Duality and effective conductivity of random two-phase flat systems

The possible functional forms of the effective conductivity σ_e of randomly inhomogeneous two-phase systems at arbitrary values of concentrations are discussed. Two explicit approximate expressions for effective conductivity are found using a duality relation, a series expansion of σ_e in the inhomogeneity parameter z, and some additional conjectures about the functional form of σ_e. They differ from the effective medium approximation, satisfy all necessary requirements, and reproduce the known formulas for σ_e in the weakly inhomogeneous case. This can also signify that σ_e of the two-phase randomly inhomogeneous systems may be a nonuniversal function, depending on some details of the structure of the random inhomogeneities.     

Percolation Thresholds and Phase Transitions in Binary Composites

The metal–insulator and metal–superconductor phase transitions related to the percolation thresholds in two-component composites are considered. The behavior of effective conductivity σ _e in the vicinity of both thresholds is described in terms of the similarity hypothesis. A one-to-one correspondence between the equations derived for σ _e in both critical regions is found for randomly heterogeneous systems.     

The influence of phonons on the optical properties and the conductivity of quasi-one-dimensional metals

The frequency dependent conductivity σ(ω) completely taking into account the interaction between electrons is studied. The shape and the temperature dependence of optical absorption near the frequency of a molecular phonon activated due to the interaction with electrons are found. For a system with attractive sign of the e-e backward scattering amplitude g₁<0 an absorption edge near the gap 2Δ in the electronic spectrum is studied. A low frequency conductivity is discussed. The properties under consideration depend essentially on the magnitudes of e-e interactions and are critical to the sign of g₁.     

Elimination of the mott interband s-d enhancement of the electrical resistance of nickel and platinum owing to the excitation of electrons by femtosecond laser pulses

The dependence of electrical, σ, and thermal, κ, conductivities of metals on the electron temperature T _e at high (～1 eV) T _e values has been calculated. The two-temperature states for which the temperature T _e of heated electrons exceeds the temperature T _i of ions in the crystal lattice result from the excitation of electrons by femtosecond laser pulses. It is well known that the existence of empty d levels with a high density of states near the Fermi surface (as, e.g., in nickel, platinum, and iron) leads to a pronounced enhancement of the electrical resistance (Mott, 1936). This is due to an increase in the statistical factor related to the electron transitions to the empty states induced by collisions with phonons. It is found that the excitation of the electron subsystem significantly reduces the electron-phonon scattering to unoccupied d states since the chemical potential μ(T _e ) rises above the upper edge of the d band. The decrease in the scattering probability leads to the anomalous behavior of the conductivity σ_el-ph, which increases with the temperature T _e . Such a behavior turns out to be inverse with respect to the usual situation in condensed matter.     

On the fermi velocity and static conductivity of epitaxial graphene

The models of the energy density of states of a metallic or semiconductor substrate, which does not further lead to divergences, have been proposed to calculate the characteristics of epitaxial graphene. The Fermi velocity of epitaxial graphene formed on a metal has been shown to be greater than that in free-standing graphene irrespective of the position of the Fermi level. On the contrary, the Fermi velocity of graphene formed on a semiconductor is lower so that the lower is the Fermi velocity, the closer is the Fermi level to the center of the band gap of the semiconductor. The zero-temperature static conductivity σ of epitaxial graphene has been calculated according to the Kubo-Greenwood formula. The quantity σ_m of undoped graphene on metal has been shown to decrease with an increase in the deviation of the Dirac point ?_D (which coincides with the Fermi level of the system) from the center of the conduction band of the substrate. In the case of the semiconductor substrate, the static conductivity σ_sc turns out to be nonzero and amounts to σ_sc = 2e ²/π?-only under the condition ?_F =?′_D, where ?′_D is the Dirac-point energy renormalized by the interaction with the substrate.     

Composition dependence of effective thermal conductivity and effective thermal diffusivity of Se100−xInx (x=0, 5, 10, 15 and 20) chalcogenide glasses

Simultaneous measurement of effective thermal conductivity (λ_e) and effective thermal diffusivity (χ_e) of twin pallets of Se_100−xIn_x (x=0,5, 10, 15, and 20) glasses, prepared under a load of 5 tons, have been made at room temperature using the Transient Plane Source (TPS) technique. The values of λ_e and χ_e were found to increase initially with the increase of concentration of In in Se-In alloy and had their maximum at 10 at.wt% of indium. For indium concentration beyond 10 at.wt% of the values of effective thermal conductivity and effective thermal diffusivity decrease linearly. This is suggestive of fact that 10 at.wt% of indium can be considered as a critical composition at which the alloy becomes, chemically ordered and maximum thermally stable than other composition. Further addition of indium in selenium decreases the values of λ_e and χ_e. The behaviour is explained on the basis of decrease of localized states and increase in disorderness for higher composition indium.     

Features of electrical conductivity in an incommensurate phase of graphite intercalation compound C10HNO3

Electrical conductivity and conduction-electron spin resonance (CESR) have been studied in stage-2 acceptor α-graphite-nitric acid intercalation compound C₁₀HNO₃. It is found that the electrical conductivity σ_c along the c axis in the structurally incommensurate phase of this compound is temperature independent, whereas the electrical conductivity σ_a along carbon layers exhibits “metallic” temperature behavior. Analysis of the temperature dependences of σ_c, σ_a, and the CESR linewidth demonstrates that, in the incommensurate phase of the graphite intercalation compound, the electrical conductivity along the c axis is realized through a nonband mechanism—the transfer of free charge carriers along thin high-conductivity channels shunting the carbon layers adjacent to the intercalate.     

Ionic conductivity of polycrystalline samples of KBrKI solid solutions

Ionic conductivity of KBr_xI_1?x(0?x?1) mixed crystals, σ_x, has been measured as a function of temperature in the range of 370°C to close to their melting points. The variation in conductivity, σ_x, with composition in the intrinsic region was found to be non-linear, having a maximum value at x=0.3. The maximum conductivity of KBr_xI_1?x mixed crystals was never far outside the range of conductivity of the component crystals. Several expressions of the relative conductivity, σ_x/σ₁ (σ₁ refers to KBr) have been suggested.     

Temperature dependence of effective thermal conductivity and effective thermal diffusivity of Se90In10 bulk chalcogenide glass

Measurement of effective thermal conductivity (λ_e) and effective thermal diffusivity (χ_e) of twin pellets of Se₉₀In₁₀ bulk chalcogenide glass has been carried out in the temperature range from 303 to 323 K and cooling from 323 to 303 K using transient plane source (TPS) technique. In the heating process variation of effective thermal conductivity (λ_e) and effective thermal diffusivity (χ_e) is observed. Both quantities are found to be maximum at 313 K, which lies in the vicinity of glass transition temperature (T_g). During the cooling process λ_e and χ_e remain same at all temperatures. Such type of behavior shows thermal hysteresis in this sample, which can be explained on the basis of structural change of the Se₉₀In₁₀ bulk chalcogenide glass.     

Effective conductivity for densely packed highly conducting cylinders

We study the effective heat conductivity λ₃ of a periodic square array of nearly touching cylinders of conductivity h, embedded in a matrix material of conductivity 1. We construct a sequence of two-point Padé approximants for the effective conductivity. As the basis for the construction we use the coefficients of the expansions of λ_e at h=1 and h=∞. The two-point Padé approximants form a sequence of rapidly converging upper and lower bounds on the effective conductivity.     

On the two definitions of nuclear surface energy and its calculation in two-component systems

It is shown that when a large nuclear system is in equilibrium there is a minimum in the Gibbs surface energy σ_μ but not generally in the droplet-model surface energy σ_e. Attempts to minimize σ_e result in the non-conservation of neutrons and protons. Nevertheless, σ_e can be calculated from the density distributions that result from the minimization of σ_μ.     

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.     

Effect of localization on the hall conductivity in the two-dimensional system in strong magnetic fields

Some exact properties of the Hall conductivity in the two-dimensional electron system in strong magnetic fields are investigated in the presence of immobile carriers. It is shown that σ_xy should be exactly the integral multiple of — $\text{e}{}^{\mathrm{<mtext>2</mtext>}}\text{h}$ when the Fermi energy lies in the localized regime.     

Commensurability oscillations by runaway and pinned electrons

We report on an investigation of commensurability oscillations in antidot square-lattices, and show that the commensurate peaks in resistivity ρ_xxderive from two kinds of electron: runaway and quasipinned electrons. The runaway electrons, which skip away along the antidot arrays, increase the value of conductivity σ_xx; and the quasipinned electrons, which orbit some antidots for a long time, decrease the value of σ_xx. Therefore, by competition between the two different types of electron, the conductivity σ_xxis determined. The conductivity σ_yxin the antidot lattice always has dips at the peaks in ρ_xx. As a result, by combining the of values of σ_xxand σ_yx, the resistivity is determined through the orthodox relation of ρ_xx =  σ_xx/ (σ_xx²  +   σ_yx²).     

Hot electron transverse conductivity in quantizing magnetic fields

In this work the general expression of the electron transverse conductivity tensor of an electron-phonon system being in crossed strong electric and quantizing magnetic fields is considered starting from the Kubo-Kalashnikov formula. An explicit formula for the hot electron transverse conductivity σ _xx is obtained and it is compared to a Titeica-type formula with the temperature of electrons replaced by an effective electron temperature depending on the electric field.     

Effects of Pauli paramagnetism on superconducting fluctuations

We discuss the effects of the Pauli paramagnetism on the excess conductivity σ^fl due to fluctuations of the superconducting order parameter. We derived a formula for σ^fl for a thin film placed in a magnetic field of an arbitrary orientation α. It was found that σ^fl has a universal behavior as a function of some parameterp which depends on α and ΔH=H-H _c. If ΔH is kept constant and σ^fl is measured as a function of α, in the absence of the Pauli paramagnetism σ^fl is maximum when the field is parallel to the film and is minimum when the field is perpendicular. But in high field superconductors due to the effect of the Pauli paramagnetism σ^fl becomes maximum at some intermediate field orientation. We also discussed the excess conductivity in magnetic alloys in which impurity spins are aligned by an external magnetic field. It was shown that in this case one should expect, with certain strengths of the external field, the excess conductivity which is non-monotonic in temperature.     

Electric conductivity of inhomogeneous two-component media in two dimensions

The electric conductivity is calculated for regular inhomogeneous two-component isotropic medium in which droplets of one phase with conductivity σ₂ are embedded in another, with conductivity σ₁. An expression is formulated that can be used in many different situations and is of particular relevance in the case where the relative proportion of the components is temperature-dependent and varies over a wide range. Behavior of the effective conductivity depends on the spatial arrangements and the shape of the inclusions.     

Thermal transport in chemically doped polyaniline materials

Simultaneous measurements of effective thermal conductivity (λ_e) and effective thermal diffusivity (χ_e) of four samples S1 (polyaniline mixed with 100% Ni), S2 (polyaniline mixed with 90% Ni and 10% Al), S3 (polyaniline mixed with 80% Ni and 20% Al) and S4 (polyaniline mixed with 70% Ni and 30% Al) of polyaniline mix with metal Ni and Al in different percentage have been made using transient plane source technique. In the temperature range from room temperature to 170 °C. Both λ_e and χ_e increase with increase in temperature. Addition of aluminium concentration at the cost of nickel in the composite increases the value of λ_e and χ_e over the entire range of temperature under investigation. It has been found that the values of λ_e and χ_e are maximum when 70% of Ni and 30% of Al are mixed in polyaniline matrix. This behaviour is also predicted by an empirical relation, which is obtained from polynomial fit of the experimental data and is explained on the basis of bond formation between metal (Ni and Al) and nitrogen of polyaniline matrix.     

Current-conducting properties of paper consisting of multiwall carbon nanotubes

Electrical conductivity σ(T) of the paper consisting of multiwalled carbon nanotubes (MWCNTs) is studied in the temperature range 4.2-295 K, and its magnetoresistivity ρ(B) at various temperatures in magnetic fields up to 9 T is analyzed. The temperature dependence of the paper electrical conductivity σ(T) exhibits two-dimensional quantum corrections to the conductivity below 10 K. The dependences of negative magnetoresistivity ρ(B) measured at various temperatures are used to estimate the wavefunction phase breakdown length L _φ of conduction electrons and to obtain the temperature dependence L _φ = constT ^?p/2, where p ≈ 1/3. Similar dependences of electrical conductivity σ(T), magnetoresistivity ρ(B), and phase breakdown length L _φ(T) are detected for the initial MWCNTs used to prepare the paper.