The far-infrared magneto-optical properties of remotely doped wide-parabolically graded AIGaAs quantum wells is reviewed. The optical response of the system is the same as that of a single-component three-dimensional plasma. Extensive studies were made of the plasma-shifted cyclotron resonance in the Voigt geometry. It is shown that the resonance frequency is independent of the areal density which demonstrates the generalized Kohn theorem. The internal modes of the system are excited by using wells with periodic and aperiodic perturbations superimposed on the parabolic potential. Dimensional modes and grating modes are identified which permit the wave-vector assignment of the internal magneto-plasma oscillations. From results on different samples and magnetic fields the dispersion relation for magnetoplasmons is measured. A roton excitation is observed at q ~2/l o which is good agreement with the predictions of calculations based on the single-mode approximation.
Strong interactions in many-body systems generally lead to highly correlated ground states. These correlations are manifest by modulations in the pair-correlation function which can be determined, in principle, from elastic diffraction measurements of the static structure factor. The interactions also lead to collective excited states of these systems that deviate significantly from the single-particle excitations of noninteracting systems. These collective excitations can be described in terms of density waves and their wavevector dispersion is expected to reflect the ground-state correlations through the spectral features in the static structure factor.
The classic example is superfluid 4He in which the collective excitations are phonons at low wave vector q and rotons at high q [1]. The dispersion relation of these excitations has been measured by inelastic neutron scattering. The