Intersubband resonances and plasmons in quasi-two-dimensional systems: A line-shape and coupling analysis

Charge- and spin-density fluctuation spectra, as they appear in electron scattering and optical experiments, have been calculated for accumulation layers at n-type GaAs interfaces. Starting with a self-consistent ground state, we employ the random-phase approximation to evaluate the nonlocal susceptibility of the anisotropic electron system. Temperature effects and excitations to the continuum of states are also included in the theory. Profiles of the complex induced potential inside the quasi-two-dimensional (2D) electron gas reveal the internal structure of the collective modes. We find substantial deviations of the 2D plasmon dispersion from strictly two-dimensional theories. Both nonlocality and coupling between the subbands play a role. Also the dispersion of the intersubband resonance has been calculated over a wide range of wave vectors q? and for various electron densities. The well-known depolarization shift at small q? is strongly influenced by the spectral proximity of the continuum of states.