  We study collective excitations of a two-dimensional electron system
consisting of two kinds of charge carriers: relativistic or Dirac electrons
with linear dispersion and non-relativistic electrons with parabolic
dispersion. We find that the strength of the interaction between the different
charge carriers, i.e., the inter-species interaction, plays a significant role
in determining the number of plasmon modes as well as their dispersions in
certain parameter regimes.
