Proposal

Interactions between ultracold cesium atoms 

Ultracold atomic cesium offers a rich opportunity to study interacting, quantum degenerate atomic gasses in controlled and varied ways. Much of this flexibility is afforded by its rich spectrum of collisional Feshbach resonances as an external magnetic field is tuned; near each resonance the effective interaction strength between atoms at long range undergoes rapid variation. This is due to a resonant coupling between free atoms undergoing a collision to molecular bound states of atoms with different internal states.

This effect is best understood by the variation of the scattering length characterizing interactions. When the scattering length is positive, long-range interaction between atoms is effectively repulsive. This naturally effects the physics of an interacting gas of many particles dramatically. Positive (repulsive) scattering length, for example, is a necessary condition for a stable Bose-Einstein condensate. When the scattering length is negative, atomic interaction is attractive.

The plot below shows the scattering length of cold Cesium atoms as a function of an applied external magnetic field. Cesium atoms have a large number of Feshbach resonances due to their large relativistic indirect spin-spin interactions. (This calculation is based on code developed by E. Tiesinga and P. Julienne at NIST.)

Full proposal: Few- and many-body physics.