their temperature is within a factor of 2 from the Bose-Einstein condensation temperature. A thermodynamical model reproduces qualitatively the experimental findings.

Figure 1

Calculated scattering length $a$ versus magnetic field for the ${}^6\mathrm{L}\mathrm{i}$ $|F,m_F⟩=|1/2,1/2⟩$, $|1/2,-1/2⟩$ mixture near the 810 G Feshbach resonance. Scanning over the resonance from 1 to 2 in 50 ms produces cold and trapped molecules with an efficiency up to $85\%$, resulting in an almost complete disappearance of the atomic signal at 2. Reversing then the scan to the initial position reestablishes the initial atomic signal. In all cases, atoms are detected and imaged at position 4 after abrupt switch-off of the $B$ field.

Figure 2

(a) Fraction of atoms in the molecular bound state, $(N_3-N_2)/N_3$, versus magnetic field at position 2 in Fig. 1. (b) Corresponding atom temperature at position 2 (squares) and position 3 (circles). Solid (dotted) line corresponds to the noninteracting (interacting) thermodynamical model described in the text.

Figure 3

Fraction of atoms bound in a molecule as a function of the depth of the dipole trap for $B=689\mathrm{G}$, $a=78\mathrm{n}\mathrm{m}$, $E_b=12\mu \mathrm{K}$.

Figure 4

Decay of ${\mathrm{L}\mathrm{i}}_2$ molecules in the optical trap for two values of the magnetic field: (a) $B=689\mathrm{G}$, $a=78\mathrm{n}\mathrm{m}$; (b) $B=636\mathrm{G}$, $a=35\mathrm{n}\mathrm{m}$. Note the difference in time scales. Solid lines are fits with two-body and one-body processes giving initial time constants of 500 ms in (a) and 20 ms in (b).