Self-diffusion and random motion in a strongly-coupled dusty-plasma: experiment

Self-diffusion and random motion in a two-dimensional (2D) dusty plasma were experimentally measured. A single-layer suspension of microsphere particles was levitated in an rf plasma, forming a strongly-coupled dusty plasma with a Yukawa interparticle potential. A pair of cw laser beams moves around the suspension and exerts radiation pressure forces on particles, providing an external control on particle temperature. A dusty-plasma solid is heated to form a dusty-plasma liquid. Particles are imaged, yielding an accurate measurement of particle position and velocity. Random motion is characterized by mean-square displacement (MSD), yielding an estimate of a diffusion coefficient, $D$. Dependence of $D$ on temperature $T$ is dominated by a power law at high $T$ and an Arrhenius form at low $T$. Particle random motion obeys Gaussian statistics at high $T$, but not at low $T$ as indicated by a probability distribution function (PDF) that resembles a kappa distribution. The PDF is self-similar for times longer than the ballistic time scale.