2.2. Materials 929 2.3. Factors Influencing Charge Mobility 931 2.3.1. Molecular Packing 931 2.3.2. Disorder 932 2.3.3. Temperature 933 2.3.4. Electric Field 934 2.3.5. Impurities 934 2.3.6. Pressure 934 2.3.7. Charge-Carrier Density 934 2.3.8. Size/molecular Weight 935 3. The Charge-Transport Parameters 935 3.1. Electronic Coupling 936 3.1.1. The Energy-Splitting-in-Dimer Method 936 3.1.2. The Orthogonality Issue 937 3.1.3. Impact of the Site Energy 937 3.1.4. Electronic Coupling in Oligoacene Derivatives 938

Charge transport in organic semiconductors.

Electrochromic tungsten oxide films: Review of progress 1993–1998

The striking similarity of ac conduction in quite different disordered solids is discussed in terms of experimental results, modeling, and computer simulations. After giving an overview of experiment, a macroscopic and a microscopic model are reviewed. For both models the normalized ac conductivity as a function of a suitably scaled frequency becomes independent of details of the disorder in the extreme disorder limit, i.e., when the local randomly varying mobilities cover many orders of magnitude. The two universal ac conductivities are similar, but not identical; both are examples of unusual non-power-law universalities. It is argued that ac universality reflects an underlying percolation determining dc as well as ac conductivity in the extreme disorder limit. Three analytical approximations to the universal ac conductivities are presented and compared to computer simulations. Finally, model predictions are briefly compared to experiment.

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Universality of ac conduction in disordered solids

Ionic conduction in space charge regions

A brief review of the history of ac ionic and electronic conduction in disordered solids is given, followed by a detailed discussion of the simplest possible realistic model: the random free‐energy barrier model. This model assumes conduction takes place by hopping, where the hopping charge carriers are subject to spatially randomly varying energy barriers. The model is solved in the continuous time random walk and in the effective medium approximation, and it is shown that the two solutions are almost indistinguishable. In the random free‐energy barrier model, the frequency‐dependent conductivity is completely determined by the dc conductivity and the dielectric loss strength. The model correctly predicts all qualitative features of ac conduction in disordered solids, and a comparison to experiment on a large number of solids shows that the model is also quantitatively satisfactory.

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The random free-energy barrier model for ac conduction in disordered solids

Basic Elements of the Theory of Hopping Transport Small Polarons Hopping Conduction in Disordered Systems Classical Hopping Transport Recent Developments Appendix 1. Diagram Technique for Strong Electron-Phonon Coupling Appendix 2. Rate Equation in the Presence of a Magnetic Field Appendix 3. Percolation Problems and the Potts Model References.

Hopping conduction in solids

Spin accumulation in lateral semiconductor superlattices induced by a constant electric field

A two-dimensional hopping system with Rashba spin-orbit interaction is considered. Our main interest is concerned with the evolution of the spin degree of freedom of the electrons. We derive the rate equations governing the evolution of the charge density and spin polarization of this system in the Markovian limit in one-particle approximation. If only two-site hopping events are taken into account, the evolution of the charge density and of the spin polarization is found to be decoupled. A critical electric field is found, above which oscillations are superimposed on the temporal decay of the total polarization. A coupling between charge density and spin polarization occurs on the level of three-site hopping events. The coupling terms are identified as the anomalous Hall-effect and the recently proposed spin Hall-effect. Thus, an unpolarized charge current through a sheet of finite width leads to a transversal spin accumulation in our model system.

V. V. Bryksin

Papers

Hopping conduction in solids

Spin accumulation in lateral semiconductor superlattices induced by a constant electric field

Spin Hall-effect in two-dimensional hopping systems

Effective-medium method for hopping transport in a magnetic field

Theory of Anderson localization in an electric field.