On the dynamic conductivity for an electron-impurity system

The correlation function formula for the dynamic conductivity of a system of non-interacting electrons in the field of impurities is analyzed in terms of proper connected diagrams. By selecting those diagrams appropriate in the region of weak coupling and low impurity concentration, a set of coupled equations for the energy broadening γ (ω, ε, n_s) and the energy shift Δ(ω, ε, n_s) is derived, where both γ and Δ depend on the frequency ω of a probing field, the energy ε of the electron, and the concentration, n_g, of impurities. With the assumption of a finite range potential, these equations are solved. It is found that γ (ω, n_s) is smaller than that extrapolated value which the conventional expression γ₀ for the low-concentration collision frequency would predict, in the entire region studied, that the difference γ₀-γ becomes appreciable when the ratio of the average time between scatterings, τ_c, to the average duration of a scattering, τ_d, is 100 or less, that γ (ω, n_s) decreases monotonically from its static value γ (0, n_s), and becomes vanishingly small in the region ω≈1/τ_d, and that in the static limit (ω=0), γ=γ₀[1?(2/π) (γ₀τ_d)+…], that the energy shift Δ is positive, and increases from 0 and reach a peak of magnitude γ₀ as ω is raised from 0. By using the γ and Δ obtained, the dynamic conductivity σ(ω, n_s) for degenerated electrons is calculated. The deviation, σ-σ₀, from the conventional expression σ₀=(?i) (n_ee²/M) [ω-iΓ₀]^?1, (n_e]=number density of electrons), for 0°K, is appreciable when the ratio τ_c/τ_d is 100 or less. The field-term correction, which arises from the modification of the scattering due to the probing field, is found to be negligible in the entire region studied.     

Self-consistent transport equations for the electron-impurity system in a magnetic field

General transport equations for a system of independent electrons in a random array of impurities of arbitrary range and in a magnetic field of arbitrary strength are derived on the basis of conventional perturbation theory, and the general structure of the transverse transport coefficients is discussed. Contrary to kinetic theory, within this framework it is easy to formulate “conserving” approximations, which include collision-broadening effects self-consistently. It is demonstrated, how for high magnetic field explicit results for the transverse transport coefficients can be obtained, which are free of the divergencies appearing in the kinetic theory.

     

Theory of cyclotron resonance lineshape for an electron-impurity system in two dimensions

Using the proper connected diagram expansion which incorporates the quasi-particle effect in a natural manner, we calculated the cyclotron resonance width Γ and frequency shift Δ in the extreme quantum limit, (a) Δ = 0 for any form of interaction potential, (b) Γ ∝ n_I^{$\text{1}\text{2}$}B^{$\text{1}\text{2}$} for a short range interaction, and (c) $\text{Γ = π}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{ze}{}^2\text{k}{}^{\mathrm{?1}}\text{n}{}^{\mathrm{?1}}\text{n}{}_{\mathrm{I}}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ for the unscreened Coulomb interaction, are obtained in the weak coupling approximation. The result (a) and the field (B)- and concentration (n_I)-dependence in case (b) are in satisfactory agreement with experimental data on the Si space-charge layers at the surface carrier densities higher than 1 × 10¹²cm^-2 and at liquid helium temperatures, where the main source of relaxation is thought to arise from a short-range interaction.     

Tabular electrical conductivity for aluminium

A new Sesame-type table for the electrical conductivity of aluminium is described. The table is based on density functional theory calculations and ranges from 10⁻³ to 1 times solid density (2.7 g/cm³), and from 10⁻² to 10³ eV in temperature. The table is compared with other those of simulations and to experiments and is generally in good agreement. The high-temperature, classical limit of the conductivity is recovered for the highest temperatures and lowest densities. The table is critically evaluated, and directions for improvements are discussed.     

Image series expansion for electrical impedance tomography

A novel electrical impedance tomography method is introduced for reconstruction of layered biological tissues. The method utilizes a recently proposed image series expansion scheme in conjunction with the WKB approximation. This results in a locality feature, assigning analytically to each image term a local impedance associated with a unique layer, and thus leading to linear, efficient, and accurate reconstruction procedures.     

Structure, oxygen stoichiometry and electrical conductivity in the system Sr-Ce-Co-O

Mixed oxides in the system S-Ce-Co-O were prepared by solid state reaction and by freeze-drying of precursor compounds followed by thermal treatment. Two types of perovskite oxides exist in the system: Solid solutions of the type Sr_{1 − y}Ce_yCoO_{3 − x} and mixed oxides of the type (1 − y)SrCeO₃ − ySrCoO_{3 − x}. Microstructures and phase compositions were determined by electron microscopy and X-ray diffraction. SrCoO_{3 − x} forms a solid solution of ceria on the A-site in the strontium cobaltite lattice up to 0.15 mol Ce. This solid solution corresponds to the high-temperature structure of pure SrCoO_{3 − x} and is characterized by high oxygen exchange and electrical conductivity. The oxygen deficiency x was measured by solid electrolyte coulometry. The oxygen deficiency of solid solutions Sr_{1 − y}Ce_yCoO_{3 − x} increases with temperature and decreases with pO₂ in the ambient atmosphere and with increasing Ce dopant concentration. The pO₂-T-x diagram of the solid solution was determined. The T, pO₂ and dopant concentration dependencies of electrical conductivity were measured by a four-point d.c. technique. By Ce doping strontium cobaltite becomes a stabilized high-conductive material (maximum conductivity: 500 S cm⁻¹ at 400 °C, E_a = 0.025 eV, p-type). Above this temperature the T-coefficient of the conductivity changes from positive (semiconducting) to negative values.     

Interpolation formula for the electrical conductivity of nonideal plasmas

On the basis of a quantum‐statistical approach to the electrical conductivity of nonideal plasmas we derive analytical results in the classical low‐density regime, in the degenerate Born limit, and for the contribution of the Debye‐Onsager relaxation effect. These explicit results are used to construct an improved interpolation formula of the electrical conductivity valid in a wide range of temperature and density which allows to compare with available experimental data of nonideal plasmas.     

The electrical conductivity of chloride melts

The interrelationship between electrical conductivity, molar volume and enthalpy of mixing was studied for molten chlorides and their mixtures. The dependence of electrical conductivity and activation energy on the molar volume is different for various groups of salts. The dependence of specific conductivity on molar volume obtained for molten alkali chlorides was found to be similar to other chloride salts. The specific conductivity of binary mixtures that lack strong chemical interactions between the components can also be described by the proposed empirical equation. The enthalpy of mixing should be taken into consideration for these chemical interactions.     

Graphene oxide with improved electrical conductivity for supercapacitor electrodes

Predominant few-layer graphene (FLG) sheets of high electrical conductivity have been synthesized by a multi-step intercalation and reduction method. The electrical conductivity of the as-synthesized FLG is measured to be ∼3.2 × 10⁴ S m⁻¹, comparable to that of pristine graphite. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman analysis reveal that the as-synthesized FLG sheets have large areas with single and double layers. The specific capacitance of 180 F g⁻¹ is obtained for the FLG in a 1 M Na₂SO₄ aqueous electrolyte by integrating the cyclic voltammogram. The good capacitive behavior of the FLG is very promising for the application for next-generation high-performance electrochemical supercapacitors.     

Ultraviolet-laser-induced permanent electrical conductivity in polyimide

When polyimide (Kapton) is irradiated by a krypton fluoride (KrF) laser, an increase of the electrical conductivity of up to 16 orders of magnitude is observed. In the high conduction regime, the resistivity is about 0.1 cm, the current voltage characteristic is ohmic and the contacts of gold and silver with the irradiated conducting polymer are also ohmic. The conduction mechanism is phonon-assisted variable range hopping, evident from the observed temperature and electric field dependence of the resistivity at low conductivities. The laser-induced conductivity depends on the ambient atmosphere during irradiation. Transmission spectroscopy in the visible region and infrared Fourier transform spectroscopy have been used to characterize the material. A thermal mechanism is proposed for the formation of conducting polyimide, by excimer-laser irradiation.     

Static electrical conductivity of a doped semiconductor

This paper evaluates corrections to the static electrical conductivity of a nondegenerate semiconductor, doped with dopants of one type, taking into account the interaction of the charge carriers both with a random field and with lattice oscillations. The results obtained may be generated to the case of an arbitrary random field.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 16, No. 7, pp. 14–21, July, 1974.The author wishes to express his profound indebtedness to V. L. Bonch-Bruevich for proposing this topic.