Time-Dependent Density Fluctuations in a Strongly Coupled Ionic Mixture

Hydrodynamic and molecular-dynamics calculations show that the charge fluctuations in a strongly coupled ${\mathrm{H}}^{+}$-${\mathrm{He}}^{++}$ mixture are dominated by a propagating plasmon mode which is damped by interdiffusion even in the long-wavelength limit. A sum-rule argument indicating that the plasmon dispersion changes from positive to negative as the coupling increases is consistent with both kinetic theory and the computer results. Plasma oscillations also contribute weakly to the mass fluctuations, but there is no hydrodynamic soundwave mode.