Magnetorotons in quasi-three-dimensional electron systems

The dispersion relation for magnetoplasmons propagating perpendicular to the magnetic field is investigated for a quasi-three-dimensional electron system. The magnetic field is in the plane of a ≃100-nm-wide electron slab which is confined by a wide ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As parabolic quantum well. A minimum is found in the magnetoplasma dispersion relation indicating the existence of a magnetoroton excitation for wave vector q about twice the inverse magnetic length. Calculations of the dispersion relation within the single-mode approximation are in good qualitative agreement with the measurements.