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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

Employing electrostatic confinement with a dual-gate device we realize periodic arrays of electron dots on Si widely tunable in diameter and electron number. From far-infrared transmission studies of dimensional resonances, we deduce dot diameters down to 40 nm for as little as 20 electrons in quantum states spaced by more than 5 meV. Excitation energies as well as mode dispersions in finite magnetic fields are found to strongly depend on the strength and the shape of the lateral confining potential. A detailed analysis of the oscillator strengths indicates a direct effect of strong quantum confinement.The high-frequency response of laterally bound electron layers has been investigated both in classically confined quasi-two-dimensional (2D) systems' and, more recently, in quantum confined systems that exhibit either quasi-one-dimensional 's (1D) or quasi-zero-dimensional (OD) properties. In particular, 1D inversion channels realized on GaAs, InSb, and Si have been extensively studied. 4 s Very recently electrons have been confined to OD quantum dots on GaAs (Refs. 8-10) and InSb. s Here we study the high-frequency response of periodic arrays of dots on Si containing few (20-350) inversion electrons.Significant quantum confinement on silicon can only be expected with dot diameters IY well below 100 nm. To meet this requirement we prepare a dual-gate structure which allows us to electrostatically define a wide range of dot diameters between 40 and 150 nm. Moreover, the main advantage of such a device is the continuous and nearly independent tunability of the depth of the lateral confining potential, the dot diameter W, and the electron number Np. This enables us to study the high-frequency response of dot arrays on Si in the transitory regime between classical and quantum confinement. Figure 1 shows a schematic cross section of our dualgate metal-oxide-silicon device. The bottom gate is a semitransparent NiCr mesh sandwiched between a thermal Si02 layer, grown on (100) p-type Si with a specific resistivity of 20 Acm at 300 K, and a plasmaenhanced chemical-vapor deposition (PECVD) Sio2 layer with a thin continuous NiCr layer of Re= 1 kQll:i on top.The scanning electron micrograph shows a top view of the bottom gate. The mesh has a periodicity of a 400 nm with circularly shaped openings of diameter t =150 nm. Different voltages Vs, and Vsb are applied between a substrate contact and the top and bottom gates, respectively.In the presence of band-gap radiation electron dots are induced at the Si-Si02 interface underneath the openings of the bottom gate via V~, . While Vg& essentially determines Np, Vgb serves to isolate the dots and to continuously vary the depth of the lateral confining potential from zero to a value exceeding the band gap of Si. As demonstrated below at fixed Vg& and Vgb the electronic diameter of the dots &can be further reduced via a substrate-bias voltage Vsg which is added to V~& in the dark and does not significantly change %p. This enhances the depletion Vgl Vgb «I d pECVD Si02 Si02 p-Si (1QO) ...

In hole space-charge layers on silicon we observe lifting of the quasispin degeneracy by the surface electric field in the absence of an external magnetic field. This lifting is detected in a splitting of intersubband resonance transitions and increases with increasing surface carrier density, reflecting an increase with quasimomentum. We have performed calculations of the subband excitation spectrum which confirm the experimentally observed quasispin splitting.PACS numbers: 71.25. Rk, 73.40.Qv An outstanding feature of the band structure in systems without inversion symmetry is that the degeneracy of the quasispin states is inherently, i.e., without external magnetic field, lifted for quasimomentum ku ^0. This is a consequence of the effect of an asymmetric potential on the spin-orbit interaction. In the bulk of zinc-blende-type compounds asymmetry effects are extremely small 1 and have only been observed by applying stress to enhance the inversion asymmetry. 2 Inversion asymmetry also exists in quasi-two-dimensional (2D) space-charge layers. There it is induced by a surface electric field in a metal-oxide-semiconductor (MOS) structure or in heterostructure interfaces. In these systems, mobile carriers are confined in a very narrow potential well which causes a 2D energy spectrum consisting of a_set of "subbands" with^ energy dispersion ^(k^cr) [/ = 1,2,3, . . . ; kn = (/c x ,/^) is the quasimomentum parallel to the interface]. 3 It was pointed out in calculations of Bangert and co-workers 4,5 and Ohkawa and Uemura 6 that the asymmetry of this potential splits the energy eigenvalues of different spin states a for k*n ^ 0. Experimental evidence for the lifting of the spin degeneracy in high-mobility space-charge layers of GaAs heterostructures has been reported recently by Stormer etal 1 and Stein, Klitzing, and Weimann. 8 However, these experiments are performed in high magnetic fields-which cause spin splitting by themselves-and are extrapolated to B = 0. We have directly observed spin splitting without the presence of a magnetic field in hole space-charge layers of Si using intersubband spectroscopy.In intersubband spectroscopy, resonant transitions between different subbands of the 2D system are studied (see, e.g., Ref. 3; for hole-intersubband spectroscopy, e.g., Refs. 9-12). Important for our investigations is the parallel excitation, which means that the electric field vector of the exciting radiation is polarized parallel to the 2D space-charge layer. For details of the experimental technique we refer to Batke and Heitmann. 13 The samples are high-mobility (intraband scattering time T = 4X10~1 3 s) w-type Si(110) MOS capacitors. The inverted p channel is isolated from the substrate by a depletion layer (JV depl = 1.6x 10 11 cm" 2 ) which causes a finite depletion field. Experimental spectra, measured with Fourier-transform spectroscopy, are shown in Figs. 1 and 2(a). AT/T=[T(V g )-T(V t )]/T(V t )is the relative change of transmission for radiation transmitted normally through the MOS sample with semit...

Cyclotron resonances of electron space-charge layers in GaAs were studied at 3.He temperatures covering a density regime from 2 to 13 x 10(exp 11) cm(exp -2). For densities higher than about 6 x 10(exp 11) cm(exp -2) many-body influences are sufficiently reduced such that line splittings due to band-structure influences were involved. At integer filling factors the spin-up and spin-down electrons are not completely decoupled as predicted by theory. Inter Landau-level optical gaps are renormalized at odd fillings, and there is a correlation gap between spin- and charge-density excitations in the long-wavelength limit