Abstract: Temperature dependence of dc conductivity in hydrogenated amorphous carbon (a-C:H) samples, deposited in the range of −200 to −700 V self-bias voltages, was studied above room temperature from 300 K to 650 K. Because a non-Arrhenius dependence on 1/T was found, actual activation energy Eact was determined by derivation of the log σ versus T−1. This Eact increased gradually with temperature by a rate depending on self bias. Strong fluctuations of Eact, observed above 500 K, indicates a transition to another conduction mechanism. Below 500 K the plot of log σ versus T−1/4 could be fitted to linear functions, with an unrealistic density of states at Fermi energy, supporting, that underlying mechanism cannot be variable range hopping near EF rather it is related to conduction by hopping of carriers excited to band tails.

Abstract: We have studied the frequency-dependent conductivity of randomly packed aggregates of ligand-stabilized, monodisperse metal clusters from dc to the infrared over 12 decades in frequency. At low frequencies the ac conductivity of these compounds is dominated by phonon-assisted hopping between localized states, and shows scaling behavior as a function of temperature and frequency. Due to the unique structure of these cluster aggregates, the transition from phonon- to photon-mediated charge transport is clearly observed. The data are found to be in excellent quantitative agreement with the two-site tunneling model. @S0163-1829~98!50324-4# Charge transport in disordered systems, where conduction proceeds by hopping between localized states, has been a widely studied subject over the last decades. A powerful method to study hopping transport is to perform conductivity experiments where both temperature and applied frequency are varied. In this way, not only the various theoretical hopping models can be tested, 1 but also insight in the structural properties of the material at mesoscopic length scales may be gained. 2,3

Abstract: Transport properties of disordered semiconductors at low temperatures are determined by hopping of electrons via localized band tail states. Much attention has been paid recently in both experimental and theoretical studies to the relation between the diffusion coefficient of carriers, D, and their mobility, μ, in the hopping regime. Rather controversial results have been reported. In particular, some computer simulations have been claimed to show that the conventional Einstein relation μ=eD/kT is violated in the hopping regime even in the case of thermal equilibrium. We study the relation between μ and D by a computer simulation and show that such statements were based on a wrong interpretation of the simulation results. In thermal equilibrium, the Einstein relation between μ and D must hold, although in a nonequilibrium system this relation can be violated.

Abstract: The electrical transport properties of polycrystalline semiconducting β-FeSi2 films have been evaluated by conductivity (σ) measurements over the temperature range 50–300 K. At low temperatures (T<200 K), a variable range hopping conduction was observed, from which the number of states near the Fermi level and the degree of disorder in the material were obtained. At moderate temperatures (200–300 K), the ln σ vs 103/T curves show anomalous features such as kinks or continuous bending. In this temperature range, the conductivity data satisfy the Meyer–Neldel rule, (MNR), which is of fundamental importance for the transport properties of the β-FeSi2. The results show that the MNR parameters are related with the degree of disorder in the material.

Abstract: This paper describes a new approach to making the active elements of gas sensors using a photochemical deposition method. The method involves making amorphous films of photochemically active precursors. These precursors are then exposed to light and, in air, convert to metal oxides. Amorphous films constructed of W(CO){sub 4}(Et{sub 2}-en) are deposited on interdigitated microelectrodes. Photolysis of these films, in air at room temperature, results in the deposition of amorphous films of tungsten oxide. This forms the sensing element of an NO{sub x} sensor. Films thus prepared were also annealed to yield sensors whose active elements were crystalline tungsten oxide. An investigation of the conduction mechanism in the sensor materials was performed. The conduction in the polycrystalline materials is controlled by grain boundaries through thermionic emission. Absorption of NO{sub 2} leads to a modification of the grain boundary resulting in the change in current. In contrast, the amorphous materials conduct via variable range hopping. In this case, absorption of NO{sub 2} leads to a reduction in the number of carriers and a change in conductivity of the material. The response of the amorphous materials has been fit to a model based on a Langmuir isotherm.

Abstract: Persistent photoconductivity has been used to probe the metal-insulator transition in the diluted magnetic semiconductor Cd1 − xMnnTe:In. We have measured the d.c. conductivity of Cd0.92Mn0.08Te:In with tunable photogenerated carrier concentration, from the insulating phase up to ~ 1.2nc in the same sample. In the insulating phase, Efros-Shklovskii variable range hopping conduction is observed. In the metallic phase the temperature dependence of the conductivity is adequately described by quantum corrections to the zero-temperature conductivity due to the effects of electron-electron interaction and weak localization. In the critical region the scaling theory of electron localization has been applied. We observe a critical carrier concentration ~ 2.3 × 1017 cm−3, and a critical conductivity exponent close to one.

Abstract: It is established that variable-range hopping conduction takes place between states localized near the Fermi level in layered TlGaS2 and TlInS2 single crystals both along and across their natural layers in a constant electric field at T⩽200 K. The densities of states near the Fermi level and the hopping distances at different temperature are estimated. The occurrence of activationless hopping conduction is established in TlGaS2 and TlInS2 single crystals in the temperature range 110–150 K.

Abstract: The electrical transport properties of have been investigated using complex-plane impedance analysis and dielectric measurements with measurements of four-probe dc conductivities and magnetic susceptibilities. The temperature dependencies of the bulk conductivities obtained by impedance analysis, and dielectric relaxation processes provide evidence of the hopping conduction below 60 K, which is remarkably different from the hopping conduction in , despite the degree of doping with Sr being very small. The features in the low-doping range are: (i) the emergence of the hopping conduction at markedly lower temperatures in comparison with the case for non-doped ; (ii) the abrupt reduction of the activation energy required for the conduction, 0.042 eV at x = 0.03 to 0.021 eV at x = 0.07, from 0.34 eV for ; and (iii) a progressive decrease in the activation energy with increasing x. The difference in conduction behaviour between and is due to the alternation of the spin state of caused by the Sr doping. The results have been discussed in terms of a process of hopping of small polarons localized by the electron-lattice interaction in tandem with the electron-magnon interaction.

Abstract: Electrical and magnetic properties of have been extensively investigated at low temperature down to 1.8 K. The irreversibility of magnetic susceptibility between zero field cooling and field cooling increases with increasing Y content. The metal-insulator transition temperature shifts towards lower temperature and the peak resistivity and magnetoresistance increase with the increase of x. Electrical conduction above is dominated by the variable range hopping mechanism. The increase in resistivity at low temperature has been interpreted in terms of electron-electron interaction. The enhancement of magnetoresistance with the increase of Y content is due to the reduction in Mn-O-Mn bond angle and the mobility of charge carriers.

Abstract: The results of the temperature dependence of dc conductivity, microwave dielectric constant, X-ray diffraction, EPR, and NMR experiments are reported for chemically synthesized polypyrrole films soluble in organic solvents. The temperature dependence of dc conductivity of these systems follows the three-dimensional variable range hopping model. The slope of the temperature dependence of dc conductivity varies with dopants and solvents used. Room temperature dc conductivities are in the range of ∼0.2−0.01 S/cm, and the localization lengths are estimated as ≤10 A, which indicates a relatively strong localization of charge. The results of microwave dielectric constant experiments support the strong localization behavior. The X-ray diffraction studies show that soluble polypyrroles are amorphous. From electron paramagnetic resonance experiments, the density of states of polypyrrole doped with naphthalenesulfonic acid in m-cresol solvent is estimated as ∼0.107 states/(eV ring). We obtain the result that the sy...

Abstract: We review some of the fundamental concepts which have been introduced into the field of amorphous semiconductors by Professor Sir Nevill Mott. These include the 8−N rule, variable range hopping, the Austin–Mott ac conductivity, the mobility edge, and the minimum metallic conductivity. We demonstrate that there are still severe problems, although there is no real alternative to Mott's concepts.

Abstract: Transport properties of disordered semiconductors at low temperatures are determined by hopping of electrons via localized band tail states. Much attention has been paid recently in both experimental and theoretical studies to the relation between the diffusion coefficient of carriers, D, and their mobility, μ, in the hopping regime. Rather controversial results have been reported. In particular, some computer simulations have been claimed to show that the conventional Einstein relation μ=eD/kT is violated in the hopping regime even in the case of thermal equilibrium. We study the relation between μ and D by a computer simulation and show that such statements were based on a wrong interpretation of the simulation results. In thermal equilibrium, the Einstein relation between μ and D must hold, although in a nonequilibrium system this relation can be violated.

Abstract: The present work investigates the effect of Sm and Co substitution in Bi 2 Sr 2 Ca 1 Cu 2 O 8+ δ system. The substitution of Sm leads to an initial increase of transition temperature followed by a decrease. No such increase of transition temperature is observed in Sm and Co co-substituted samples, thus reflecting localization of excess holes in these samples. The suppression of superconductivity in both kind of samples occurs due to hole filling and disorder effects. The insulating state of the samples is described by 3D variable range hopping of conduction mechanism with energy dependent density of states.

Abstract: Composites containing copper particles with nanometer dimensions in a silica gel medium have been synthesized by an electrodeposition technique. The precursor composition of the gel was in the system Cu(NO3)2–SiO2 and the copper particle diameters were in the range of 3.2–11.4 nm. The dc electrical resistivity of pellets obtained from the nanocomposite powders was measured in the temperature range of 110–300 K. A temperature dependence with a fractional exponent of 0.25 was observed. This behavior has been explained on the basis of a variable range hopping mechanism.

Abstract: DC conductivity measurements were performed by modified four-probe rig on electrochemically synthesized polypyrrole films at a temperature range of -30◦C to 120◦C. Conductivity increased with temperature. The temperature dependence of conductivity was very high for lightly doped polymers, decreasing as the doping level increased. The model used to describe the conduction process was the conduction model originally developed for amorphous silicon by Mott and Davis. When applied to conducting polymers, it assumes that electron transport originates from localized or fixed states within the polymer chain. The charge transfer between the chains takes place by hopping, referred to as phonon-assisted hopping, between two localized states. Plots of DC conductivity versus temperature can be parametrized by Mott’s Variable Range Hopping conduction model. The DC conductivity of polypyrrole films doped from light to intermediate levels with p-toluene sulphonic acid were measured in the temperature range of 77K to 300K. The localization length of localized electrons was assumed to be 3A, which is approximately equal to the length of the pyrrole monomer. Mott parameters of polypyrrole films doped with p-TS were evaluated at 300K and 10K. Results were found to be consistent with the Mott’s requirement that αR >> 1. Theoretical values of α and N(EF ) have been determined at approximately 10 cm−1 and 10 -10 cm−3 eV−1 , respectively. Hence according to Mott parameters determined by the experimental data for the p-TS doped polypyrrole samples, Mott parameters are seen to have a better agreement with those expected from disordered systems, particularly for lightly doped samples, indicating the suitability of Mott’s model to these samples. The average hopping distance R decreased from 16Ato 4.4Awith the increase in the doping level from 0.006 M to 0.03 M at 300K, whereas at 10K, R decreased from 37A to 10A over the same dopant range.