Collective excitations in imperfect parabolic quantum wells with in-plane magnetic fields

Calculations of far-infrared optical absorption for ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As perturbed parabolic quantum wells (PQW) with a magnetic field in the plane of the electron slab are presented within the linear response theory. The nonparabolicities associated with the perturbation allow the coupling of long-wavelength radiation to collective excitations of the electron gas other than the center-of-mass mode, otherwise forbidden in pure PQW’s by virtue of Kohn’s theorem. We employ a quantum-mechanical self-consistent-field approach which makes use of the density functional formalism within the local-density approximation. We study two different types of samples employed in recent magneto-optical absorption measurements. The first consists of PQW’s with controlled δ-planar perturbations located at the center of the well or forming superlatticelike periodic arrays. These samples were recently used in an experimental study aimed at indirectly measuring the magnetoroton dispersion relation of a three-dimensional electron gas. We construct a magnetoplasma dispersion relation and critically discuss whether the experimental results are consistent with the bulk magnetoroton picture originally invoked to understand the data. The second system we study is asymmetric parabolic wells, obtaining good agreement with experiment at high and low areal densities, but quantitative discrepancies at intermediate density.