A High-Phase Space Density Gas of NaCs Feshbach Molecules*

We report on the production of ultracold ensembles of sodium-cesium Feshbach molecules. We have located and characterized an s-wave Feshbach resonance at 864.12(2) G between sodium and cesium in their respective hyperfine ground states. We demonstrate that this resonance is well-suited for the creation of Feshbach molecules. By varying the atom number and temperature of the atomic clouds, we have wide-ranging control over the properties of the resulting molecular ensembles. We demonstrate the creation of large molecular ensembles with up to 4 × 10⁴ molecules at 2 μK, as well as ensembles of about 1 × 10³ molecules at temperatures below 100 nK, when associating from dual condensates. This is a key step towards the creation of sodium-cesium ground state molecules.