On the question of short-period oscillations of the collector voltage during transverse electron focusing

Giant short-period oscillations during transverse electron focusing at high emitter voltages have been observed in bismuth samples with a superlattice on the surface. A model is proposed which explains the onset of the oscillations, their position on the magnetic-field scale, and their intensification and shift along the magnetic-field scale with increasing current (this shift depending on the direction of current flow) and which can also account for the absence of oscillations in fields which are multiples of the field of the first oscillation. In this model the oscillations are attributed to the appearance of resonant surface (edge) states and their contribution to the electron transport. Pis'ma Zh. éksp. Teor. Fiz. 68, No. 12, 887–892 (25 December 1998)     

High-frequency modulation of current in tunnel-coupled quantum wells by transverse radiation in the terahertz range

The modulation of a longitudinal current in double quantum wells by a transverse voltage in the terahertz frequency range is calculated. A quantum kinetic equation which allows for the contributions of a pair of asymmetric tunnel-coupled levels to the resonant response is solved (the difference in effective mass or the asymmetric scattering in the left-and right-hand quantum wells is also taken into account). These structural features lead to conversion of the modulating voltage into a high-frequency longitudinal current. The calculated longitudinal-transverse nonlinear susceptibility determines the conditions needed for creating an efficient field-effect transistor circuit that would use pumping in the terahertz range instead of a transverse control voltage. Zh. éksp. Teor. Fiz. 114, 322–332 (July 1998)     

Effect of electron-phonon interaction on the thermoelectric properties of superlattices

The electron relaxation time on acoustical phonons, the electrical conductivity, and the phonon-drag thermopower of a semiconductor superlattice with quasi-two-dimensional quantum wells are calculated. The inelasticity of the scattering of charge carriers is taken into account. It is shown that the phonon-drag thermopower of a superlattice can be an order of magnitude greater than the corresponding thermopower of a bulk semiconductor. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 4, 290–295 (25 February 1999)     

Electric-field distribution in a quantum superlattice with an injecting contact: Exact solution

A very simple model describing steady-state electron transport along a quantum superlattice of a finite length taking into account an arbitrary electrical characteristic of the injecting contact is considered. In the singleminiband approximation, exact formulas for the spatial distribution of the electric field in the superlattice are derived for different types of contact. Conditions under which the field is uniform are identified. Analytical expressions for the current–voltage characteristics are obtained. In the context of the developed theory, the possibility of attaining uniform-field conditions in a diode structure with a natural silicon-carbide superlattice is discussed.     

Minibands of p-type δ-doping superlattices in GaAs

A microscopic theory is presented for high-field miniband transport in a two-dimensional superlattice. The energy transfer to the lateral electron motion is taken into account as well as scattering on polar optical phonons. Oscillatory current anomalies appear when the optical phonon frequency is a multiple of the Bloch frequency. The current oscillations, which are due to Wannier–Stark localization, are much more pronounced in a two-dimensional than in a three-dimensional system with a superlattice structure in one direction.     

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)     

Thermoelectric power in quasi-two-dimensional systems with scattering of current carriers by phonons

The thermopower α in electron systems with a quasi-two-dimensional energy spectrum is investigated in the relaxation-time tensor approximation. The longitudinal and transverse components of the thermopower are calculated for scattering of the current carriers by different types of phonons. It is shown that the anisotropy of the thermopower in such systems is substantial. The dependence of a on the ratio of the Fermi level ɛ _F to the half-width ɛ ₀ of the one-dimensional conduction band is considered. For scattering by acoustical and nonpolar optical phonons, the thermopower changes sign: α becomes positive for ɛ _F<ɛ ₀. Comparison of the theory with published experimental data demonstrates good qualitative agreement. Fiz. Tverd. Tela (St. Petersburg) 39, 1857–1858 (October 1997)     

Absolute negative conductivity and spontaneous current generation in semiconductor superlattices with hot electrons

We study transport through a semiconductor superlattice with an electric field parallel to and a magnetic field perpendicular to the growth axis. Using a semiclassical balance equation model with elastic and inelastic scattering, we find that (1) the current-voltage characteristic becomes multistable in a large magnetic field and (2) "hot" electrons display novel features in their current-voltage characteristics, including absolute negative conductivity and a spontaneous dc current at zero bias. We discuss experimental situations providing hot electrons to observe these effects.     

Effects of electron levels broadening and electron temperature in tunnel structures based on metal nanoclusters

We study the influence of energy levels broadening and electron subsystem overheating in island electrode (cluster) on current-voltage characteristics of three-electrode structure. A calculation scheme for broadening effect in one-dimensional case is suggested. Estimation of broadening is performed for electron levels in disc-like and spherical gold clusters. Within the two-temperature model of metallic cluster and by using a size dependence of the Debye frequency the effective electron temperature as a function of bias voltage is found approximately. For helium temperature of ion subsystem, the heating temperature of electrons in a quantum disc is almost one order of magnitude higher than that in a sphere; it achieves thousands of Kelvin. We suggest that the effects of broadening and electron overheating are responsible for the smoothing of current-voltage curves, which is observed experimentally at low temperatures in structures based on clusters consisting of accountable number of atoms. However, a role of the broadening is much more significant.     

Size effect in the problem of thermal stabilization of superconducting composites

The conditions for thermal stabilization of composite superconductors are investigated in the model of a continuous medium with allowance for the transverse thermal conductivity of the components under the assumption that the current is uniformly distributed over the cross section of the current lead. The conditions for complete (steady-state) and partial (unsteady) stability are analyzed. Numerical simulations are carried out for current-carrying elements of circular and rectangular cross section with various transverse dimensions and for various cooling intensities, and the results are compared with the known results of the one-dimensional theory. It is shown that the current of steady-state stabilization in the two-dimensional approximation is always less than the corresponding value calculated in the one-dimensional model. The critical energies of disturbances are found, giving an upper bound on the admissible intensities of heat release distributed uniformly over the cross section of a composite superconductor. For cooled leads a nonmonotonic dependence of the critical energies on the radius of the composite is observed. This function has a minimum which is shifted to larger values of the radius as the heat removal is improved. It is found that in an extremely narrow interval of initial parameters the one-dimensional model gives a slight underestimate of the level of admissible disturbances. On the whole the one-dimensional calculations give higher values than the two-dimensional calculations. The size effect alters the stability conditions to a greater degree when the heat transfer coefficient is increased. The underlying cause of this effect is an increase in temperature at the center of the lead and a simultaneous decrease in temperature at the surface. Zh. Tekh. Fiz. 67, 51–56 (January 1997)     

Differential thermopower of a superlattice in a strong electric field

The effect of an electric field on the differential thermopower α(E) of a one-dimensional superlattice is investigated in the semiclassical approximation. A nonmonotonic temperature dependence of α(0) is established for a degenerate electron gas. It is shown that, in principle, an electric field can be used to control the thermoelectric properties of superlattices. Fiz. Tverd. Tela (St. Petersburg) 41, 1314–1316 (July 1999)     

Phonons and periodons in IV–VI semiconductor superlattices

We have calculated the phonon and periodon dispersion relations in IV–VI semi-conducting bulk PbTe and SnTe and their superlattice structure. The model used here is a one-dimensional lattice which includes harmonic interactions up to second neighbours as well as on-site nonlinear electron-ion interactions at the anion site. We calculate the phonon and periodon dispersion relations in bulk and PbTe-SnTe superlattice for the transverse optic and acoustic modes using the transfer matrix method. Our analysis has predicted correct nature of the folding of acoustic and confinement of optical phonons at various frequency intervals corresponding to pass and stop bands of the superlattices.     

Microscopic Three-Dimensional Theory of Space-Charge-Wave Modes in GaAs-Based Planar Superlattices

We report a systematic, fully three-dimensional thkoreticd examination of convective space-charge wave modes of a planar superlattice subject to negative differential miniband conductance (NDC) in a dc bias electric field. Our analysis is based on a set of hydrodynamic balance equations for system having an arbitrary energy band, with an accurate microscopic treatment of phonon and impurity scatterings. When applied to the longitudinal transport of an unconfined planar superlattice, these equations, which take full account of the role of carrier transverse motion, give rise to bulk NDC in the dc steady state conduction and provide a unified formulation to analyze the small-signal ac response in the space homogeneous case and the drift-relaxational modes under spatidy inhomogeneous condition.     

Electron spectrum in a quantum superlattice with cylindrical symmetry

A quantum superlattice with axial symmetry, a heterostructure in which two semiconductor materials in the form of coaxial wires with a nanosize cross section are in contact with one another and form a periodic structure in the radial direction, is studied. It is shown that the electron energy spectrum consists of alternating allowed and forbidden bands. The electron dispersion law is studied for different values of the period of the potential, thicknesses of the semiconductor layers, and radius of the inner crystal of the system. It is shown that the coherent electron effective mass of the quantum superlattice is a tensor: The longitudinal component is close in value to the electron effective mass of the semiconductor material characterizing the quantum well of the superlattice and the radial component depends strongly on the period of the potential, the thicknesses of the coaxial semiconductor layers, and the core radius of the heterosystem, taking on positive or negative values in different allowed bands. Fiz. Tverd. Tela (St. Petersburg) 40, 557–561 (March 1998)     

Ultrafast Fiske effect in semiconductor superlattices

The current flowing across a semiconductor superlattice in tilted electric and magnetic fields is known to exhibit resonant enhancement, when Landau states of neighboring wells align at certain ratios of the field strengths. We show that the ultrafast version of this effect, in which coherent electron wave packets are involved, has a profound analogy to the Fiske effect in superconductor Josephson junctions and superfluid weak links, in that the coupling of the tunneling-induced charge oscillations (magneto-Bloch versus Josephson oscillations) to another oscillator (in-plane cyclotron oscillations versus external oscillator modes) opens an elastic rectifying transport channel. We explore the superlattice effect both theoretically and experimentally, and find that the transient self-induced current can be adequately modeled if the damping of both types of coupled electron oscillations is properly taken into account.     

Hopping mobility in semiconductor superlattices

We present results on the electrical transport perpendicular to interfaces in GaAs/Al_xGa_1?xAs superlattices. We have measured the current-voltage characteristics on a series of superlattices. This has been simulated numerically, the superlattice being replaced by an effective medium. Using this model we obtain the values of the effective mobility as a function of the superlattice period. Our data are in good agreement with a theory of phonon-assisted hopping transport between localized states, rather than the theory of phonon-limited band transport of Bloch waves.     

Scaling of the current-voltage characteristics of superconducting films in the flux creep model

On the basis of the magnetic flux creep model, taking viscous vortex motion and the spatial shape of the pinning potential into account, we have constructed a model explaining the shape of the current-voltage characteristics (CVC’s) of high-temperature superconducting films and the sign change of the curvature of these characteristics with change in temperature. The model given also allows one to explain the scaling of these curves. Fiz. Tverd. Tela (St. Petersburg) 40, 989–992 (June 1998)     

Anisotropy of long-wavelength optical phonons in GaAs/AlAs superlattices

The angular anisotropy of optical phonons in GaAs/AlAs (001) superlattices is investigated by Raman scattering spectroscopy. Scattering configurations allowed for phonons with wave vectors oriented along the superlattice layers and normally to them are used. For phonons localized in GaAs layers, the theoretically predicted mixing of the LO1 longitudinal modes with TO1 transverse modes in which atomic displacements occur along the normal to the superlattice is observed experimentally. These modes possess noticeable angular anisotropy. For transverse modes in which atoms move in the plane of the superlattice, the angular anisotropy is small.     

Kinetics of an irreversible transition to a normal state by a multiconductor superconducting composite

A model that takes into account the discrete character of a multiconductor superconducting medium is used to study the characteristic features of transition processes which occur when a region of normal conductivity appears. This analysis is based on numerical simulations of the propagation of a normal zone inside a superconducting region consisting of rectilinear superconductors, with a model for the kinetics of the irreversible loss of the superconducting properties of a single-ply, large radius winding. The thermophysical aspects of the irreversible propagation of a thermal perturbation within the transverse cross section of a multiconductor superconducting medium are analyzed. The major features of the phenomena taking place during the loss of superconductivity by a discrete region are formulated as functions of the conditions for heat transfer between its elements, the character of the perturbation, the magnitude of the flowing current, and the conditions for stabilization. The influence of longitudinal heat conduction on the magnitude of the transverse propagation velocity of a normal zone is examined. Zh. Tekh. Fiz. 69, 40–51 (May 1999)