Magnetoplasma resonance in a GaAs/AlGaAs quantum well in a strong parallel magnetic field

The dispersion of magnetoplasma excitations in two-dimensional electron systems in a strong parallel magnetic field has been studied. A considerable increase in the electron cyclotron mass with an increase in the parallel component of magnetic field has been detected. It has been found that the cyclotron mass increment is a quadratic function of the magnetic field parallel to the interface. It has been shown that the mass anisotropy of 2D electrons induced by the parallel magnetic field reaches nearly 2.5 in B _‖ = 7 T. The energy of space quantization of the electron in the quantum well has been estimated from the magnetic field dependence of the anisotropy.     

Magnetoplasma excitations and the effect of electron and hole velocity renormalization in free-hanging graphene studied by Raman scattering

The properties of plasma and magnetoplasma excitations in free-hanging graphene have been studied for the first time by Raman scattering. In addition to single-particle excitations associated with transitions between empty Landau levels of electrons and holes, collective plasma and magnetoplasma excitations in the system of electrons (and holes) of various densities have been discovered for the first time. Hybridization of plasma and cyclotron modes corresponding to the Kohn law has been shown to occur in the limit of high filling factors, which allows measuring directly the plasma and cyclotron energies. The dependence of the electron and hole velocities on their density has been investigated via the magnetic-field dependence of the cyclotron energy in free-hanging graphene. The effect of strong renormalization of the electron and hole dispersion relations seen as an increase in the velocity (by 40–50%) with a decrease in the charge-carrier density to 10¹¹ cm^–2 has been discovered. The charge-carrier density dependences of the widths of magnetoplasma resonances in free-hanging graphene and graphene lying on a silicon dioxide surface have been measured and shown to be at least 3.5 and 14.8 meV, respectively.     

Alfvén waves in the electron-nuclear plasma in the peripheral crust of a neutron star as a residual effect of a supernova explosion

The conservation of magnetic flux in the evolution and collapse of massive stars suggests that Alfvén magnetoplasma oscillations can be excited in an isolated neutron star by residual (after the supernova explosion) disturbances of the magnetized electron-nuclear plasma localized in the peripheral crust of the star. The frequencies of the poloidal Alfvén oscillations are calculated in the uniform magnetic field approximation, and it is found that the periods of the oscillations fall into the time interval of the periodicity of radio pulsar radiation. This coincidence of the periods could mean that, at least for some pulsars observed in the radio range, the electromagnetic activity is due to converstion of the energy of magnetoplasma oscillations into electromagnetic radiation. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 9, 593–598 (10 November 1996)     

Collective magnetoplasma excitations in two-dimensional electron rings

The spectra of magnetoplasma excitations in two-dimensional electron disks and rings are studied by optical detection of resonance microwave absorption. For ring-shaped structures, two types of edge magnetoplasma modes localized along the inner and outer boundaries of the ring are observed. It is shown that the interaction between these modes leads to a strong modification of their magnetic-field dependences as compared to disks. In addition to the longitudinal edge magnetoplasma excitations, transverse plasma modes associated with the electron density oscillations along the ring radius are revealed. The spectra of magnetoplasma excitations are calculated in terms of the electrodynamic theory for both ring-shaped and disk-shaped structures. The classification of all modes of collective magnetoplasma excitations observed in the experiment is performed on the basis of the comparison between experimental and theoretical results.     

Manifestation of exitonic effects in the magneto-oscillations of the intensity of the recombination radiation of 2D electrons

A study is made of the temperature dependence of the magnetooscillations of the recombination radiation of 2D electrons from the photoexcited size-quantization subband in an isolated GaAs/AlGaAs quantum well. It is shown that at high temperatures (T>10 K) the period of the oscillations is determined by the ratio of the intersubband energy splitting and the sum of the electron and hole cyclotron energies. It is found that as the temperature decreases (T<5 K), a new series of oscillations (with the same period but with a larger phase shift), which are associated with the appearance of excitonic states under the Landau levels, appears. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 10, 719–724 (25 November 1996)     

Absolute negative resistance in a nonequilibrium two-dimensional electron system in a strong magnetic field

The effect of microwave electromagnetic radiation on the resistance of the 2D electron gas in a GaAs/AlAs heterostructure in a strong magnetic field is investigated. It is shown that, under the nonequilibrium conditions caused by microwave radiation, the aforementioned 2D system exhibits giant oscillations of its resistance with varying magnetic field. When the measuring current density is small, an increase in the microwave power leads to the appearance of an absolute negative resistance at the main minimum of these oscillations, which lies near the cyclotron resonance. The experimental data are found to be in qualitative agreement with the theory of multiphoton photoinduced impurity scattering [J. Inarrea and G. Platero, Appl. Phys. Lett. 89, 052109 (2006)]. Original Russian Text ? A.A. Bykov, D.R. Islamov, D.V. Nomokonov, A.K. Bakarov, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 9, pp. 695–698.     

Coexistence of collective and single-particle effects in the photoresponse of a 2D electron gas to microwave radiation

The photoresponse of magnetoresistance of a high-density two-dimensional electron system to microwave electromagnetic radiation is studied. The damping of the Shubnikov-de Haas oscillation by radiation with a non-monotonic dependence of this effect on the magnetic field and the radiation-induced oscillations of magnetoresistance are observed. The damping is most pronounced within isolated narrow magnetic field intervals that closely correspond to the expected positions of magnetoplasma resonances in the sample under study and also near the cyclotron resonance position. A “window” is observed in the photoresponse near the field value predicted on the basis of a single-particle electron spectrum consisting of broadened Landau levels. The radiation-induced oscillations, the window in the photoresponse, and the damping of the Shubnikov-de Haas oscillations near the cyclotron resonance are described in terms of the theory based on the concept of the nonequilibrium filling of single-electron states. Thus, it is demonstrated that the photoresponse pattern observed in the experiment is formed by both single-particle and collective (magnetoplasma) effects.     

Nonlinear effects in a two-dimensional electron gas with a periodic lattice of scatterers

The magnetoresistance of two-dimensional (2D) electrons in a periodic lattice of antidots is found to be substantially influenced by an applied electric field. The non-Ohmic behavior of the resistance in the region of commensurability oscillations originates from the electric-field-induced breakdown of the trajectories skipping along the lattice arrays. In the region of magnetic fields where the cyclotron diameter is less than the distance between antidots the breakdown of the orbits skipping around antidots is responsible for the nonlinear behavior of the magnetoresistance. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 3, 237–241 (10 February 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Raman spectroscopy of magnetoplasma excitations in a system of two-dimensional electrons in inclined and parallel magnetic fields

The dispersion of magnetoplasma excitations of two-dimensional electrons in oblique and parallel magnetic fields is investigated by the method of Raman scattering of light. The inclination of the field is used to distinguish the signals due to volume hot luminescence from the signals due to Raman scattering by two-dimensional electrons. The dependence of the magnetoplasmon energy on the inclination angle of the field is measured for different momentum transfers. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 12, 974–978 (25 June 1996)     

Dimensional magnetoplasma resonance of 2D holes in (001) GaAs/AlGaAs quantum wells

Dimensional magnetoplasma resonance is observed and studied in a spatially confined, two-dimensional hole system in (001) GaAs/AlGaAs single quantum wells. From the analysis of the field dependence of the magnetoplasma resonance on the diameter of the 2D system, the semiclassical cyclotron hole mass is determined. Its value is found to be equal to 0.26m ₀ (m ₀ is the free electron mass), which considerably exceeds the theoretically predicted value. A method is proposed for a direct determination of the concentration and mobility of 2D holes from the analysis of the magnetoplasma resonance.     

Aharonov-Bohm effect for plasmons in a finite-width quantum ring

The magnetoplasma oscillation frequencies of the 2D electrons in a quantum ring have been found taking into account the finite width of the ring in the model allowing for an exact solution for the single-particle spectrum. It is shown that instead of the periodic oscillations of the plasmon frequency ω typical of the 1D ring, ω in the case under consideration depends on the magnetic flux with the frequency and amplitude modulations: the period of oscillations as a function of the magnetic flux and their amplitude depend on the magnetic field strength.     

Effect of switching a magnetoplasma medium on the duration of a monochromatic pulse

The main effect of switching a magnetoplasma medium is to split the source wave into new waves whose frequencies are different from the source wave. In addition, if the source is a monochromatic pulse, the duration of the pulse is altered. Analytical expressions for the pulse duration of the various characteristic waves in a magnetoplasma are derived. The variations of the pulse duration with the source frequency, the plasma frequency and the cyclotron frequency are illustrated. The principle of the change in the pulse duration may be used to diagnose dynamically the time — varying parameters of a magnetoplasma medium.     

Oscillations of the magnetoresistance in an inclined magnetic field for MIS structures on (100) silicon with a high electron density

At electron densities N _S>6×10⁻² cm⁻²² a second series of oscillations, which are tentatively attributed to population of the second energy subband, is observed in addition to the main series of Shubnikov-de Haas oscillations. A change in phase of the oscillations of the second series is observed at some angle of inclination α_e of the field. The measured value of α_e is used to calculate the ratio of the cyclotron mass to the effective g factor. The maximum possible cyclotron mass is also determined as m _H< 0.32m _e. On this basis it is concluded that the second series of oscillations is due to electrons which have an in-plane effective mass m*≈0.2m _e and which belong to the same valleys of the Fermi surface as in the case of the main oscillations. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 2, 136–140 (25 January 1998)     

Magnetoplasma oscillations of a two-dimensional electron layer in a bounded system

The spectrum of magnetoplasma oscillations of a two-dimensional electron layer in a transversal magnetic field is studied under the condition that the electron system is unbounded along the layer plane and screened in the perpendicular direction. It is shown that under certain conditions oscillation frequencies much lower than the electron cyclotron frequency exist. Also the electromagnetic wave-guided oscillations in the system are described. It is shown that a strong magnetic field causes a frequency shift and splitting, depending inversely on the external magnetic field and the transversal specific dimension.     

Excitonic effects in the recombination radiation spectra of completely filled Landau levels of two-dimensional electrons

The recombination radiation spectra of two-dimensional electrons in an asymmetrically doped GaAs/AlGaAs quantum well are investigated at different temperatures and laser-excitation energies. At low temperatures and in high magnetic fields the recombination lines of the electrons from completely filled Landau levels are split into narrow sublevels. It is shown that this fine structure of the Landau levels is due to the presence of excitonic effects in the initial and final states of the photoexcited system. It is demonstrated that the recombination process is accompanied by the excitation of intersubband and cyclotron magnetoplasma modes. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 38–43 (10 January 1997)     

Total conversion of the polarization of electromagnetic waves during excitation of cyclotron polaritons in a two-dimensional electron system

The reflection of an electromagnetic wave from a two-dimensional (2D) electron system in a magnetic field is studied. It is predicted that a p (s) polarized incident wave will be totally converted into a reflected wave with the orthogonal polarization when cyclotron polaritons are excited in the 2D system. For a high electron density in the 2D system, the effect remains very substantial in magnitude even in the presence of electron scattering. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 4, 247–252 (25 August 1999)     

On the nature of the oscillations of cyclotron absorption in InAs/GaSb quantum wells

The mechanism of oscillations of the half-width and intensity of the cyclotron resonance (CR) line of electrons in a semimetal quantum well based on an InAs/AlSb/GaSb heterostructure is investigated experimentally and theoretically. It is shown that the oscillations of the CR spectrum are due to mixing of states of the spatially separated two-dimensional electrons and holes. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 10, 753–758 (25 November 1998)     

Kinetics of two-dimensional electrons on a curved surface

A number of effects associated with the curvature of the surface on which a two-dimensional (2D) electron gas is placed are studied. The most significant effect in an external magnetic field (which for 2D electrons becomes effectively nonuniform) is the lifting of the degeneracy of the Landau levels. The intensity and shape of the cyclotron resonance line (inhomogeneously broadened) for different polarizations and the corrections to the Hall constant are found for the example of a circular cylinder. A picture of the quantization of the conductance that is qualitatively different from the case of a flat strip is obtained for a quasi-one-dimensional quantum wire in the form of a hollow cylinder. It is shown that in contradistinction to the planar case the spectrum of 2D electrons on the curved surface is sensitive to the sign of the spin-orbit coupling constant (for a fixed sign of the curvature). For hetero-junctions, for example, this opens up new possibilities for extracting information about their “hidden parameters.” Pis’ma Zh. éksp. Teor. Fiz. 64, No. 6, 421–426 (25 September 1996)     

Coherent and Incoherent Contributions to the Damping of Cyclotron Magnetoplasma Resonance for Two-Dimensional Electrons

The spectrum of resonant microwave absorption for a two-dimensional electron disk is studied by the optical detection technique. The dependence of the width of cyclotron magnetoplasma resonance on the electron density and on the size of the two-dimensional electron system is analyzed. The contributions of the coherent radiative and incoherent collision mechanisms to the relaxation of magnetoplasma excitations are measured. It is demonstrated that the ratio of the length of the electromagnetic wave and the lateral size of the twodimensional electron system is an important parameter determining the width of the resonance.     

Magnetoquantum de Haas-van Alphen oscillations in spin-split two-dimensional Fermi liquid

Theory of magnetoquantum oscillations with spin-split structure in strongly anisotropic (two-dimensional (2D)) metal is developed in the formalism of level approach. Parametric method for exact calculation of oscillations wave forms and amplitudes, developed earlier for spin degenerate levels is generalized on a 2D electron system with spin-split levels. General results are proved: 1) proportionality relation between magnetization and chemical potential oscillations accounting for spin-split energy levels and magnetic field unperturbed levels (states of reservoir), 2) basic equation for chemical potential oscillations invariant to various models of 2D and 1D energy bands (intersecting or overlapping) and localized states. Equilibrium transfer of carriers between overlapping 2D and 1D bands, characterizing the band structure of organic quasi 2D metals, is considered. Transfer parameter, calculated in this model to be of the order of unity, confirms the fact that the wave form of oscillations in organic metals should be quasisymmetric up to ultralow temperature. Presented theory accounts for spin-split magnetization oscillations at magnetic field directions tilted relative to the anisotropic axis of a metal. Theoretical results are compared with available experimental data on organic quasi-2D metal α-(BEDT-TTF)₂KHg(SNC)₄ explaining the appearance of clear split structure under the kink magnetic field and absence above by the corresponding change in the electron g-factor rather than cyclotron mass. Received 20 December 2000 and Received in final form 13 July 2001