Collisional Stability of Fermionic Feshbach Molecules

Using a Feshbach resonance, we create ultracold fermionic molecules starting from a Bose-Fermi atom gas mixture. The resulting mixture of atoms and weakly bound molecules provides a rich system for studying few-body collisions because of the variety of atomic collision partners for molecules; either bosonic, fermionic, or distinguishable atoms. Inelastic loss of the molecules near the Feshbach resonance is dramatically affected by the quantum statistics of the colliding particles and the scattering length. In particular, we observe a molecule lifetime as long as 100 ms near the Feshbach resonance.

Figure 1

Molecule loss coefficient $\beta$ for collisions with distinguishable atoms (K in the $|9/2,-7/2⟩$ state) as a function of the heteronuclear scattering length. For large $a$, we observe $\beta$ to decrease with increasing scattering length in agreement with numerical calculations (solid curve). We observe $\beta$ saturating for small $a$ ($a\sim 100a_0$), where the molecular size, $⟨r_{\mathrm{mol}}⟩$, no longer has a strong dependence on $a$. Inset: $\beta$ vs molecule size $⟨r_{\mathrm{mol}}⟩$ as extracted from the measured binding energy of the Feshbach molecules [18]. The dashed curve is a power law fit ${\beta }_{\mathrm{fit}}\propto ⟨r_{\mathrm{mol}}{⟩}^p$ to the experimental data; we obtain $p=-0.97\pm 0.16$.

Figure 2

Molecule loss coefficient for collisions with indistinguishable bosons versus heteronuclear scattering length. For large $a$, $\beta$ increases with increasing $a$. In this regime, the molecular loss is enhanced compared to loss due to molecule collisions with distinguishable atoms (dashed curve and Fig. 1). For small $a$, $\beta$ increases with decreasing scattering length (see text).

Figure 3

Molecule loss coefficient $\beta$ for collisions with indistinguishable fermions. For large $a$, we observe loss rates scaling as $a^{-1.6\pm 0.2}$. The power law dependence is consistent with numerical calculations for this scattering length regime (solid curve). In addition, the molecular loss is suppressed compared to inelastic decay of molecules due to collisions with distinguishable atoms (dashed curve and Fig. 1). Inset: Molecule decay at $a\approx 1300a_0$. The $1/e$-lifetime of the molecular cloud is $\tau =100\pm 20\mathrm{ms}$.

Figure 4

Three-body recombination near the heteronuclear Feshbach resonance. The measured $K_3$ varies as $a^4$ and saturates at large $a$ due to the unitary limit. The dashed curve is an $a^4$ fit to the data for $|a|<4000a_0$. The solid line is the unitarity limit at 150 nK.