Production of an ultracold mixture of 23Na40K and 40K

We report on the production of an ultracold mixture of 2.8 × 10423Na40K molecules and 3.3 × 10540K atoms at a temperature of about 250 nK. Compared with previous studies, the number of atoms and molecules improved by a factor of about 2, and the temperature reduced by a factor of about 2. These improvements occur mainly because of the use of a crossed large-volume horizontal dipole trap to load the atoms from a cloverleaf-type magnetic trap, and a large-volume three-beam dipole trap to perform optical evaporative cooling. Besides achieving the mode-matching loading, this method avoids evaporative cooling in a decompressed cloverleaf magnetic trap, which is sensitive to magnetic field fluctuations. We characterize an atom-molecule Feshbach resonance using the ultracold atom-molecule mixture. The enhancement of particle number and temperature significantly improves the signal-to-noise ratio, and enables us to refine the location and width of the Feshbach resonance.