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Two-dimensional plasmons have been investigated in AloaAs-GaAs heterostructures with transmission spectroscopy. In a magnetic field B, we observe an interaction of the plasmon excitation with harmonics of the cyclotron-resonance frequency cu, =eB/m, which arises from nonlocal effects on the two-dimensional-plasmon excitation and is governed by the parameter (quF/ao, ) (q is the plasmon wave vector; up is the Fermi velocity). The dispersion and the excitation strengths of the combined resonances are in excellent quantitative agreement with a quasiclassical nonlocal theory.PACS numbers: 71,45.Gm, 73.40.Lq The dynamic spatial modulation of the charge density, which is induced by a plasmon excitation in a plasma, causes in a magnetic field 8 a nonlocal interaction of the plasmon with harmonics n to, (n~2) of the cyclotron resonance (CR) co, =eB/m. The strength of this nonlocal effect is governed by the parameter (q uF/to, ), where q is the plasmon wave vector and vF is the Fermi velocity. For two-dimensional (2D) plasmons nonlocal effects on the dispersion have been theoretically studied in great detail. Previous experimental investigations of magnetoplasmon excitations in Si-metal-oxide-semiconductor (MOS) structures7 s demonstrated an interaction of plasmons with harmonics of the cyclotron resonance. However, it has been shown that under the experimental conditions of Refs. 7 and 8, the nonlocal corrections are too small to explain the observed interaction. Rather the interaction observed in the Si inversion layer has been linked to the effect of scatterers on the line profile of Landau levels. 4 This interaction has the same origin as the excitation of so-called "subharmonic cyclotron resonances, " ' which are strictly forbidden in a uniform, translationally invariant plasma.We have investigated 2D plasmons in electron space-charge layers of A16aAs-6aAs heterostructures" ' using frequency-domain transmission spectroscopy and grating-coupler' techniques. In a magnetic field perpendicular to the 2D plane, we observe an interaction of the plasmon resonance with the n = 2 harmonic of the CR that is in excellent quantitative agreement with a quasiclassical nonlocal theory.The experiments are performed on modulationdoped A16aAs-6aAs single-quantum-well structures. The samples are grown by molecular-beam epitaxy on (100) GaAs substrates and consist of a l-p, m GaAs buffer layer, a 5-nm undoped space layer of Alz 3Gao 7As, a n-doped Ala 3Gao 7As (n = 3 x 10' cm 3) layer of thickness 60 nm, and a thin (15-nm) GaAs cover layer. Lateral Ag stripes with periodicity a =1.15 p, m are prepared on top of the heterostructure (see inset of Fig. 1). The Ag grating modulates normally incident far-infrared (FIR) radiation in the near field and couples radiation to plasmons of wave vectors q = n 2m/a (n = 1, 2, . . . ). '3 The excitation of plasmons is studied in a Fourier-transform spectrometer. Since here, in contrast to Si-MOS structures, the charge density N, cannot be switched between 0 and N, without complicated gate technique...

We find that the long-wavelength magnetoplasmon, resistively detected by photoconductivity spectroscopy in high-mobility two-dimensional electron systems, deviates from its well-known semiclassical nature as uncovered in conventional absorption experiments. A clear filling-factor dependent plateau-type dispersion is observed that reveals a so far unknown relation between the magnetoplasmon and the quantum Hall effect.

We report here the first observation of the cyclotron resonance Landau splitting for low density electron inversion layers in Al Ga& As-GaAs heterojunctions.Even at temperatures close to 100 K electron-electron interactions couple the electrical dipole transitions from the ground and first excited Landau levels, with a coupling strength comparable to the one found at liquid helium temperatures. The experiment can be explained only in a single-particle approximation at sufficiently low densities and/or magnetic field strengths where the resonant polaron effect is important. PACS numbers: 73.20.Dx, 78.20.Ls Cyclotron resonance (CR) in a translational invariant system is a center-of-mass motion and independent of the electron-electron interaction. This famous result, known as Kohn's theorem [1], has been an important guideline

The interaction coupling of cyclotron transitions with different spin orientation was investigated as a function of density for electron inversion layers in GaAs in the magnetic quantum limit. The spatial electron distribution strongly influences the coupling of the electrical dipole transitions, resulting in deviations from the interaction strength for ideally two-dimensional electrons. At the higher densities the coupling is essentially reduced due to the finite thickness of the inversion layer, whereas at sufficiently low densities the coupling reflects the influence of disorder