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.     

Microwave-induced magnetic field state with zero conductivity in GaAs/AlAs Corbino disks and hall bars

The microwave photoconductivity of the 2D electron gas in GaAs/AlAs heterostructures has been investigated at a temperature of 4.2 K in magnetic fields up to 1.5 T. It has been found that the magnetic field state with zero conductivity appears in GaAs/AlAs Corbino disks irradiated by 130.70-GHz microwave radiation. This state was previously observed only in GaAs/AlGaAs Corbino disks with much higher electron mobility and lower density. It has been shown that the microwave photoconductivity measured in high magnetic fields on Corbino disks can significantly differ from the value calculated from the results of the measurements on Hall bars. This difference is explained by the fact that the conditions of the appearing magnetoplasmons that affect the magnitude and character of the microwave photoconductivity (photoresistance) in the Corbino disks are nonequivalent to those in the Hall bars.     

Spatial inhomogeneity of the microwave-induced electronic states with zero conductivity in Corbino disks at high filling factors

The fluctuations of the microwave electromotive force in the magnetic field state with an extremely low conductivity have been revealed in 2D electronic Corbino disks prepared based on GaAs/AlAs heterostructures. The experimental data are attributed to the spatial inhomogeneity and time instability of the electronic states that have zero conductivity and are induced by microwave radiation in the 2D Corbino disks at high filling factors.     

Population inversion and novel magnetoconductivity oscillations induced by resonant microwave irradiation in multisubband two-dimensional electron systems

The possibility of a population inversion in a multisubband two-dimensional (2D) electron system exposed to resonant microwave (MW) radiation and a static magnetic field is shown. Theoretical analysis given here indicates that the resonant excitation of the third subband, necessary for the population inversion, is accompanied by novel magnetoconductivity oscillations in the 2D electron system formed on the surface of liquid helium.     

Zener tunneling between the landau levels in quasi-two-dimensional electronic Corbino disks at large filling factors

The magnetotransport characteristics of quasi-two-dimensional electronic Corbino disks based on a wide GaAs quantum well with two filled subbands of size quantization are studied. At low temperatures and high magnetic fields, the Corbino disks exhibit the oscillations of differential conductivity, which are periodic with respect to dc electric field E _dc. It is shown that the observed oscillations are related to the Zener tunneling induced by E _dc. The tunneling occurs between the filled and empty Landau levels under conditions corresponding to the absence of the Hall field.     

Investigation of the current injection properties of ohmic spikes in nanostructures

We investigate the mechanism by which annealed Ni–Au–Ge metallizations establish electrical contact to the two-dimensional electron gas (2DEG) in AlGaAs/GaAs heterostructures and we assess the suitability of this contact process for defining patterned contacts in semiconductor nanostructures. We present a picture where electrons are injected from an array of spikes which extend below the surface-metal pattern to penetrate the 2DEG and the associated quantum waves determine the contact resistance through phase-coherent interference processes at low temperatures. To probe this mesoscopic current injection process, we induce an evolution between two distinct geometries traditionally used to characterize semiconductor systems—the Corbino disc and the Hall bar. We also discuss refinements to the injection process which will facilitate the incorporation of shaped ohmic contacts into future nanostructure designs.     

Interference of ballistic electrons in an open quantum dot at high temperatures

The Aharonov-Bohm-type oscillations in the ballistic magnetoconductivity of an open quantum dot ～1 μm in size created in the GaAs/AlGaAs heterostructure by local anode oxidation have been studied. The measurements have been performed at temperatures of 4.2 and 1.5 K, which are high enough to expect a considerable suppression of the oscillations with period h/e (in magnetic flux units). The magnetoconductivity oscillations with a period less than the quantum h/e with respect to the magnetic field are observed at 1.5 K. The explanation is proposed on the basis of the interference of the electrons moving along the time-reversed paths inscribed into the quantum dot that have their initial and final points at one of the contacts, i.e., corresponding to a period of h/2e.     

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)].     

Observation of Microwave-Induced Magnetoresistance Oscillations in a ZnO/Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>O Heterojunction

Microwave-induced magnetoresistance and magnetoconductance oscillations in the two-dimensional electron system have been detected in a ZnO/Mg _x Zn_1–x O heterojunction. The structure has a high electron density of n = 6.5 × 10¹¹ cm^–2 and a very low mobility of 10⁵ cm² V^–1 s^–1. This clearly indicates that the mobility of two-dimensional electrons is not a critical parameter for the observation of microwave-induced oscillations. The samples were both Hall bars and Corbino disks. At least four oscillations have been resolved. Their amplitude increases with the magnetic field and all oscillations with numbers >1 have the same phase of 0.25. It has been shown that the microwave-induced photoelectromotive force is also a periodically oscillating function of the inverse magnetic field.     

Universal relation between hall conductivity and the damping constant of edge magnetoplasma resonances

The spectra of edge magnetoplasma excitations in two-dimensional (2D) electron disks have been analyzed by the method of optical detection of resonant microwave absorption. The magnetic dispersion of an edge magnetoplasmon in samples with a high 2D electron density is found to be poorly reproduced by existing theoretical models. Analysis of the magnetic-field dependence of the linewidth of resonant microwave absorption for samples with various 2D electron densities shows that the inverse width of the main mode of resonant microwave absorption is universally proportional to the Hall resistance of 2D electrons.     

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.     

硅衬底GaN基LED N极性n型欧姆接触研究

在Si衬底GaN基垂直结构LED的N极性n型面上,利用电子束蒸发的方法制作了Ti/Al电极,通过了I-V曲线研究了有无AlN缓冲层对这种芯片欧姆接触的影响.结果显示,去除AlN缓冲层后的N极性n型面与Ti/Al电极在500到600 ℃范围内退火才能形成欧姆接触.而保留AlN缓冲层的N极性n型面与Ti/Al电极未退火时就表现为较好的欧姆接触,比接触电阻率为2×10^-5 Ω·cm²,即使退火温度升高至600 ℃,也始终保持着欧姆接触特性.因此,AlN缓冲层的存在是Si衬底GaN基垂直结构LED获得高热稳定性n型欧姆接触的关键. 关键词： 硅衬底 N极性 AlN缓冲层 欧姆接触     

Experimental investigation of plasma excitations in asymmetric stripes of two-dimensional electrons

The plasmon microwave response of symmetric and asymmetric stripes of two-dimensional electrons with different boundary conditions has been studied. The symmetric case corresponds to stripes either without ohmic contacts or with ohmic contacts on both sides, while the contact in asymmetric stripes is present only on one side. It has been shown that there is a frequency shift of the observed plasma modes in asymmetric stripes. The lowest-frequency mode becomes a previously unobserved mode for which the quarter wavelength of the plasmon coincides with the length of the stripe. The behavior of the lowest mode and its multiple frequencies in asymmetric stripes has been studied under variation of the magnetic field, electron density, and temperature and size of the stripes. The results indicate that all plasma modes in symmetric and asymmetric stripes have the same physical nature.     

Net negative charge of electron-hole drops in germanium

The giant fluctuating photocurrent has been observed in highly excited germanium samples with ohmic contacts at 1.6 K. From the observation of this fluctuating photocurrent spike with varying static electric field, it is found that the electron-hole drop is negatively charge as a whole.     

Magneto-oscillations due to electron-electron interactions in the ac conductivity of a two-dimensional electron gas

Electron-electron interactions give rise to the correction, deltasigma(int)(omega), to the ac magnetoconductivity, sigma(omega), of a clean 2D electron gas that is periodic in omega_(c)(-1), where omega_(c) is the cyclotron frequency. Unlike conventional harmonics of the cyclotron resonance, which are periodic with omega, this correction is periodic with omega(3/2). Oscillations in deltasigma(int)(omega) develop at low magnetic fields, omega_(c)相似文献   

Oscillatory magnetoconductivity of a two-dimensional electron system due to phonon scattering

Summary A quantum-statistical theory of magnetophonon resonance oscillations in two-dimensional systems has been developed, starting from the resolvent representation of Kubo's formula and its proper connected diagram expansion. Non-polar and polar optical-phonon scattering has been considered and the results show, as anticipated based on the physical considerations and experimental observations, conductivity oscillations as a function of magnetic field with the magnetophonon resonances occurring at the phonon frequencies {ie1539-1} {ie1539-2}=cyclotron frequency). Divergences occurring in the magnetoconductivity near the magnetophonon resonances are removed by using the full resolvent operator in the tetradic self-energy operator of an electron. These additional terms provide necessary damping of the magnetophonon resonance oscillations. The present results are also shown to be qualitatively similar to those obtained by others using quantum Boltzmann's equation approach to quantum transport theory.     

On the electrostatic potential distribution in a screened Corbino disk carrying a transport current under conditions of the quantum Hall effect

It is shown that the characteristic features of the chemical potential for 2D electrons in a magnetic field, which lead to sharp dips in the magnetic field dependence of the capacitance of a 2D system, also affect the electrostatic potential distribution in the direction of the transport current flowing through a 2D Corbino disk under conditions of integral magnetic filling factor. The associated details of the temperature dependence of the electrostatic potential distribution, the distances to the screening electron, and the transport potential difference at the Corbino edges are investigated. The possibilities of experimentally observing these features of the electrostatic potential distribution along a Corbino disk with a transport current under conditions of the quantum Hall effect are discussed. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 8, 545–550 (25 October 1997)     

Microwave ponderomotive action on the inhomogeneous collisionless and collisional plasmas

The nonlinear interaction of a high-power microwave (MW) with an unmagnetized inhomogeneous plasma is investigated in collisionless and collisional regimes. The electron density distribution and the nonlinear wave equation in an inhomogeneous plasma are obtained by taking into account the ponderomotive force due to the high-power MW. It is shown that the electron density distribution becomes very steepened in the presence of the ponderomotive force. In the collisional regime, the expression for electron temperature is also found by considering ohmic heating. It is indicated that the amplitude of oscillations of the electron temperature and dielectric permittivity increases and the wavelength of these oscillations decreases with increasing energy flux, hence modulation occurs.     

Magnetotransport in thin epitaxial Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> films

The magnetoconductivity of thin Bi₂Se₃ films covered by a protective Se layer and grown at (111) BaF₂ substrates is studied. It is shown that the negative magnetoconductivity observed at low magnetic fields and caused by the effect of weak antilocalization, as well as the Shubnikov?de Haas oscillations at higher fields, is determined only by the magnetic field component perpendicular to the film plane. The obtained experimental results can be reasonably interpreted under the assumption that the studied films exhibit two-dimensional topologically protected electron states. Moreover, the contribution of these states to the total conductivity turns out to be the dominant one.     

Conductance oscillations of a spin-orbit stripe with polarized contacts

We investigate the linear conductance of a stripe of spin-orbit interaction in a 2D electron gas; that is, a 2D region of length l\ell along the transport direction and infinite in the transverse one in which a spin-orbit interaction of Rashba type is present. Polarization in the contacts is described by means of Zeeman fields. Our model predicts two types of conductance oscillations: Ramsauer oscillations in the minority spin transmission, when both spins can propagate, and Fano oscillations when only one spin propagates. The latter are due to the spin-orbit coupling with quasibound states of the non propagating spin. In the case of polarized contacts in antiparallel configuration Fano-like oscillations of the conductance are still made possible by the spin orbit coupling, even though no spin component is bound by the contacts. To describe these behaviors we propose a simplified model based on an ansatz wave function. In general, we find that the contribution for vanishing transverse momentum dominates and defines the conductance oscillations. Regarding the oscillations with Rashba coupling intensity, our model confirms the spin transistor behavior, but only for high degrees of polarization. Including a position dependent effective mass yields additional oscillations due to the mass jumps at the interfaces.