On the theory of galvanomagnetic properties of composites

The conductivity of composites in the presence of a magnetic field H is considered. The galvanomagnetic characteristics for a weakly inhomogeneous medium are determined in explicit form in an approximation quadratic in the deviations of conductivity tensor $\hat \sigma $ (r) from its mean value 〈 $\hat \sigma $ 〉. The contribution to the effective conductivity tensor $\hat \sigma _e $ linear in concentration c of inclusions for a composite with a small value of c is expressed in terms of the dipole polarizability of an individual inclusion, which is defined in the transformed system in which it is surrounded by an isotropic matrix with a scalar conductivity. Transition to this system is performed using a symmetry transformation that does not change the dc equations. An approximate approach proposed for describing the galvanomagnetic properties of composites in the wide range of parameters appearing in the problem generalizes the standard theory of an effective medium to the case of anisotropic systems with inclusions of arbitrary shape in field H ≠ 0.     

Non-ohmic properties and electric breakdown of quasi one-dimensional organic conductors

The dependence of the d.c. conductivity of single crystals of TTF-TCNQ on the electric field strength has been investigated. At 4.2 K drastic deviations from Ohm's law are observed. At an electric field strength of 300–600 V/cm, dependent on the room temperature conductivity, a reversible breakdown occurs connected with a rise of the conductivity by about three orders of magnitude. Similar effects have been found in single crystals of (TTF)₇J₅ and (TTF)J₂ in the temperature range around 100 K. Possible mechanisms responsible for this large conductivity changes are discussed.     

Beweglichkeitsanisotropie und Relaxation warmer Elektronen inn-Typ Germanium

The nonohmic part of the electric conductivity ofn-type germanium in a weak electric field depends on the field direction in the crystal. Measurements were made both with d.c. and microwave fields at lattice temperatures between 85 and 273° K. The anisotropy decreases with increasing field frequency and temperature. The data show that the effect of the effective mass anisotropy is enhanced by intervalley scattering. From the measured frequency dependence of the conductivity anisotropy the intervalley relaxation time is calculated and compared with results obtained from the acoustoelectric effect. In 〈001〉-direction where all 〈111〉-valleys of the conduction band are equally populated, the conductivity between 200 and 273° K is in accordance with theoretical results obtained byAdawi for an isotropic model; at lower temperature there are deviations even if ionized impurity scattering is included in the theory. The energy relaxation time is calculated from the measured frequency dependence of the 〈001〉-conductivity and compared with previous results.     

Thermophoresis of touching solid spheres in the direction along the line joining their centers

A theory of the steady motion of an aggregate of two touching solid, nonvolatile, low-thermalconductivity, spherical particles in the direction along the line joining their centers in a nonuniformly heated viscous gas is constructed in a hydrodynamic regime with slipping at low Reynolds and Peclet numbers. The thermophoretic transport velocity of an aggregate is determined in an approximation linear in the small parameters. The small parameters are the relative deviations of the thermal conductivity of the constituent particles of an aggregate from the thermal conductivity of the external medium. Zh. Tekh. Fiz. 68, 25–31 (June 1998)     

Transport coefficients in superionic α-AgI

A phenomenological theory is given for the time dependence of the relaxation process of an α-AgI sample which has been previously polarized by the application of a voltage. The electromotive force caused by the internal electric field due to the polarized charge distribution is given by a series of exponential terms as a function of Dt, with D being the effective diffusion constant and t the time. The diffusion constants are obtained at several temperatures. In addition, the AC conductivity was measured for the same specimen. The results are analyzed with the help of a generalized Drude theory. Using both the results of the diffusion constant D and the AC conductivity, deviations from the Nernst-Einstein's relation are derived. Furthermore, computer simulations on these quantities were carried out by using well-established pairwise potentials. The agreement is satisfactory. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997.     

Interlayer quasiparticle transport in the vortex state of josephson coupled superconductors

We calculate the dependence of the interlayer quasiparticle conductivity, sigma(q), in a Josephson coupled d-wave superconductor on the magnetic field B parallelc and the temperature T. We consider a clean superconductor with resonant impurity scattering and a dominant coherent interlayer tunneling. When pancake vortices in adjacent layers are weakly correlated, at low T the conductivity increases sharply with B over a field scale determined by the impurity scattering, before reaching an extended region of slow linear growth. At high T the conductivity initially decreases and then reaches the same linear regime. For correlated pancakes, sigma(q) increases much more strongly with the applied field.     

Thermal conductivity of superconducting lead-indium alloys near the upper critical field

The thermal conductivity of a series of lead/indium alloys was measured in the normal, superconducting and mixed states. The electronic part of the thermal conductivity near the upper critical field was compared with the theory of Caroli and Cyrot. There is good agreement in the dirty limit but large deviations are observed as the indium concentration is reduced.     

Phenomenological model of nonlinear dielectric losses related to the intrinsic conductivity of dielectrics

A phenomenological approach was applied to the dynamics of the intrinsic electric conductivity of dielectrics in an electric field in order to analyze its contribution to dielectric losses. The proposed differential equation contains only two parameters - the effective time of conductivity decay in an electric field, and , the time describing the recovery rate of the conductivity after switching off the field. The proposed approach predicts a linear dependence of specific conductivity of dielectrics on the sample thickness, as experimentally confirmed by Du Pont [1] for Teflon FEP. The field and time (or frequency) dependences of intrinsic conductivity and related dependences of dielectric losses were calculated, analyzed and illustrated with the published experimental data. The results show that the discussed contribution in dielectric losses is characterized by two hyperbolas (instead of one) while the distance between the branches depends on the intensity of applied electric field.Received: 3 November 2003, Published online: 29 June 2004PACS: 72.20.Ht High-field and nonlinear effects - 72.80.Sk Insulators - 77.22.Gm Dielectric loss and relaxation     

The effect of a magnetic field on the crossover from the linear to quadratic frequency dependence of phononless hopping conductivity in disordered systems

The effect of an external magnetic field on the frequency dependence of phononless hopping conductivity in disordered system is studied. The dependence of the low-temperature phononless conductivity on the applied magnetic field is established. It is shown that in the limit of a strong magnetic field, the frequency of the transition (crossover) from linear to quadratic frequency dependence of phononless conductivity grows logarithmically with the value of a magnetic field.     

The influence of impurities on the quasiparticle tunneling current between weakly coupled quasi-one-dimensional charge density waves

The dc and ac conductivity of a tunneling junction between two impure quasi-one-dimensional charge density wave (CDW) systems is calculated. The non-magnetic impurities are considered in the self-consistent Born approximation (SCBA). Impurities modify the density of states (DOS) of the pure CDW system for quasiparticles inside the energy region of the gap 2(T). As in the pure case, the theory predicts in addition to a tunneling current which is proportional to the product of the DOS a term proportional to the cosine of the order parameter phase difference. In the case of a normal state/CDW junction, analytical expressions are obtained forT=0 showing deviations from the pure case. The linear ac conductivity is obtained by the scaling relation between the dc and the ac response.     

Nonlinear electronic transport in TTF-TCNQ observed by microwave harmonic mixing

The field dependent electronic conductivity of single crystals of TTF-TCNQ has been studied in both the metallic and the semiconducting range by means of microwave harmonic mixing, a method which excludes a contribution to the non-linearity by lattice heating. Because of the small strength of the applied field, the deviations from Ohm's law are also small such that they are proportional to the square of the field strength. The results in the semiconducting range are explained by the assumption of pinned charge density waves which are depinned by means of the microwave electric field.     

Conductivity of a “colored” plane

The conductivity of a “colored” plane, i.e., a plane divided into domains differing in conductivity, is calculated. The exact relation between the effective conductivities of the cited and dual (with inverse conductivities) systems is derived for the isotropic case (i.e., the effective conductivity tensor is proportional to the unit matrix). The conductivity of two-colored systems such as a “chessboard” or triangular lattice is exactly calculated to give σ=(σ₁σ₂)^1/2. The particular case of a “hexagon, ” as well as the duality relations for anisotropic systems and for a system in a magnetic field are discussed.     

Effective electrical and thermal conductivity of multifilament twisted superconductors

The effective electrical and thermal conductivity of composite wire with twisted superconducting filaments embedded into normal metal matrix is calculated using the extension of Bruggeman method. The resistive conductivity of superconducting filaments is described in terms of symmetric tensor, whereas the conductivity of a matrix is assumed to be isotropic and homogeneous. The dependence of the resistive electrical conductivity of superconducting filaments on temperature, magnetic field, and current density is implied to be parametric. The resulting effective conductivity tensor proved to be non-diagonal and symmetric. The non-diagonal transverse–longitudinal components of effective electrical conductivity tensor are responsible for the redistribution of current between filaments. In the limits of high and low electrical conductivity of filaments the transverse effective conductivity tends to that of obtained previously by Carr. The effective thermal conductivity of composite wires is non-diagonal and radius-dependent even for the isotropic and homogeneous thermal conductivities of matrix and filaments.     

Electrical properties and conduction mechanism of [N(C2H5)4][N(CH3)4]CuCl4 compound

The [N(CH₃)₄][N(C₂H₅)₄]CuCl₄ single crystal has been synthetized in order to determinate the temperatures transition and to study the electrical properties and the conduction mechanism. At room temperature, this compound crystallizes in the tetragonal system with P-42₁m space group. The calorimetric study shows three anomalies at 248, 284 and 326 K. Electrical conduction and dielectrical relaxation mechanisms at various frequencies and temperatures were analyzed by impedance spectroscopy and the equivalent circuit based on the Z-View-software was proposed. The variation of f_p relaxation determinate by the modulus study and σ_dc specific to the AC conductivity as a function of temperature and confirm the all transitions for our sample. The values of the activation energy are determined and compared by those, which are found in the similar compound. Frequencies dependence of alternative current (AC) conductivity is interpreted in terms of Jonscher's law and the conduction mechanisms for each phase are determined with the Elliot's theory.     

The masses of charged leptons and quarks from superposition self-interference of their Dirac fields

Without Higgs field interaction, accurate pole mass values are obtained for the charged leptons and quarks from a Z₃-symmetric linear superposition self-interference of the Dirac fields in the effective free-field Lagrangian. The charged lepton and quark pole masses evidence the discrete Z₃ symmetry, the theoretical-experimental deviations δm/m are $ \mathcal{O} $ \mathcal{O} (10⁻⁵) for all three charged leptons, and the quark pole masses are in very satisfactory overall agreement with the experimental data.     

Observation of the Photoinduced Conductivity in a Regular Domain Structure with Tilted Walls in MgO:LiNbO3 at a Wavelength of 632.8 nm at Bragg Diffraction

A 632.8-nm radiation-induced change in the conductivity of a regular domain structure (RDS) formed in a 5% MgO:LiNbO₃ crystal has been detected for the first time. As a result, the relaxation rate for the Bragg diffraction efficiency on the RDS, which is observed after the application of an external electric field, increases with the intensity of a probe beam. This dependence is linear in the initial stage of relaxation caused by the screening of the external field because of the redistribution of charges over tilted conductive domain walls of the RDS. For the probe beam with an intensity of 49 mW/mm², the induced effective conductivity of the RDS, which is estimated as σ_eff = 3.5×10⁻⁹Ω⁻¹m⁻¹, is more than four orders of magnitude higher than the dark conductivity of the single-domain MgO:LiNbO₃ sample.

     

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.     

The conductivity of the two component system of the chessboard type

The effective conductivity of the sample with a chessboard structure is found. The corner points are singular for the electric field and charge density. The effective conductivity, charge density and electric potential are expressed in terms of a Weierstrass elliptic function. The results are valid for films and for cylindrical samples with the same cross-section structure. We confirm the conjecture of Dykhne about the value of the effective conductivity for the chessboard structure.     

Charge density wave transition and nonlinear conductivity in NbS3

We report dc conductivity (σ) measurements on the linear chain compound NbS₃. The temperature dependence of σ indicates a phase trànsition at T_MI = 155 K with strong one-dimensional fluctuations above T_MI. Below T_MI the conductivity is strongly increasing with increasing electric field above a threshold field E_T, and is also strongly frequency dependent. We argue that the nonlinear conductivity is due to sliding charge density waves.     

Determination of thiophanate-methyl using UV absorption spectra based on multiple linear regression

As one worldwide used agent for fungi control in the fruit and vegetable field, thiophanate-methyl has been constituted a significant health risk. To determinate thiophanate-methyl, the method using UV absorption spectra based on multiple liner regression is used. Principal absorption wavelengths related closely with thiophanate-methyl ethanol solution are confirmed through analyzing the absorption spectra of thiophanate-methyl. Utilizing multiple linear regression, mathematical model between absorbance obtained from fourteen absorption wavelengths and thiophanate-methyl concentration is established. The result shows concentrations between the predicted values and measured values are well coincident and the related coefficient is 0.9853 over the concentration range 2–35 ppm.