High frequency properties of a quasi-two-dimensional conductive film

The propagation of a monochromatic longitudinal acoustic wave along the low conductivity axis of a thin quasi-two-dimensional conductive film, in the absence of an external magnetic field, is studied theoretically. It is shown that under certain conditions the formation of both a standing ordinary wave (OAW) and an anomalous acoustic wave (AAW) is possible. The frequency dependence of the amplitudes of both waves is derived. For certain values of the characteristic parameters, the AAW in the film may be dominant. From the resonance conditions for the formation of standing OAW and AAW waves (especially the AAW), it is possible to obtain information about the electronic structure of the quasi-two-dimensional conductors, e.g. the corrugation parameter η or the relaxation properties of the charge carriers.     

Anisotropy of impurity scattering and electrical conductivity in quasi-two-dimensional electronic systems

Scattering of charge carriers by impurity ions in quasi-two-dimensional electronic systems is investigated. The dependences of the longitudinal and transverse components of the relaxation time on the wave vector components and the screening length are analyzed. The anisotropy of electrical conductivity of a degenerate quasi-two-dimensional electron gas is studied as a function of the ratio between the energy at the Fermi level and the miniband width, the impurity concentration, and the lattice constant.     

Simple formulas for the oscillating component of the electrical conductivity of layered chargeordered crystals

In the present paper, oscillations of the longitudinal component of the electrical conductivity of layered crystals are examined in electric and quantizing magnetic fields perpendicular to the layers. It is demonstrated that frequencies and amplitudes of longitudinal conductivity oscillations can be determined with sufficient accuracy through the chemical potential of the electron gas and effective width of the miniband caused by the charge ordering. In addition, based on an analysis of formulas for the transverse conductivity, it is established that the applicability limits for the transverse conductivity in the semiclassical approximation (for the magnetic field induction) in the field perpendicular to the layers are much wider than for the longitudinal conductivity. An immediate reason for this is the zero longitudinal velocity of current carriers in the extreme cross sections, which leads to the field dependence of the amplitudes of longitudinal conductivity oscillations stronger than of transverse ones. Calculated results are used to interpret experimental data obtained for the β-(ET)2IBr2 synthetic metals. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 34–43, May, 2006.     

Influence of topological transitions in a quantizing magnetic field and anisotropy of current carrier scattering by acoustic phonons on the longitudinal electrical conductivity of layered crystals with open fermi surfaces

It is demonstrated that the dependence of Fermi’s energy on the magnetic field causes a set of the Shubnikov – de Haas (SDH) oscillation frequencies to change, and their relative contribution to the total longitudinal conductivity of layered crystals depends on whether the scattering of current carriers is isotropic or anisotropic. Owing to the topological transition in a strong magnetic field, Fermi’s surface (FS) is transformed from open into closed one and is compressed in the magnetic field direction. Therefore, in an ultraquantum limit, disregarding the Dingle factor, the longitudinal electrical conductivity of the layered crystal tends to zero as a reciprocal square of the magnetic field for the isotropic scattering and as a reciprocal cube of the magnetic field for the anisotropic scattering. All calculations are performed in the approximation of relaxation time considered to be constant versus the quantum numbers for the isotropic scattering and proportional to the longitudinal velocity of current carriers for the anisotropic scattering.     

Stability of Self-Gravitating Magnetized Hall Plasma with Electrical and Thermal Conductivity through Porous Medium

The problem of stability of a self-gravitating, infinite homogeneous gas in the presence of magnetic field is investigated. The medium is assumed electrically and thermally conducting. The effect of porosity, electrical conductivity, thermal conductivity and Hall current is investigated on the self-gravitating plasma flowing through porous medium. The relevant linearized equations of the problem are stated and dispersion relation is obtained. The effect of Hall current on the condition of the instability of the system is examined for both longitudinal and transverse mode of propagation and found that in longitudinal propagation Hall effect does not change the condition of instability but modifies the Alfvén wave mode. The stability of the system is discussed by applying Routh-Hurwitz criterion and it is found that Jeans criterion determines the stability of the system. Thermal conductivity and porosity have a destabilizing influence on the medium. The general condition for instability of the system is also derived.     

Dispersive effects and correction term in two-mode phonon conduction model for Ge

A complete analysis of the phonon conductivity κ, still lacking in the literature, is presented in the two-mode conduction model for Germanium. First a method is derived from which the correction term κ_c of the Callaway model is separated into its longitudinal and transverse parts and then the effects of strong phonon dispersion and the role of longitudinal and transverse phonons on κ_c are studied. For this purpose we have also proposed some new empirical expressions for the three phonon relaxation rates τ_3ph⁻¹'s which are valid in the entire temperature range. This improvised model, when applied simultaneously to the phonon conductivity data of both normal and enriched Ge, yields some new results. These are (i) κ_c neglected by the earlier workers in the two-mode phonon conduction model, gives a substantial contribution beyond the conductivity maximum and (ii) the longitudinal phonons are the major carriers of heat at high temperature.     

碲化铋薄膜载能粒子相互作用的超快研究

碲化铋禁带宽度非常窄而具有高电导率和塞贝克系数,同时具有低热导率,成为已知室温下优值系数最高的热电材料。已有研究表明,纳米薄膜和超晶格是进一步提高材料热电性能的可行途径。因此超快研究碲化铋纳米薄膜中载能子间的相互作用过程对开发高性能热电材料有重要意义。本文采用飞秒激光泵浦-探测技术,实验研究了沉积在硅基底上厚度为100 nm碲化铋薄膜中各载能粒子的相互作用过程。通过改变延迟时间步长,分别观察到价带电子被光子激发跃迁至导带,激发电子在导带内与声子的能量弛豫及导带电子与空穴复合跃迁至价带,并将能量传递给声子导致声子温度升高的过程。此外,还观察到热应力产生的声波,并据此得到了碲化铋薄膜中纵波声速为2649 m s^-1。     

Coherent tunneling between elementary conducting layers in the NbSe3 charge-density-wave conductor

Characteristic features of transverse transport along the a* axis in the NbSe₃ charge-density-wave conductor are studied. At low temperatures, the I–V characteristics of both layered structures and NbSe₃-NbSe₃ point contacts exhibit a strong peak of dynamic conductivity at zero bias voltage. In addition, the I–V characteristics of layered structures exhibit a series of peaks that occur at voltages equal to multiples of the double Peierls gap. The conductivity behavior observed in the experiment resembles that reported for the interlayer tunneling in Bi-2212 high-T _c superconductors. The conductivity peak at zero bias is explained using the model of almost coherent interlayer tunneling of the charge carriers that are not condensed in the charge density wave.     

Second harmonic wave generation from Joule heating in layered organic conductors

The amplitude of a second harmonic wave (SHW) generated from Joule heating as a heat source in organic conductor β-(BEDT-TTF)₂IBr₂ is analyzed as a function of the magnetic field strength and its orientation with respect to the plane of the layers. Angular oscillations of the SHW amplitude are correlated with the angular changes of in-plane conductivity that arise from the periodic dependence of charge carriers velocity on the field orientation. It was found that the nonlinear effect of wave generation leads to a shift between the position of the peaks of the wave amplitude and in-plane conductivity. This allows an important information on the parameter values of organic conductors as well as wave velocity to be obtained. Magnetic field dependence shows that the wave is not strongly attenuated with increasing field and might give insights on the interactions between the electromagnetic, temperature and acoustic oscillations. We found that these observations are completely different compared to those of linear acoustic wave generation. It has been shown that the necessary conditions for observing the nonlinear acoustic wave generation are fulfilled in a wide range of fields and angles that allow the acoustic properties of organic conductors to be studied in detail.     

Gap opening in the zeroth Landau level in gapped graphene: pseudo-Zeeman splitting in an angular magnetic field

We present a theoretical study of gap opening in the zeroth Landau level in gapped graphene as a result of pseudo-Zeeman interaction. The applied magnetic field couples with the valley pseudospin degree of freedom of the charge carriers leading to the pseudo-Zeeman interaction. To investigate its role in transport at the charge neutrality point (CNP), we study the integer quantum Hall effect in gapped graphene in an angular magnetic field in the presence of pseudo-Zeeman interaction. Analytical expressions are derived for the Hall conductivity using the Kubo-Greenwood formula. We also determine the longitudinal conductivity for elastic impurity scattering in the first Born approximation. We show that pseudo-Zeeman splitting leads to a minimum in the collisional conductivity at high magnetic fields and a zero plateau in the Hall conductivity. Evidence for activated transport at CNP is found from the temperature dependence of the collisional conductivity.     

Radioelectric effect in a superlattice under the action of an elliptically polarized electromagnetic wave

The density of the current associated with the drag of charge carriers in a superlattice by an elliptically polarized electromagnetic wave is calculated. Two cases of the mutual orientation of the Umov-Poynting vector and the superlattice axis, i.e., their parallel and perpendicular orientations, are analyzed. It is shown that, for the parallel orientation, the radioelectric effect can change sign. The influence of the longitudinal dc electric field on the radioelectric effect is investigated under the conditions where a circularly polarized electromagnetic wave propagates along the superlattice axis. The current density is studied as a function of the electric field strength and the electromagnetic wave intensity. It is demonstrated that the direction of the electric current is changed at specific values of the dc field strength and the wave intensity.     

The role of the longitudinal component of the electric field vector in two-wave mixing in photorefractive BaTiO3

It is shown that for waves incident with a polarisation in the plane of the grating the component of the electric field in the direction of propagation must be included in the calculations. Coupled differential equations for both the transverse and longitudinal components are derived for wave interaction in photorefractive BaTiO₃. Analytic solutions for the electric field of the signal wave are obtained under the assumption that the pump wave is undepleted. For the parameters considered the longitudinal component is shown to be negligible.     

A composite theoretical model for the thermal conductivity of nanocrystalline materials

In order to study the thermal conductivity of nanocrystalline (NC) materials, a two-phase composite model consisting of grain interior (GI) regarded as an ordered crystal phase and plastically softer grain boundary-affected zone (GBAZ) phase was presented. The effects of GI and GBAZ on thermal conduction were considered, respectively. In this work, time independent Schrodinger’s wave equation (TISWE) was used to study the carriers’ transmission in a crystal particle, through which we can get the thermal conductivity of the GBAZ. The thermal conductivity of GI was calculated based on a kinetic theory. The whole effective grain thermal conductivity was simulated by a modified formula for composite materials. The results showed that as the grain size decreases to 80 nm, it has a strong size effect, and the thermal conductivity decreases with the decreasing of grain size.     

Gravitational Instability of a Rotating Viscous Thermally Conducting Plasma

This paper deals with the gravitational instability of an infinite homogeneous viscous rotating plasma of finite electrical conductivity in the combined presence of effects of Hall currents, finite Larmor radius (FLR) and thermal conductivity. The ambient magnetic field is assumed to be uniform and acting along the vertical direction. Both longitudinal and transverse modes of wave propagation have been studied. It is shown that Jean's criterion determines the gravitational instability even in the presence of the effects of thermal conductivity, viscosity, finite electrical conductivity, FLR, rotation and Hall currents. Further it is found that while FLR, viscosity and rotation have a stabilizing influence, both the thermal and the electrical conductivities have a destabilizing influence on the gravitational instability of a plasma.     

Solutions of the Maxwell equations and photon wave functions

Properties of six-component electromagnetic field solutions of a matrix form of the Maxwell equations, analogous to the four-component solutions of the Dirac equation, are described. It is shown that the six-component equation, including sources, is invariant under Lorentz transformations. Complete sets of eigenfunctions of the Hamiltonian for the electromagnetic fields, which may be interpreted as photon wave functions, are given both for plane waves and for angular-momentum eigenstates. Rotationally invariant projection operators are used to identify transverse or longitudinal electric and magnetic fields. For plane waves, the velocity transformed transverse wave functions are also transverse, and the velocity transformed longitudinal wave functions include both longitudinal and transverse components. A suitable sum over these eigenfunctions provides a Green function for the matrix Maxwell equation, which can be expressed in the same covariant form as the Green function for the Dirac equation. Radiation from a dipole source and from a Dirac atomic transition current are calculated to illustrate applications of the Maxwell Green function.     

Magnetic semiconductors

As far as the electrical conductivity is concerned, solids are usually classified as metals, semiconductors, or insulators. In metals the concentration of the charge carriers responsible for the electrical conductivity is large, whereas in semiconductors and insulators the carrier concentration is much smaller. The distinction between semiconductors and insulators is based on a difference in the nature of the conductivity. For semiconductors the charge carriers (electrons or holes) occupy the states of energy bands; these states are not Iocalized on particular atoms, but spread throughout the entire crystal. In such a situation the mobility of the carriers can be quite high and would, in fact, be infinite in a rigid periodic lattice; in this model the thermal motion of the atoms induces a scattering of the carriers and thus limits the conductivity to finite values. The classical examples of semiconductors are the elements Si and Ge and compounds such as GaAs, InSb, CdTe, ZnS, etc.     

The electric conductivity of a laminated metal system (alternating magnetic and nonmagnetic layers)

A set of kinetic equations for the distribution functions of carriers differing both by the energy spectrum and by the spin projection is used to investigate the conductivity of a multilayer sample (alternating layers of magnetic (m) and nonmagnetic (_n) metals). The boundary conditions on the interlayer surfaces are derived in an approximation in which the surface scattering is divided into “specular” and “diffuse” scattering and is characterized by scattering parameters (reflection and transmission) which are related to each other by relations dependent on spin projections and on the type of spectrum. The problem on the longitudinal (with respect to the layers) current is treated; situations are analyzed in which the variation in conductivity due to the change of mutual orientation of magnetization in successive m layers from antiparallel to parallel may be of the order of the values of the conductivity proper (the so-called giant magnetoresistance effect). This is possible only in the case of thin (compared with the free path) n layers (in m layers, the ratios of the characteristic dimensions may be arbitrary) and in the mandatory presence of specular surface scattering. Results are given for different possible ratios of Fermi momenta of electron groups and for different fractions of specular and diffuse scattering. The possibility of realizing the effects of both signs is demonstrated.     

Kinetic study of instability growth rate in a helicon plasma discharge source

In this work, a kinetic model is developed to study the effects of the radio frequency antenna wavenumber, helicon plasma electron density, as well as their drift velocity and temperature on the instability increment rate of the helicon wave in both longitudinal and transverse directions. The ion acoustic (IA) wave frequencies and wavenumbers of the helicon waves are obtained when the maximum wave energy is deposited on the plasma ions. Moreover, it is shown that, at the IA wavenumber and frequencies, while the longitudinal instability increment rates for both the helicon and IA waves are ignorable, the transverse instability increment rate for both the helicon and IA wave increases. Besides, the longitudinal instability increment rate for the helicon or IA wave has non‐zero resonant frequencies. On the other hand, the transverse instability increment rate of helicon or IA wave can be neglected. Furthermore, it is observed that, while both the imaginary part of longitudinal permittivity and longitudinal instability increment rate are not influenced by the electron temperature, their transverse component increases linearly with the electron temperature. In addition, the imaginary part of transverse permittivity increases almost linearly with the drift velocity of the plasma electrons.     

Surface effects on optical phonons and on phonon-plasmon modes

The size and shape dependence of the properties of long wave optical phonons in polar crystals is discussed. The main effect occurring due to the presence of surfaces is the appearance of a surface optical phonon band in the frequency region between the frequencies ω_t and ω_l of the long wave transverse and longitudinal bulk modes. The surface modes give rise to strong absorption peaks in the infrared absorption spectra of small samples. For very small crystallites the surface mode absorption is dominant, and as the size of the specimens is increased, the ratio of surface to bulk mode absorption decreases. It is shown that the large spread in particle sizes usually encountered in experimental work and the increased damping of the phonon modes in small samples both tend to obscure the fine structure of the absorption spectrum. The surface phonon-plasmon modes in polar crystals containing free carriers are treated in an analogous manner.