Resonant collisions in many-body and complex systems

Resonant exchange is a general process playing a key role in many-body and complex systems, such as in spin, charge, or excitation diffusion. The underlying process is described by the resonant exchange cross section σ_exc. A prime example is the diffusion of an ion A⁺ in its parent neutral gas A. In fact, the charge actually behaves as a hole (h) at ultralow temperatures, hopping from atom to atom instead of staying on its heavy center (the ion). We have predicted a faster diffusion coefficient for the hole (D_h) than if the charge was diffusing via collision (D_coll). We model charge hopping in the dynamics of an ultracold ^6/7Li⁺ ion immersed within an ultracold gas of ^6/7Li atoms at micro-Kelvin temperatures and show that the charge hopping and collisional diffusion compete, giving unique results leading to charge trapping in regions of high atomic density gradient. We also explore resonant collision involving much larger projectile and target, namely C₆₀ scattering with C⁺₆₀. We find that in such complex molecules, internal degrees of freedom play an important role, and that the formation of “temporary bridges” between the two molecular systems must be accounted for to explain experimental measurements.