Influence of the beam-forming conditions on the development of space-charge oscillations in a long-pulse relativistic electron beam

The characteristics of the space-charge oscillations of a long-pulse relativistic electron beam in magnetically insulated diodes are determined for different geometries of the electron acceleration section and for explosive-emission cathodes of different materials. The important role of the stream of electrons having high transverse velocities in the evolution of the oscillations is demonstrated, and the laws governing the generation of this stream are determined. Possible mechanisms of the space-charge oscillations are described, taking into account the interaction of the electron stream in the beam halo with the main electron stream, the development of diocotron instability in the stream of electrons emitted by the outer lateral surface of the plasma emitter, and the instability of the space charge of “long-lived” electrons in the beam transport channel. Zh. Tekh. Fiz. 68, 102–106 (April 1998)     

Observation of a hybridization gap in cyclotron resonance spectra of semimetallic InAs/GaSb quantum wells

Splitting of the cyclotron resonance (CR) line is observed in the electron CR spectra of InAs/GaSb heterostructures containing tunneling-coupled electron and hole layers. This splitting is interpreted to be a manifestation of a hybridization gap arising as a result of anticrossing of the Landau levels of electrons and holes when their wave functions overlap. The energy splitting of the CR lines is correlated with the magnitude of the overlap and agrees with theoretical estimates of the hybridization gap width. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 4, 313–317 (25 February 1999)     

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)     

Stability of a quasineutral beam of negative ions

The stability of a beam of negative ions propagating in the parent gas is investigated. It is shown that growth of plasma oscillations with a small growth rate is possible at subcritical currents in a quasineutral regime. The influence of traveling secondary electrons, whose density is small, is also studied. The low-frequency oscillations considered pose the greatest danger to a beam whose velocity is close to the velocity of the secondary electrons. Zh. Tekh. Fiz. 68, 96–98 (September 1998)     

Spectrum of polariton excitations of a two-dimensional electron plasma in a magnetic field

Polariton excitations associated with magnetoplasma and cyclotron oscillations in a two-dimensional (2D) electron plasma are studied. In contrast to previous works by other authors, it is concluded that there exists a low-frequency nontransmission band in the spectrum of 2D surface magnetoplasma polaritons. Radiative polariton excitations associated with nonuniform cyclotron oscillations of electrons in a 2D system are investigated. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 3, 200–204 (10 August 1998)     

Effect of the spin-orbit interaction on the cyclotron resonance of two-dimensional electrons

In the cyclotron resonance (CR) spectra of two-dimensional (2D) electrons in InAs quantum wells, the CR line splitting is observed. The splitting is found to be an oscillating function of magnetic field. The oscillations do not correlate with the filling factor. The experimental results are interpreted in terms of the spin-orbit splitting in the presence of a built-in electric field appearing due to the asymmetry of the quantum-well potential. From the splitting of the CR line, the spin-orbit coupling constant α_so is determined. The resulting value agrees well with the value obtained for the same sample from the Shubnikov-de Haas oscillations. The role of the resonance interaction of charge carriers in the well with the interface donor states is discussed.     

Electronic topological transition in an <Emphasis Type="Italic">n</Emphasis>-BiSb semiconductor alloy in the quantum limit range of magnetic fields for H‖<Emphasis Type="Italic">C</Emphasis><Subscript>2</Subscript>

The galvanomagnetic properties of single-crystal samples of the Bi_0.93Sb_0.07 semiconductor alloy with the electron density n = 1.6 × 10¹⁷ cm⁻³ in magnetic fields up to 14 T at T = 1.6 K have been investigated. The resistivity ρ and Hall coefficient R have been measured as functions of the magnetic field directed along the binary axis of a crystal for a current flowing through a sample along the bisector axis; i.e., the components ρ₂₂ and R _{32, 1} have been measured. The strong anisotropy of the electron spectrum of the samples makes it possible to separately observe quantum oscillations of the magnetoresistance ρ₂₂(H) for H ‖ C ₂ in low magnetic fields for two equivalent ellipsoids with small extremal cross sections (secondary ellipsoids) and in high magnetic fields for electrons of the ellipsoid with a large extremal cross section (main ellipsoid). An increase in the energy of the electrons of secondary ellipsoids in the quantum limit magnetic fields is accompanied by the flow of electrons to the main ellipsoid; i.e., an electronic topological transition occurs from the three-valley electron spectrum to the single-valley one. After the flow stops, the Fermi energy E _F increases from 18 meV to 27.8 meV. With an increase in the quantizing magnetic field, the Fermi energy of the electrons decreases both in the region of quantum oscillations of the resistance that are attributed to the electrons of the secondary ellipsoids and in the region of oscillations associated with the electrons of the main ellipsoid. The Hall coefficient R _{32, 1} decreases in high magnetic fields; this behavior indicates the absence of the electron magnetic freezing effect.     

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)     

Excitation of plasma oscillations during the motion of Josephson vortices in layered superconductors

The electric and magnetic fields arising during uniform motion of a vortex lattice in a magnetic field oriented parallel to the conducting layers are calculated in an exactly solvable model. For low temperatures and high velocities of the lattice, features due to the excitation of plasma oscillations of the superconducting electrons appear in the current-voltage characteristic. Peaks associated with plasmon excitation and the Cherenkov effect are present in the radiation spectrum. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 12, 811–816 (25 December 1997)     

Limiting nonideal metastable supercooled plasma

Direct modeling of the dynamics of a system of many Coulomb particles is applied to analyze the formation stage of a metastable plasma state from an initial, highly nonideal state, and also to consider some properties of this metastable supercooled state. It is shown that relaxation of the average particle kinetic energy may be characterized by a universal dimensionless function and in particular, there is a limiting degree of plasma nonideality which may be achieved in the metastable state, in the absence of any external influence. The calculated pair correlation functions agree with the results of the Debye model, even outside its limits of validity. The time dependence of the total dipole moment of the particle system is investigated. It is shown that oscillations of the total dipole moment are observed. These collective oscillations take place at a frequency slightly below the Langmuir frequency and the oscillations of free and bound electrons are in antiphase. The hypothesis is put forward that recombination relaxation is frozen as a result of interaction between quasibound electrons and Langmuir oscillations of free electrons. Zh. Tekh. Fiz. 67, 42–52 (August 1997)     

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)     

Shubnikov-de Haas effect in tricrystals of the system of alloys Bi1−x Sbx (x=0.1; 0.13) with n-type conductivity

The Shubnikov-de Haas effect has been investigated in tricrystals of the alloy system Bi_1−x Sb_x (x=0.1; 0.13) with n-type conductivity in stationary (up to 14 T) and pulsed (up to 40 T) magnetic fields. Reconstruction of the internal boundary of the tricrystals was observed, along with a number of new component oscillations of ρ(B) indicating a rotation of the constant-energy surfaces of the L electrons on the intercrystallite boundary at an angle of ∼74° in the binary-trigonal plane. Fiz. Tverd. Tela (St. Petersburg) 41, 991–993 (June 1999)     

Co/Pd and Co/Pd/CoNi multilayer films: Determination of the sign and magnitude of the exchange interaction of ferromagnetic layers separated by palladium layers

It is found that the exchange interaction of Co (5 nm)ferromagnetic layers via Pd in Co/Pd multilayer films and of Co (5 nm)and CoNi (5 nm) layers via the same nonmagnetic metal in Co/Pd/CoNi films can be both ferro-and antiferromagnetic. The period of the AF-F-AF oscillations is of the order of 0.8 nm, and the amplitude of the oscillations decays as a power-law function ~d _Pd ⁻² . Pis’ma Zh. éksp. Teor. Fiz. 66, No. 7, 487–491 (10 October 1997)     

Determination of the sign of the conduction-electron g factor in semiconductor quantum wells by means of the Hanle effect and spin-quantum-beat techniques

A method of measuring the sign of the conduction-electron g factor in semiconductor quantum-wells is proposed, based on determination of the sense of electron-spin Larmor precession by the Hanle effect or spin-quantum-beat techniques under oblique incidence of pump light on the sample, with the luminescence detected at an angle to the pump beam. This method has been used to measure the sign of the transverse electronic g-factor component in GaAs/Al_0.3Ga_0.7As quantum-wells of various widths. It has been shown experimentally that the average spin of electrons photocreated in quantum-wells may not coincide with the pump light direction. Expressions for the oscillations of the luminescence circular polarization in the spin-quantum-beat regime and in the Hanle effect have been obtained, taking into account the electron spin relaxation anisotropy. Fiz. Tverd. Tela (St. Petersburg) 39, 768–773 (1997)     

Numerical simulation of space-charge dynamics in a gyrotron trap

The particle-in-cell (PIC) method is used to simulate the self-consistent accumulation and bunching of space charge in the trap of a gyrotron electron-optical system. It is shown that it is possible to generate charge bunches that oscillate along the direction of the magnetic field. The dependence of the characteristics of these oscillations on the magnitude of the electron current into the trap is determined, along with the effect of the accumulated charge on the velocity distribution of electrons in the current passing through the magnetic mirror. Satisfactory agreement with the experimental data is obtained. Zh. Tekh. Fiz. 67, 98–101 (September 1997)     

Beam-plasma discharge in the propagation of a long-pulse relativistic electron beam in a medium-pressure rarefied gas

A theoretical model is given, along with a numerical analysis of the evolution of beam-plasma discharge in the propagation of a long-pulse relativistic electron beam in a rarefied gas at medium pressure. It is shown that the self-stabilization of beam-plasma discharge as a result of longitudinal inhomogeneity of the density of the discharge plasma makes it possible for the beam to traverse the beam chamber with relatively low total energy losses, including ionization losses and energy losses in the generation of oscillations. During the dissociative recombination of electrons and ions of the discharge-driven plasma, heat is released and spent in raising the temperature of the gas. The investigated collective-discharge mechanism underlying heating of the gas for a relativistic beam can be more efficient than the classical heating mechanism due to ionization losses of the beam in pair collisions of its electrons with gas particles. Zh. Tekh. Fiz. 67, 94–98 (May 1997)     

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.     

Direct observation of standing electron waves in diffusively conducting inas nanowire

We investigate the disturbance of the InAs nanowire resistance by a conductive tip of a scanning probe micro-scope at helium temperature as a function of the tip position in close vicinity to the nanowire. At the tip displacement along the wire the resistance (R _wire ∼ 30 kΩ, what is typical for diffusive regime) demonstrates quasi-periodical oscillations with an amplitude about 3%. The period of the oscillations depends on the number of electrons in the nanowire and is consistent with expected for standing electron waves caused by ballistic electrons in the top subband of the InAs nanowire.     

Anharmonicity of Bloch oscillations in GaAs/AlAs superlattices in the case of inhomogeneously broadened excitonic states in a weak electric field

The variation of the spectral shape of the four-wave mixing signal in GaAs/AlGaAs superlattices was studied experimentally and theoretically. It was shown that in an external electric field, leading to anti-crossing of the levels of heavy and light excitons, sawtooth oscillations of the energy position of the levels are observed. These oscillations are due to the inhomogeneous broadening of the states participating in the Bloch oscillations. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 10, 820–825 (25 May 1998)     

Total quantum-current yield in the soft x-ray region

The emission of slow secondary electrons excited in efficient photocathodes by fast internal x-ray electrons upon absorption of x-ray photons having energies in the range 1–10 keV is analyzed. Analytical expressions are derived for the quantum current yield of the x-ray photoelectric effect for a “point” model and a “non-point” model of energy exchange of fast internal x-ray electrons. We present some estimates for its parameters in a CsI photocathode. Fiz. Tverd. Tela (St. Petersburg) 40, 1042–1046 (June 1998)