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.     

与核密度及温度相关的核子相对论微观光学势

在Walecka模型、热场动力学和相对论Dirac-Bruckner-Hartree-Fock计算结果基础上,研究了在不同核密度和各种温度情况下的核子相对论微观光学势及其相应的薛定谔等效势和平均自由程,计算结果表明,对于不同的温度,核子的薛定谔等效势与平均自由程随核密度变化较为敏感,而核温度对核子的薛定谔等效势和平均自由程的影响随着核密度的增加变大.     

Zur Mikrowellendiagnostik der Rauschtemperatur und der Trägerdichte eines Neonplasmas im Übergangsbereich zwischen der Freifall- und diffusionsbestimmten Säule

This paper deals with the experimental determination of the noise temperature and electron density. These plasma parameters were determined in the pressure region between diffusion and free-fall conditions with the aid of the microwave resonator method. In most cases the calculations of noise temperature and electron density were made using the energy distribution functions following from the electron Boltzmann equation. Great differences occur between calculated and measured values when the mean free path length of electrons becomes comparable with the dimensions of the discharge tube. These differences may be caused by a greater radial decrease of the noise temperature and/or by an increased interaction between boundary layer oscillations and plasma.     

Measurement of the mean free path of edge magnetoplasmons revealed in the spectra of magnetic oscillations of photovoltage in a two-dimensional electron system under microwave radiation

In the spectrum of photovoltage oscillations that are periodic with respect to the magnetic field and appear on Hall structures under microwave irradiation, two frequency components of oscillations are observed and analyzed. The appearance of these two frequencies in photovoltage oscillations is explained by the existence of two trajectories of edge magnetoplasmons and by the effects of the interference of collective excitations on these trajectories. The effects of the temperature, microwave radiation frequency, and magnetic field strength on the mean free path of edge magnetoplasmons are analyzed.     

Oscillations of the magnetoresistance of a two-dimensional electron gas under microwave irradiation: Influence of the irradiation frequency

We present measurements of the magnetoresistance of a two-dimensional electron gas (2DEG) under continuous microwave as a function of the irradiation frequency. In a previous work by Simovič et al. [Phys. Rev. B 71 (2005) 233303], the magnetoresistance under microwave was shown to be modulated by oscillations of large amplitude that are periodic with magnetic field, their period and phase depending strongly on the electron density. Here we show that the phase and the amplitude of the microwave-induced oscillations also depend on the frequency of irradiation and the sign of the magnetic field.     

New type of B-periodic magneto-oscillations in a two-dimensional electron system induced by microwave irradiation

We observe a new type of magneto-oscillations in the photovoltage and the longitudinal resistance of a two-dimensional electron system. The oscillations are induced by microwave radiation and are periodic in magnetic field. The period is determined by the microwave frequency, the electron density, and the distance between potential probes. The phenomenon is accounted for by interference of coherently excited edge magnetoplasmons in the contact regions and offers perspectives for developing new tunable microwave and terahertz detection schemes and spectroscopic techniques.     

Oscillations of the magnetoresistance of a two-dimensional electron gas in a GaAs quantum well with AlAs/GaAs superlattice barriers in a microwave field

The effect of microwave radiation in the frequency range from 1.2 to 10 GHz on the magnetoresistance of a high-mobility two-dimensional electron gas has been studied in a GaAs quantum well with AlAs/GaAs superlattice barriers. It has been found that the microwave field induces oscillations of this magnetoresistance, which are periodic in the reciprocal magnetic field (1/B). It has been shown that the period of these oscillations in the frequency range under study depends on the microwave radiation power.     

Comparative Study of the Two-Dimensional Plasma Excitations in the Heterostructures ZnO/MgZnO,AlAs/AlGaAs,and GaAs/AlGaAs

The plasma oscillations in new advanced two-dimensional electron systems (2DESs) based on the heterostructures ZnO/MgZnO, AlAs/AlGaAs, and GaAs/AlGaAs are studied and compared. The relaxation times and the effective masses in samples with various electron densities in these 2DESs are found by microwave plasma spectroscopy. The specific features of the plasma oscillations in the AlAs/AlGaAs quantum wells that are induced by the filling of several valleys with electrons are revealed. The possibility of adjusting a plasmon spectrum via changing the electron concentrations in valleys is demonstrated.     

Quantum interference and decoherence in hexagonal antidot lattices

The Altshuler–Aronov–Spivak (AAS) oscillations and the Aharonov–Bohm (AB) type oscillations both at low and high magnetic fields were observed in hexagonal antidot lattices fabricated from a GaAs/AlGaAs two-dimensional electron gas sample. The periodicities in the magnetic field and in the gate bias voltage, of the high field AB oscillation furnish information on the edge states localized around the antidots. The temperature dependences of these quantum oscillations are studied.     

Contactless measurement of the conductivity of two-dimensional electrons in the regime of microwave-induced giant magnetoresistance oscillations

Magnetic-field dependences of the conductivity of a two-dimensional electron system obtained by contact and contactless measurements in the regime of microwave-induced giant magnetoresistance oscillations have been comparatively analyzed. The contactless technique for studying the conductivity of two-dimensional electrons is based on measuring the attenuation of the RF signal propagating along a coplanar waveguide manufactured using lithography on the sample surface. It has been found that Shubnikov-de Haas oscillations of conductivity are observed in both techniques, whereas the microwave-induced giant magnetoresistance oscillations appear only in the contact measurements. This contradiction indicates that the contact and/or boundary regions of the two-dimensional system with a strong potential gradient play an important role for the observation of the induced magnetoresistance oscillations.     

Relativistic graphene ratchet on semidisk Galton board

Using extensive Monte Carlo simulations we study numerically and analytically a photogalvanic effect, or ratchet, of directed electron transport induced by a microwave radiation on a semidisk Galton board of antidots in graphene. A comparison between usual two-dimensional electron gas (2DEG) and electrons in graphene shows that ratchet currents are comparable at very low temperatures. However, a large mean free path in graphene should allow to have a strong ratchet transport at room temperatures. Also in graphene the ratchet transport emerges even for unpolarized radiation. These properties open promising possibilities for room temperature graphene based sensitive photogalvanic detectors of microwave and terahertz radiation.     

Impact of nonlinear absorption on propagation of microwave in a plasma filled rectangular waveguide

In collisional and ponderomotive predominant regimes, the propagation of microwave in rectangular waveguide filled with collisional plasma is investigated numerically. The dominant mode is excited through an evacuated waveguide and then enters a similar and co-axis waveguide filled with plasma. In collisional predominant regime, the amplitude of electric field is oscillated along propagation path; outset of propagation path due to the electron-ion collision, the intensity oscillations are reduced. Afterward, under competition between the collisional nonlinearity and absorption, the intensity is increased, so the electron density peak is created in middle of waveguide. In ponderomotive predominant regime, the intensity is slowly decreased due to collision, so the electron density is ramped. Control parameters, like the frequency, input power, collision frequency, and background electron density are surveyed that can be used to control propagation characteristics of microwave. This method can be used to control heating of fusion plasma and accelerate charged particle.     

Thermodynamic properties of a magnetically modulated graphene monolayer

The effect of magnetic modulation on thermodynamic properties of a graphene monolayer in the presence of a constant perpendicular magnetic field is reported here. One-dimensional spatial electric or magnetic modulation lifts the degeneracy of the Landau levels and converts into bands and their bandwidth oscillates with magnetic field, leading to Weiss-type oscillations in the thermodynamic properties. The effect of magnetic modulation on the thermodynamic properties of a graphene sheet is studied and then compared with electrically modulated graphene and magnetically modulated conventional two-dimensional electron gas (2DEG). We observe Weiss-type and de Haas-van Alphen oscillations at low and high magnetic fields, respectively. There is a definite phase difference in Weiss-type oscillations in thermodynamic quantities of magnetically modulated graphene compared to electrically modulated graphene. On the other hand, the phase remains the same and the amplitude of the oscillation is large when compared with the magnetically modulated two-dimensional electron gas (2DEG). Explicit asymptotic expressions of the density of states and the Helmholtz free energy are provided to understand the phase and amplitude of the Weiss-type oscillations qualitatively. We also study thermodynamic properties when both electric and magnetic modulations are present. The Weiss-type oscillations still exist when the modulations are out-of-phase.     

Magnetoresistance oscillations due to Zener tunneling and microwave radiation in a 2D electron gas in GaAs quantum well with AlAs/GaAs superlattices barriers

The effect of microwave radiation in the frequency range from 1.2 to 10 GHz on the magnetoresistance of a high-mobility two-dimensional electron gas has been studied in a GaAs quantum well with AlAs/GaAs superlattice barriers. It has been found that the microwave field induces magnetoresistance oscillations periodic in the reciprocal magnetic field (1/B). It has been shown that the period of these oscillations in the covered frequency range depends on the microwave radiation power.     

Microwave-induced magnetic field oscillations of the electromotive force in a two-dimensional Corbino disk at large filling factors

Microwave-induced magnetic field oscillations of the electromotive force are observed at large filling factors in two-dimensional Corbino disks fabricated on the basis of GaAs/AlAs heterostructures. It is shown that these oscillations of the electromotive force are periodic in the inverse magnetic field and are in antiphase with oscillations of microwave photoconductivity. The experimental data are explained by the inhomogeneous distribution of the microwave field between the coaxial metal contacts to the two-dimensional electron system.     

Electric-field-induced impact ionization of excitons in GaN and GaN/AlGaN quantum wells

Impact ionization of exciton states in epitaxial GaN films and GaN/AlGaN quantum-well structures was studied. The study was done using an optical method based on the observation of exciton photoluminescence quenching under application of an electric field. It was established that electron scattering on impurities dominates over that from acoustic phonons in electron relaxation in energy and momentum. The mean free path of the hot electrons was estimated. The hot-electron mean free path in GaN/AlGaN quantum wells was found to be an order of magnitude larger than that in epitaxial GaN films, which is due to the electron scattering probability being lower in the two-dimensional case.     

Quantum size effects in the elastic scattering of light from small metal particles

A quantum mechanical calculation of the differential elastic scattering cross-section of light from a metal microparticle is presented. The scattering intensity is found to exhibit oscillations as a function of the frequency due to the discreteness of the electron energy levels. The magnitudes of the oscillations have a sensitive dependence on the size of the electron mean free path relative to the diameter of the particle.     

Maximum of the mobility of two-dimensional electrons upon scattering by a correlated distribution of impurity ions

The concentration dependences of the mobility of two-dimensional electrons in heterostructures with selective doping are investigated. Correlations of impurity ions in the volume of the doped layer are considered. The structure factor, which characterizes the influence of correlations in the arrangement of scatterers on the electron mobility, is described by the analytical expression derived in the framework of the hard-sphere model. The effect of oscillations of the structure factor on the electron mobility is evaluated.     

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.     

A model for the dependence of the dielectric loss factor of a ferroelectric on the microwave signal amplitude

The nonlinear phenomena occurring in ferroelectrics were studied using a model for the dependence of the ferroelectric polarization on the amplitude of the applied field. The inclusion of the static internal defect field made it possible to determine the dependence of the complex permittivity of a ferroelectric on the dc electric field strength. The dependences of the complex permittivity and the dielectric loss factor on the amplitude of microwave oscillations were obtained using the Poynting theorem for harmonic microwave oscillations.