Decay of an Ultracold Fermionic Lithium Gas near a Feshbach Resonance

We studied the magnetic field dependence of the inelastic decay of an ultracold, optically trapped fermionic ${}^{\mathrm{6}}\mathrm{L}\mathrm{i}$ gas of different spin compositions. The spin mixture of the two lowest hyperfine states showed two decay resonances at 550 and 680 G, consistent with the predicted Feshbach resonances for elastic $s$-wave collisions. The observed lifetimes of several hundred ms are much longer than the expected time for Cooper pair formation and the phase transition to superfluidity in the vicinity of the Feshbach resonance.

Figure 1

Magnetic field dependence of inelastic decay in clouds of fermionic ${}^{\mathrm{6}}\mathrm{L}\mathrm{i}$. The fraction of atoms remaining after the 500 ms magnetic field pulse is shown for different spin compositions of the cloud. (a) For the state $|2⟩$, no significant loss was observed. (b) The energetically lowest state $|1⟩$ exhibits a weak decay resonance at $\approx 680\mathrm{G}$. (c) The $50\%\mathrm{\text{−}}50\%$ mixture of two spin states shows two decay resonances, at 550 and 680 G. (d) The two resonances are shown with higher density of data points and for 2 s magnetic field pulses. Each data point represents an average of three measurements.

Figure 2

Decay of the atom number at 680 G. (a) The data plotted as $1/N$ show a linear time dependency, consistent with two-body decay. (b) The same data plotted as $1/N^2$ clearly show nonlinear dependency. For the resonance at 550 G, the comparison of least square fits also revealed consistency with a two-body decay.