BEC of Potassium-41: towards creation of ultracold polar molecules

One of the major goals in the field of ultracold gases is the production of ultracold polar molecules. Due to anisotropic, long-range interaction, a polar molecular gas is expected to show us a rich variety of new phenomena, including anisotropic collapse and a super-solid phase. We work with a two-species ultracold atomic gas of $^{41}$K and $^{87}$Rb. We successfully produced a BEC of $2\times10^5$ $^{41}$K atoms. A BEC of $^{41}$K was first realized by the LENS group[1]. However, since that experiment was based on sympathetic cooling with $^{87}$Rb, the characteristics of $^{41}$K under evaporative cooling were unknown. Furthermore, producing a high phase-space density $^{41}$K cloud by laser cooling alone was expected to be difficult because of the small hyperfine splitting of the excited states (13MHz). We show that compressed-MOT and Doppler cooling stages are essential for achieving a 100uK cloud with a number density of $5\times10^{10} $cm$^{-3}$. Typically, $9\times10^8$ atoms are loaded in a magnetic trap. We see a difference in efficiency of evaporative cooling using an rf-transition between magnetic sublevels and between hyperfine states. The underlying physical mechanism is discussed. [1] G. Modugno et al., Science 294, 1320 (2001).