The purpose of this paper is to explore the magnetoconductance (MC) effect in an organic semiconductor. By taking into account the external magnetic field and the hyperfine interaction, we determine the magnetoconductance (MC) from a numerical solution by using a steady-state rate equation for correlated electrons in Gaussian disordered systems. The ingredient of this model is that at thermal equilibrium, the spins of the polarons relax in the same direction as the local magnetic fields. We have investigated the MC dependence of the external magnetic field, the hyperfine interaction, the carrier densities, the intra-site coulomb energy, and the applied electrical field. We show that the interplay between the Hubbard energy and the charge-carrier density leads to positive and negative magnetoconductance. The theoretical calculations are in qualitative agreement with the experimental results.

From the numerical solution of the master equation for hopping transport in disordered materials, we determine the effect of the density of states (DOS) on the dependence of the charge carrier mobility on the carrier concentration, on the electric field, and on the temperature. We consider the three principal DOS: Gaussian, exponential, and constant. These DOS lead to the same dependence over a large range of the concentration, temperature, or applied electric field. We find that there is no single DOS shape which can capture the full physical picture. Moreover we find that the phenomenon of negative differential resistance observed in disordered organic materials and amorphous semiconductors, seems to be an inherent signature of a constant DOS. Furthermore, we show that the Poole-Frenkel behavior is a common feature for the three densities of states, i.e., a common feature for all disordered systems.

The isotope coefficient α has been studied in the context when the Fermi level is close to the van Hove singularity. In this approach, we consider the interaction is related to the magnetic excitations at high temperature. An analytical expression for the isotope effect is obtained in the weak coupling limit. Our results show that when the superconducting transition temperature increases the isotope coefficient decreases in qualitative agreement with experimental results.

Recently the change in temperature dependence of conductivity observed in polythiophene field-effect transistors has been attributed to field-induced metal-insulator transition. Under a high source-drain voltage, the conductivity becomes independent from temperature and depends exponentially on the inverse of the square root of electric field. In this paper we present a theoretical interpretation of this behavior in the framework of hopping mechanism. The ingredient of this model, is the energy gained from the electrical field is always larger than the energy difference between the localized states involved into the charge transitions, which induces a crossover from phonon-assisted hopping to field-assisted hopping.

Analytical expressions for the superconducting transition temperature Tc and the isotope coefficient α have been obtained for the case where the Fermi level is close to the van Hove singularity. In this approach, we consider two interactions, the first related to the phonons and the second relevant to the magnetic excitations. Our result shows that the isotope coefficient α decreases with the superconducting transition temperature Tc in qualitative agreement with experimental data.

The superconducting gap ratio R = 2Δ(0)/kBTc has been studied in the context when the Fermi level is close to the van Hove singularity. In this approach, we consider the interaction is related to the phonons at low temperature but at high temperature, it is relevant to the magnetic excitations. Our results show that the superconducting gap ratio R takes large values in qualitative agreement with experimental data.

Recently the change in temperature dependence of conductivity observed in polymer memory device prepared from polythiophene, has been attributed to a field-induced transition from a charge-injection-limited current to a trap controlled current. Under a high electrical field, the conductivity becomes independent on temperature and depends exponentially on the inverse of the applied field. In this paper we present a theoretical interpretation of this behavior in the framework of hopping mechanism, where the physical feature is the field effect on the tunneling probability.

In this paper, we study the structural, elastic and electronic properties of perovskite LaAlO3 using two different methods: the full-potential linearized augmented plane wave (FP-LAPW) method and the pseudopotential plane wave (PP-PW) scheme in the frame of generalized gradient approximation (GGA). We have evaluated the ground-state quantities such as lattice parameter, bulk modulus and its pressure derivative as well as the elastic constants. Also, we have presented the results of the band structure, densities of states and charge densities. These results were in favourable agreement with previous theoretical works and the existing experimental data. To complete the fundamental characteristics of this compound we have analyzed the thermodynamic properties using the quasi-harmonic Debye model.

The full-potential linearized augmented plane waves (FP-LAPW) method based the on density functional theory (DFT) using the generalized gradient approximation (GGA) is applied to study the structural, mechanical, and electronic properties of BaTiO3, BaZrO3, and BaNbO3 cubic perovskites. The quasi-harmonic Debye model, by means of total energy versus volume calculations obtained with the FP-LAPW method, is applied to study the thermal and vibrational effects. Predicted temperature and pressure effects on the structural parameters, thermal expansions, heat capacities, and Debye temperatures are determined from the non-equilibrium Gibbs functions.

We present a model to describe electrical transport in disordered materials. Solving the rate equation by the Green function technique and taking into account the electric field effect on the effective dimension of the transport path, we obtain a different law for the conductivity field dependence. This law could be confounded with the Poole-Frenkel law in the range of field, corresponding to the experimental observations in organic materials and conjugated polymers. We show that this behavior is a common feature of all disordered systems. We find a field-induced crossover on the temperature dependence of the conductivity.

The electric field dependence of the tunnelling probability between localized states in a disordered solid is presented. The dc conductivity has been determined as a function of the dimension d of the system. We find that, at high fields ( E U 106Vcm-1), the conductivity decreases with increasing field as ln - E 1/2/ T, independently of the dimension of the system.

An attempt is made to formulate a theoretical approach to electronic transport in disordered materials. The conductivity is expressed in terms of a Green function corresponding to the linearized rate equation and is solved using diagrammatic methods. We find a result similar to that of Movaghar and Shirmacher (1981, J. Phys. C, 14, 859), deduced by a renormalized perturbation expansion. The hopping conductivity and thermopower are examined. For temperature and field-dependent conductivity, the results are in complete agreement with others reported in the literature.

The transport phenomena of disordered systems are examined. We transform the rate equation to a form similar to the master equation. This results in an expression of the conductivity in the form of a Green's function, which is evaluated by the diagrammatic technic of Gouchanour et al. For strongly localized systems, we obtain an analytic solution of the DC conductivity, which agrees nicely with literature. We also examine the thermopower.

The purpose of this paper is to present a discussion of the non-Ohmic conductivity of disordered systems in terms of the theories of Butcher, on the one hand, and of Movaghar and Schirmacher, on the other. The linearization of the master equation by Butcher led to an expression the validity of which is extended here to the high-field regime. The hopping conduction theory of Movaghar and Schirmacher founded on this expression can therefore be generalized to the non-Ohmic region. With an adequate choice of a weak disorder in energy and in the low-temperature limit, our approach leads to the following: (1) the law j approximately exp (AE2), for the dependence of the current density j on the electrical field E, as predicted by Apsley and Hughes; (2) under slightly different conditions, the law of Pollak and Riess, j approximately exp (c\e\Er(c)/kT), appears. It seems therefore that the theories of Butcher and of Movaghar and Schirmacher remain valid for the investigation of non-Ohmic effects in disordered systems.

In the present work, we deal with the non-ohmic conduction problem in granulars. We focus on the effects of the electrical field on the tunnelling probability. This effects are rather rarely studies. The classical percolation criterion is extended to the non-ohmic regime. The extreme cases of weak and high fields are examined.