Non-Markovian Dynamics and Self-Diffusion in Strongly Coupled Plasmas

In weakly coupled plasmas, collisions are dominated by long range, small angle scattering, and each collision is an uncorrelated binary event. In contrast, collisions in strongly coupled plasmas (coupling parameter $\Gamma > 1$) are dominated by short range, large angle scattering in which the collisions may be correlated and non-independent in time, \textit{i.e.}, non-Markovian. In this work, we present experimental results indicative of non-Markovian processes in a strongly coupled ultracold neutral plasma (UCNP) created by photoionizing strontium atoms in a magneto-optical trap. We use optical pumping to create spin ``tagged'' subpopulations of ions having non-zero average velocity ${\langle}v{\rangle}$, and use laser induced fluorescence (LIF) imaging to measure the relaxation of ${\langle}v(t){\rangle}$ back to equilibrium. We observe clear non-exponential decay in ${\langle}v(t){\rangle}$, which indicates non-Markovian dynamics. We further demonstrate there is a theoretical basis to consider ${\langle}v(t){\rangle}$ as an approximation to the ion velocity autocorrelation function (VAF). We then calculate diffusion coefficients from our data, demonstrating experimental measurement of self-diffusion coefficients for $0.3 < \Gamma < 3.5$.