Plasmon excitations in a two-dimensional electron gas with spin-orbit interactions: Zero magnetic field

We report on a theoretical investigation of plasmon excitations in a quasi-two-dimensional electron gas in the presence of a spin-orbit (SO) interaction induced by the Rashba effect. We derive and discuss the dispersion relations for both intra-SO and inter-SO charge-density excitations within the framework of the Bohm-Pines random-phase approximation. The zero-field (or Rashba) spin splitting gives rise to three branches of plasmon excitations, even when only the lowest electric subband is occupied. However, not all three plasmon branches survive the Landau damping as is discussed in detail. It is found that, in the long-wavelength limit, the two branches of inter-SO plasmons are optic-like, propagate with negative group velocity, and remain stronger functions of the Rashba parameter than their intra-SO counterpart. We discuss the dependence of the plasmon energy on the propagation vector, the two-dimensional charge density, and the effective Rashba wave vector.