Contact phenomena in a semiconductor film with activational conduction

It is noted that the contact of an unscreened, two-dimensional electronic system with “external” metallic electrodes destroys the spatial uniformity of its electron density, which decreases slowly (inversely as the distance from the interface) away from the contact zone. These effects are especially pronounced in systems with a small number of carriers, for example, semiconductor films with an exponentially low electron (hole) density, when the nonuniform part of the contact density competes without difficulty with its initial, uniform component. The effect of contact phenomena on the conductivity of a semiconductor film, which is the central part of a Corbino disk, for different temperatures, doping levels and doping composition, sample dimensions in the direction of the current, and so on is described in detail. A comparison with existing experiments is presented. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 2, 176–181 (25 January 1997)     

Conductance of a 2D Corbino disk under QHE conditions in the presence of contact phenomena

It is noted that contacts of a two-dimensional (2DEG) electronic system with “external” metallic electrodes destroy the spatial uniformity of the density of the system. This gives rise to specific dimensional effects for the conductance of an unscreened Corbino disk in a magnetic field sufficient for the appearance of local regions with an integer-valued filling factor in the electron density profile. The overall pattern of the transport voltage distribution along the disk is seriously transformed. It becomes possible to explain the large (compared with the cyclotron energy) values of the critical transport voltage, leading to breakdown of the QHE regime. The experiment qualitatively confirms the predictions of the theory. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 1, 64–69 (10 January 1999)     

Two-dimensional electron gas in double quantum wells with tilted bands

When a voltage is applied to double quantum wells based on AlGaAs/GaAs heterostructures with contact regions (n-i-n structures), a two-dimensional (2D) electron gas appears in one of the quantum wells. Under illumination which generates electron-hole pairs, the photoexcited holes become localized in a neighboring quantum well and recombine radiatively with the 2D electrons (tunneling recombination through the barrier). The appearance, ground-state energy, and density of the degenerate 2D electron gas are determined from the structure of the Landau levels in the luminescence and luminescence excitation spectra as well as from the oscillations of the radiative recombination intensity in a magnetic field with detection directly at the Fermi level. The electron density is regulated by the voltage between the contact regions and increases with the slope of the bands. For a fixed slope of the bands the 2D-electron density has an upper limit determined by the resonance tunneling of electrons into a neighboring quantum well and subsequent direct recombination with photoexcited holes. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 11, 840–845 (10 June 1997)     

Contribution to the theory of “incompressible” regions in an inhomogeneous magnetized 2D electronic system

A modification of the theory of “incompressible” regions in an ideal spinless inhomogeneous magnetized 2D electronic system near points on the electron density profile n(x) with an integer filling factor is proposed. Such regions leads to the appearance of a finite capacitance between the parts of the 2D system that are separated by an incompressible channel, so that capacitive methods can be used to investigate such a system. The Corbino configuration is especially convenient for these purposes. The parameters of the “incompressible” channel in a Corbino disk with a spatially inhomogeneous 2D electronic system in the presence of an individual point, near the channel, on the electron density profile with an integer magnetic filling factor are determined. The magnetocapacitance between the edges of the Corbino disk separated by an incompressible interlayer is found for cases of practical interest. It is shown that this magnetocapacitance contains direct information about the width of the integer strip. Fiz. Tverd. Tela (St. Petersburg) 41, 1103–1109 (June 1999)     

Evolution of vacancy defects in the surface layers of a metal irradiated with a pulsed electron beam

The distribution of vacancy defects in the surface layers of α-Fe after irradiation with a high-current pulsed electron beam is studied experimentally by unique nuclear-physical methods — low-energy positron annihilation, Rutherford backscattering (RBS), and proton-induced x-ray emission (PIXE). Regions with low local density, which are sources of crater formation on the surface of the irradiated sample, are observed by scanning a proton microbeam. Positron lifetime measurements reveal that as the electron beam power increases, nonequilibrium vacancies tend to be captured by carbon impurity atoms. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 618–622 (25 April 1997)     

Local surface segregations of implanted aluminum in an iron crystal with a low density of defects

The surface distribution of elements is studied by scanning a 3-MeV proton beam along the surface of a bcc-Fe sample implanted with aluminum ions in the dose interval (1–50) · 10¹⁶ cm⁻². Ring-shaped regions, up to 30 μm in diameter, with a high density of aluminum, which appear at implantation doses (5–20) · 10¹⁶ cm⁻², are observed. These regions appear as a result of radiation-stimulated segregation processes. A mechanism based on the existence of a low density of dislocations in the initial crystal is proposed to explain the implanted impurity segregation processes. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 86–89 (10 January 1997)     

Negative persistent photoconductivity in GaAs (δ-Sn) structures

The effect of illumination with various wavelengths λ (770 nm<λ<1120 nm) on the conductivity of GaAs structures with tin δ-doping of the vicinal faces was investigated in the temperature range 4.2–300 K. Negative persistent photoconductivity was found in strongly doped samples. It was shown on the basis of the results of investigations of the Hall and Shubnikov-de Haas effects that the negative photoconductivity is due to a large decrease in the electron mobility with increasing electron density. The decrease of electron mobility is explained by ionization of DX centers, which destroys the spatial correlation in the distribution of positively charged donors and negatively charged DX centers. Zh. éksp. Teor. Fiz. 116, 2130–2139 (December 1999)     

Nonuniform distribution of absorbed energy in high-resistance materials excited by an electron beam

Experimental results are presented on the changes in the optical characteristics of lithium fluoride induced by an electron beam with time-varying density and pulse energies close to the threshold for destruction of the material. The spatial distribution of color centers is investigated, especially near breakdown channels. Mechanisms for nonuniform accumulation of defects are discussed, along with the fundamental causes of the inhomogeneous energy distributions induced by the high-current electron beam. Concrete results of calculations of the field intensity distribution in LiF crystals during irradiation are presented, based on models of “uniform” and nonuniform charging of the sample. An abrupt increase in the electric field intensity is predicted near the breakdown channel. Zh. Tekh. Fiz. 68, 53–59 (April 1998)     

Role of electron “lakes” in the negative magnetoresistance effect in the region of Mott hopping conductivity

It is shown for doped and compensated germanium that the appearance of negative magnetoresistance under the conditions of Mott hopping conductivity may be due to the presence of a nonuniform spatial distribution of the electron density, the temperature at which the effect appears apparently being determined by the temperature at which the electron gas condenses into electron “lakes.” A “dead zone” effect was also observed in weak magnetic fields, the threshold field increasing with the nonuniformity of the electron distribution. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 3, 187–191 (10 February 1996)     

On the electron mobility of δ layers in the presence of diamagnetic “ejection” of size-quantization levels

The Hall mobility of electrons is investigated as a function of the population of size-quantization subbands in the two-dimensional electron gas of a δ-doped layer in GaAs with constant total electron density N _s =3.2×10¹² cm⁻² (three initially filled subbands) at T=4.2 K. The population of the subbands is varied by diamagnetic “ejection” of size-quantization levels (i.e., pushing them over the Fermi level) by a magnetic field oriented parallel to the plane of the δ-doped layer. The measurements are made in magnetic fields making small angles (5°) with the plane of the doping. The magnetic field component normal to the plane was used to measure the Hall mobility and density. It is found that the measured Hall mobility as a function of the ejecting magnetic field has a distinct maximum. This maximum is due to an increase in the electron mobility in the first subband (the ground subband is assigned the index 0) and electron redistribution between subbands with in increasing ejecting magnetic field parallel to the plane of the δ layer. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 704–708 (10 December 1997)     

Experiments on two-step heating of a dense plasma in the GOL-3 facility

This paper presents the results of experiments on two-stage heating of a dense plasma by a relativistic electron beam in the GOL-3 facility. A dense plasma with a length of about a meter and a hydrogen density up to 10¹⁷ cm⁻³ was created in the main plasma, whose density was 10¹⁵ cm⁻³. In the process of interacting with the plasma, the electron beam (1 MeV, 40 kA, 4 μs) imparts its energy to the electrons of the main plasma through collective effects. The heated electrons, as they disperse along the magnetic field lines, in turn reach the region of dense plasma and impart their energy to it by pairwise collisions. Estimates based on experimental data are given for the parameters of the flux of hot plasma electrons, the energy released in the dense plasma, and the energy balance of the beam-plasma system. The paper discusses the dynamics of the plasma, which is inhomogeneous in density and temperature, including the appearance of pressure waves. Zh. éksp. Teor. Fiz. 113, 897–917 (March 1998)     

Suppression of the equilibrium tunneling current between slightly disordered two-dimensional electron systems with different electron concentrations in a high magnetic field

Tunneling between parallel two-dimensional electron gases (2DEG) in accumulation layers formed on both sides of the single doped AlGaAs barrier are examined in both zero and high magnetic field. Accumulation layers are separated from highly n-doped contact regions which freely supply electrons to the 2DEGs via 80 nm thick lightly n-doped spacer layers. Strongly oscillating current with magnetic field along the 2DEGs is absent in this arrangement. Without magnetic field resonant tunneling between 2DEGs with different as grown electron concentration could be settle by application of external voltage bias. High magnetic fields (ν<1) shift resonant tunneling to zero external bias and suppresses tunneling current, creating wide gap in the tunneling density of states at the Fermi level arisen from the in-plane Coulomb interaction in the 2DEGs. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 3, 236–241 (10 February 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

High-frequency asymptotic behavior of T-odd optical effects

It is shown that the contribution of low-frequency excitations with characteristic energy ℏω _l to T-odd (nonreciprocal) optical effects, including spatial dispersion effects, at optical frequencies ω≫ω _l can be calculated in the zeroth-order approximation with respect to the parameter ω _l/ω. This greatly simplifies their analysis. Some of these effects are found to be frequency independent in the spectral regions where the refractive index n(ω)≈ const. It is shown that frequency-independent Faraday rotation can be observed in media with zero magnetization, including in media with zero microscopic magnetic moment density. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 7, 510–513 (10 April 1999)     

Domain Patterns in the Microwave-Induced Zero-Resistance State

It has been proposed that the microwave-induced “zero-resistance” phenomenon, observed in a GaAs two-dimensional electron system at low temperatures in moderate magnetic fields, results from a state with multiple domains, in which a large local electric field E(r) is oriented in different directions. We explore here the questions of what may determine the domain arrangement in a given sample, what do the domains look like in representative cases, and what may be the consequences of domain-wall localization on the macroscopic dc conductance. We consider both effects of sample boundaries and effects of disorder, in a simple model, which has a constant Hall conductivity, and is characterized by a Lyapunov functional.     

Landau quantization and equatorial states on the surface of a nanosphere

The Landau quantization for the electron gas on the surface of a sphere is considered. It is shown that in the regime of strong fields the lowest energy states are those with magnetic quantum numbers m of order of Φ /Φ₀, the number of magnetic flux quanta piercing the sphere. For an electron gas of low density (semiconducting situation) it leads to the formation of an electronic stripe on the equator of the sphere in high fields. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 6, 405–409 (25 September 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Studies of the electron density at the nucleus and radial expectation values of the ground state for lithium-like systems from Z=11 to 18

The electron density at the nucleus ρ(0) and radial expectation values 〈rⁿ〉 (n=-2–10) of the ground state for lithium-like systems from Z=11 to 18 are calculated with the full core plus correlation wave functions. By using these obtained expectation values, accurate inequalities of radial expectation values derived by Gálvez and Porras [Phys. Rev. A 44, 144 (1991)] are examined and verified. The final results show that the full core plus correlation wave functions for lithium-like systems with higher nuclear charge are also accurate in the full configuration space and can give satisfactory electron density at the nucleus and radial expectation values.     

Temperature dependence of the kinetic coefficients of interference ballistic structures

The effect of the temperature on the kinetic coefficients of a mesoscopic sample in contact with two electron reservoirs is considered for the case in which the electron transmission coefficient of the sample undergoes oscillations near the Fermi energy. Zh. éksp. Teor. Fiz. 114, 1827–1835 (November 1998)     

Contributions to the theory of ferromagnetism in the degenerate Hubbard model

The possibility of ferromagnetic ordering in a generalized Hubbard model with allowance for degeneracy and for infinite Hubbard energy is studied. The region of existence of ferromagnetism for electron density greater than 1 is determined. It is shown that for electron density less than 1 ferromagnetism exists only in special cases when the Fermi surface passes near van Hove singularities. Zh. éksp. Teor. Fiz. 112, 2223–2236 (December 1997)     

Spontaneous interference bremsstrahlung effect in the scattering of a relativistic electron by a nucleus in the field of two light waves

This paper presents, for the general relativistic case, a theoretical study of nonresonance spontaneous bremsstrahlung by an electron scattered by a nucleus in the field of two elliptically polarized light waves propagating in the same direction. We show that there are two significantly different kinematic regions: the noninterference region where the main multiphoton parameters are the Bunkin-Fedorov quantum parameters γ _1,2, and the interference region where interference effects play an important role and where the quantum interference parameters α(±) act as multiphoton parameters. We encounter the spontaneous interference bremsstrahlung effect in two cases: in the special case of the same linear polarization of both waves, and in the general case of elliptical polarization of the waves. The effect manifests itself in the interference region and is due to stimulated, correlated emission and absorption of photons of both waves. For moderately strong fields, we find the cross sections of spontaneous bremsstrahlung by an electron scattered by a nucleus in the given kinematic regions. Finally, we show that the differential cross section in the interference region with correlated emission (absorption) of equal numbers of photons of both waves can be much greater than the corresponding cross section in any other geometry. Zh. éksp. Teor. Fiz. 116, 1210–1240 (October 1999)