Strong coupling arises when interaction energies are comparable to, or exceed, kinetic energies. This occurs in a diverse group of systems, including dense white dwarf stars, strongly correlated electron systems, and cold quantum gases. Ultracold neutral plasmas (UNPs), generated by photoionization of a laser-cooled gas, are ideal for exploring the effects of strong coupling. In this talk, I will describe experimental studies of self-diffusion and equilibration after a fast quench in interaction energy, and show how strong coupling modifies these phenomena in ways that invalidate standard theoretical tools such as magneto-hydrodynamics and kinetic theory. I will also present results from the first application of laser-cooling to a neutral plasma, which increases the achievable coupling strength. Although the technique we use, optical molasses, is well established, the high collision rates and rapid hydrodynamic expansion of the plasma create a uniquely challenging environment for laser cooling. Nevertheless, through laser-cooling we have achieved a factor of 4 enhancement in the coupling strength post-equilibriation, placing the laser-cooled UNP in the same coupling regime as white dwarf stars and allowing for experimental benchmarking of new models and of molecular dynamics simulations of transport in strongly coupled systems.
Host: David DeMille