Collective modes of spatially separated, two-component, two-dimensional plasma in solids

Longitudinal collective spectrum of a spatially separated, two-component, two-dimensional plasma is investigated using a generalized random-phase approximation. Such plasmas can be realized in semiconductor heterojunctions, superlattices, and inversion or accumulation layers. In general two modes are found to exist: the one with energy proportional to $\sqrt{q}$ at long wavelengths is the usual optical plasmon and the other with energy proportional to $q$ at long wavelengths is the acoustic plasmon. It is shown that the spatial separation between the two charge components makes it possible for the acoustic branch to move out of the electron-hole continua of both the components provided it exceeds a critical value. Consequently the acoustic-plasmon mode is totally undamped at long wavelength. The dynamic structure factor of the system is analyzed and the feasibility of observing an acoustic plasmon in GaAs-${\mathrm{Ga}}_x{\mathrm{Al}}_{1-x}\mathrm{As}$ double quantum well is discussed.