Abstract
The magneto-plasmon dispersion relation of a quasi-three-dimensional electron gas confined in a wide Al xGa 1− x As parabolic quantum well is investigated in the magnetic quantum limit. Magneto-plasma excitations polarized perpendicular to the magnetic field and with wave vector q of the order of the inverse magnetic length 1 l 0 are measured in far infrared magneto-transmission experiments on 100 nm thick electron slabs. A minimum is found in the magneto-plasma mode frequency at ql 0 ≈ 2 suggesting the existence of a magneto-roton excitation. The collective excitation spectrum is calculated in analogy with Feynman's theory for rotons in helium, and is found to be in good qualitative agreement with the measurement.
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