Formation of Ultracold NaLi Molecules

We have succeeded in forming NaLi molecules from an ultracold mixture of Na and Li atoms. This is done by carefully sweeping a magnetic field around a Feshbach resonance, where the energy for a state of two free atoms becomes degenerate with the energy for the bound molecular state of the two atoms, but with different internal electronic and nuclear spins. A magnetic field can be used to tune the energy difference between these two states to zero because the different spin orientations lead to different magnetic moments of the two-atom system.

Usually, when the magnetic field is swept across a Feshbach resonance, the atom pair is adiabatically transferred to the molecular bound state because the two are coupled by the hyperfine interactions in the system. However, in the Na + Li system, such hyperfine-induced Feshbach resonances are at very high magnetic fields that are out of experimental range. Instead, we worked around a Feshbach resonance at 745G that is produced by weak dipole-dipole coupling between the atoms. This coupling term is orders of magnitude weaker than the hyperfine interaction, meaning that the requirements for successful adiabatic conversion of atom pairs to molecules become extraordinarily demanding. However, by carefully controlling our magnetic field stability and sweep sequence, we were able to rapidly jump the field near resonance, do a slow, adiabatic sweep across the very narrow, ~mG wide range of the Feshbach resonance, and then immediately jump the field away from resonance again to isolate the molecules for imaging. This produced a fraction of NaLi molecules from our initial atomic mixture, and perhaps represents successful molecule formation around the most difficult Feshbach resonance ever used.

In the near future, we plan to take advantage of the fact that NaLi is the lightest heteronuclear alkali molecule. This means that it is expected to have a long-lived metastable spin-triplet ground state, with decay to lower lying spin-singlet states suppressed by the smallness of second-order spin-orbit coupling in systems with small constituent atomic charges. In the spin-triplet ground state, NaLi would be the first molecule with both a significant electric dipole moment and a nonzero magnetic dipole moment. The simultaneous presence of two independently tunable dipole moments opens the door to engineering exotic new Hamiltonians in optical lattices, as well as the possibility of exploring new ways to tune chemical reactions with applied fields. In particular, because of the small mass of the system, NaLi has an exceptionally low density of states, meaning that there can be the possibility of observing discrete, well-separated resonances in molecule-molecule collisions, which would enable resonant tuning of reaction rates for ultracold molecules.

The paper we wrote about our work can be found here.