Ultracold Heteronuclear Molecules in a 3D Optical Lattice

We report on the creation of ultracold heteronuclear molecules assembled from fermionic ${}^{40}\mathrm{K}$ and bosonic ${}^{87}\mathrm{Rb}$ atoms in a 3D optical lattice. Molecules are produced at a heteronuclear Feshbach resonance on both the attractive and the repulsive sides of the resonance. We precisely determine the binding energy of the heteronuclear molecules from rf spectroscopy across the Feshbach resonance. We characterize the lifetime of the molecular sample as a function of magnetic field and measure lifetimes between 20 and 120 ms. The efficiency of molecule creation via rf association is measured and is found to decrease as expected for more deeply bound molecules.

Figure 1

rf spectroscopy of ${}^{40}\mathrm{K}\mathrm{\text{−}}{}^{87}\mathrm{Rb}$ in a 3D optical lattice on the ${}^{40}\mathrm{K}|9/2,-7/2⟩\to |9/2,-9/2⟩$ transition (see inset) at a lattice depth of $U_{\mathrm{lat}}=27.5E_r$ and a magnetic field of 547.13 G, where the interaction is attractive. $E_r={\hslash }^2{k}^2/2m_{\mathrm{Rb}}$ is the ${}^{87}\mathrm{Rb}$ recoil energy. The spectrum is plotted as a function of detuning from the undisturbed atomic resonance frequency and clearly shows the large atomic peak at zero detuning. The peak at 13.9 kHz is due to association of ${}^{87}\mathrm{Rb}|1,1⟩{}^{40}\mathrm{K}|9/2,-7/2⟩$ atom pairs into a bound state. The right part of the inset schematically shows the dependence of the pair energies on the magnetic field (see Fig. 2).

Figure 2

Binding energy of heteronuclear ${}^{40}\mathrm{K}\mathrm{\text{−}}{}^{87}\mathrm{Rb}$ molecules in an optical lattice for two different lattice depths $U_{\mathrm{lat}}$ in units of the ${}^{87}\mathrm{Rb}$ recoil energy $E_r={\hslash }^2{k}^2/2m_{\mathrm{Rb}}$. The center of the Feshbach resonance is located at 546.8(1) G [18]. We observe attractively bound molecules, which are confinement-induced at a positive detuning with respect to the resonance center, and real molecules, which are stable in free space below the center of the resonance. In addition, we observe repulsively interacting pairs with a positive “binding energy” below the resonance.

Figure 3

Lifetime of heteronuclear molecules in the optical lattice as a function of magnetic field.

Figure 4

Transfer efficiency of the rf association process as a function of magnetic field. The transfer efficiency is normalized to its peak value on the attractive side of the Feshbach resonance, where we have observed the highest efficiency within the plotted magnetic field range.