Lifetime of Molecule-Atom Mixtures near a Feshbach Resonance in ${}^{\mathrm{40}}\mathrm{K}$

We report a dramatic magnetic-field dependence in the lifetime of trapped, ultracold diatomic molecules created through an $s$-wave Feshbach resonance between fermionic atoms. The molecule lifetime increases from less than 1 ms away from the Feshbach resonance to greater than 100 ms near resonance. We also have measured the trapped atom lifetime as a function of magnetic field near the Feshbach resonance; we find that the atom loss is more pronounced on the side of the resonance containing the molecular bound state.

Figure 1

Remaining atom number after creating molecules by lowering the magnetic field across the Feshbach resonance to $B$. For this measurement we start with a two-component Fermi gas in the $|9/2,-7/2⟩$ and $|9/2,-9/2⟩$ states. The line is a fit to an error function from which the center position is determined to be $202.15\pm 0.06\mathrm{G}$.

Figure 2

Remaining total atom number without (open triangles) and with (solid triangles) transferring the molecules back to atoms. These data are plotted as a function of the amount of time spent at a magnetic field of $B=201.48\mathrm{G}$ ($\Delta B=-0.6\mathrm{G}$). The curves are subtracted to determine the molecule decay rate.

Figure 3

(a) Decay rate of molecules near a Feshbach resonance. These data are plotted as a function of magnetic field detuning from the $|9/2,-7/2⟩$, $|9/2,-9/2⟩$ Feshbach resonance peak (solid circles) and $|9/2,-5/2⟩$, $|9/2,-9/2⟩$ Feshbach resonance peak (open circles). The bars indicate a lower limit on the decay rate of $|9/2,-7/2⟩$, $|9/2,-9/2⟩$ molecules. The fraction of atoms present for these measurements ranged from 0.4 to 0.6, as indicated in Fig. 1. (b) Decay rate of atoms in the $|9/2,-7/2⟩$, $|9/2,-9/2⟩$ states. The black circles are data with molecules present; the grey squares are data taken with no deliberate creation of molecules. All of these measurements start with the Fermi gas in a relatively shallow optical trapping potential at a typical temperature of 70 nK and a typical density of $n_{\mathrm{p}\mathrm{k}}=1.5\times {10}^{13}{\mathrm{c}\mathrm{m}}^{-3}$ .

Figure 4

(a) Anisotropic expansion of a nearly equal mixture of the $|9/2,-7/2⟩$ and $|9/2,-9/2⟩$ states of ${}^{40}\mathrm{K}$ near a Feshbach resonance. The aspect ratio of the cloud ${\sigma }_z/{\sigma }_r$ decreases at the Feshbach resonance peak due to an enhanced elastic collision rate. The line is a Lorentzian fit of the data to determine the resonance position, $B_0=202.1\mathrm{G}$. (b) Heating of the $|9/2,-7/2⟩$ and $|9/2,-9/2⟩$ gas near the Feshbach resonance. The typical initial temperature is 670 nK. (c) Loss of the $|9/2,-7/2⟩$ and $|9/2,-9/2⟩$ gas near the Feshbach resonance. The typical initial density of the cloud is $n_{\mathrm{p}\mathrm{k}}=1.4\times {10}^{13}{\mathrm{c}\mathrm{m}}^{-3}$. The dotted line corresponds to the Feshbach resonance peak ($\Delta B=0$).