Second Plasmon and Collective Modes in Binary Coulomb Systems
  In a system consisting of two different charged species we identify the excitation of a second, low frequency plasmon. At strong coupling the doublet of high frequency (first) and low frequency (second) plasmons replaces the single plasmon excitation that prevails at weak coupling. We observe the formation of the second plasmon from the acoustic Goldstone type mode associated with short range interaction as the range is extended to infinity. 
Gabor J. Kalman, Zoltan Donko, Peter Hartmann, Kenneth I. Golden
physics.plasm-ph