Understanding Magnetic Feshbach Resonances in Collisions of Ultracold NaK with K

Magnetic Feshbach resonances are a key feature in the scattering of ultracold atoms and have been pivotal in their study. Now that ultracold molecules can be made, their scattering properties are of great interest, both with themselves and with ultracold atoms. Feshbach resonances have been observed in ultracold collisions of Na⁴⁰K with ⁴⁰K by Zhao and coworkers in Shanghai [Yang et al Science 363, 261], and if these resonances can be understood they will provide insight into and control over these collisions. We use extended results from the Shanghai group to arrive at a detailed model for a low-field group of resonances and suggest possible interpretations for another group of resonances. The states causing such resonances have often been assumed to be highly complicated or even chaotic in nature due to the deep and anisotropic short-range potentials. However, our suggested interpretation is based on weakly bound long-range states which are highly ordered and relatively simple. Such states and corresponding low-field Feshbach resonances may be expected in other similar alkali atom+diatom collision systems.