Abstract: Computer modeling of the variable-range-hopping (VRH) conductivity in two-dimensional systems has been done by a kinetic Monte Carlo method, which includes some new elements. Study of the temperature dependence of the conductivity, testing of the different scaling relations, and study of the size effect show the detailed validity of the Efros-Shklovskii theory of the VRH in the system of interacting electrons. It has also been shown that simultaneous transitions of many electrons are not important. The reasons for disagreement with previous computational works are thoroughly analyzed.

Abstract: We studied the electronic conductivity and magnetic properties of quasi-one-dimensional ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{2}{\mathrm{O}}_{6}$ single crystals. The results evidence a variable range hopping conductivity with temperature-induced crossover between one-dimensional (intrachain) and three-dimensional (3D) transport and the opening of a Coulomb gap in the d bands along with the ferromagnetic intrachain ordering. At low temperatures, an applied magnetic field induces a large negative magnetoresistance (MR) in apparent dissociation with the 3D magnetic ordering. Both spin-dependent hopping and field-induced suppression of the Coulomb gap are discussed. The electronic hopping parameters we infer agree remarkably with the accessible Co sites. Surprising narrow peaks corresponding to a transient resistivity decrease are observed on the MR curves. We discuss these in terms of peculiar in-plane magnetization states in an Ising-like Heisenberg antiferromagnetic triangular lattice during the magnetization reversal.

Abstract: This note discusses the relationship between a given bridge-assisted electron transfer rate and the corresponding zero-bias molecular conduction of the same molecular species, in the limit where both processes occur by sequential hopping. It follows a previous publication (A. Nitzan, J. Phys. Chem. A 2001, 105, 2677-2679) in which the same issue was discussed for coherent tunneling transfer.

Abstract: Experimental investigations of the transport phenomena (electric conduction; Seebeck, Hall, and Nernst-Ettingshausen effects) in PbTe and Pb1−xSnxTe solid solutions with high content of In impurity (up to 20 at. %) at temperatures of up to 400 K are reviewed. Many properties of these substances are similar to those of noncrystalline materials. The experimental data are analyzed in terms of hopping conduction via strongly localized impurity states related to In atoms. Temperature dependences of transport coefficients, which are uncommon to IV–VI compounds; inversion of thermoelectric power with negative Hall coefficient; and a positive Nernst-Ettingshausen coefficient are accounted for. The activation energy of hopping conduction, which characterizes the effective energy spread of impurity levels; the effective radius of the wave function; and the energy dependence of the density of localized states are found from experimental data. The discussion of the experimental data on hopping conduction is preceded by a brief description of resonance and deep localized electron states related to indium impurities in IV–VI compounds. Particular attention is given to the specific features of impurity states in samples with high In content, in which the hopping conduction is observed.

Abstract: Polypyrrole nanocylinders were fabricated by chemically synthesizing polypyrrole within the pores of nanoporous polycarbonate particle track-etched membranes. The morphology of the nanostructures was characterized by transmission electron microscopy and scanning electron microscopy. The nanocylinders were observed to be cigar-shaped, with the diameter at the center being up to 2.5 times the diameter at the ends. The electrical conductivity of the nanocylinders was measured by leaving the nanocylinders embedded in the insulating template membrane and measuring the trans-membrane resistance. The cigar-like shape of the nanocylinders was taken into account in calculating the conductivity. Contrary to previous reports, the smallest diameter nanocylinders exhibited a slightly lower conductivity relative to the larger diameter nanocylinders. The temperature dependence of the resistance and magnetoresistance was in accordance with Mott variable range hopping at temperatures above 5±1 K and Efros–Shklovskii variable range hopping at temperatures below 5±1 K. Based on the measurements in the Mott regime, the localization length, the density of states at the Fermi energy, and the temperature dependence of the average hopping distance were calculated.

Abstract: The temperature dependence of the magnetoresistance (MR) of insulating ${\mathrm{Al}}_{70}{\mathrm{Pd}}_{22.5}{\mathrm{Re}}_{7.5}$ quasicrystals taken at liquid-helium temperatures can be explained by the theories of MR---the forward interference and the wave-function shrinkage---in the variable-range hopping (VRH) regime. By analyzing the MR data with the theories mentioned above, a crossover from Mott VRH conduction to Efros-Shklovskii VRH conduction at liquid helium temperatures was identified in a highly resistive ${\mathrm{Al}}_{70}{\mathrm{Pd}}_{22.5}{\mathrm{Re}}_{7.5}$ sample. The rapid decrease in the negative MR of highly resistive samples at low temperatures might be attributed to the conduction via the states in the Coulomb gap.

Abstract: In the course of developing microcalorimeters as detectors for astronomical X-ray spectroscopy, we have undertaken an empirical characterization of non-ideal effects in the doped semiconductor thermometers used with these detectors, which operate at temperatures near 50 mK. We have found three apparently independent categories of such behavior that are apparently intrinsic properties of the variable-range hopping conduction mechanism in these devices: 1/f fluctuations in the resistance, which seems to be a 2D effect; a departure from the ideal coulomb-gap temperature dependence of the resistance at temperatures below T 0 /24; and an electrical nonlinearity that has the time dependence and extra noise that are quantitatively predicted by a simple hot electron model. This work has been done largely with ion-implanted Si:P:B, but similar behaviors have been observed in transmutation doped germanium.

Abstract: Our previous modeling of hopping in localized bandtails has improved the description of electrical transport in amorphous carbon films and elemental amorphous semiconductors. This model is further developed to understand the role of the wavefunction localization radius γ -1 and density of states (DOS) distribution N(E) on the T dependence of the conductivity and the thermopower in disordered solids. Using values of the localization parameter LP = N(E F ) γ -3 in the range from 10 -4 to 1 eV -1 , our simulations show that: (i) The assumption of extremely low LP values made in previous works is often irrelevant for comparison with experiments. (ii) For low temperatures or large LP values, a linear relation holds between the dominant transport energy (E t - E F ) and temperature (with a stronger dependence as LP decreases) while a logarithmic dependence is obtained in the high-T range for small LP values. (iii) In the weak localization condition, a T-independent thermopower S(T) is predicted with an absolute value of eS/k (LP) -1/3 . (iv) The relationship σT 1/2 = σ 00 exp (-(T 0 /T) 1/4 holds, as expected for 3D hopping, over a wide temperature range (50-500 K); using the LP value as a parameter, the positive linear correlation between In (σ 00 ) and T 1/4 0 is a signature of an exponential localized bandtail distribution, clearly incompatible with Mott's hypothesis for 'Variable Range Hopping' in an energy-independent DOS distribution near the Fermi level.

Abstract: We study localization in two- and three-channel quasi-one-dimensional (1D) systems using multichain tight-binding Anderson models with nearest-neighbor interchain hopping. In the three-chain case we discuss the cases of both free and periodic boundary conditions between the chains. The finite disordered wires are connected to ideal leads and the localization length is defined from the Landauer conductance in terms of the transmission coefficients matrix. The transmission and reflection amplitudes in properly defined quantum channels are obtained from S matrices constructed from transfer matrices in Bloch wave bases for the various quasi-1D systems. Our exact analytic expressions for localization lengths for weak disorder reduce to the Thouless expression for 1D systems in the limit of vanishing interchain hopping. For weak interchain hopping the localization length decreases with respect to the 1D value in all three cases. In the three-channel cases it increases with interchain hopping over restricted domains of large hopping.

Abstract: The equilibrium carrier mobility in an energetically disordered and positionally random hopping system is analytically calculated by direct averaging of carrier hopping rates and by the use of the effective transport energy concept. In good quantitative agreement with experimental data and results of Monte-Carlo simulation, the temperature and concentration dependencies of the mobility can be almost perfectly factorized, i.e., represented as a product of two functions one of which depends solely upon the temperature while another one governs the dependence on the density of localized states.

Abstract: Using an rf magnetron reactive sputtering technique thin films (<~ 1 mu-m) of amorphous silicon suboxides (a-SiOx, 0 < x < 2) have been deposited. The substrate temperature during deposition remained below 100oC. The films appear rather homogenous in composition (O/Si ratio), with an incorporated fraction of hydrogen and argon atoms of about 2-5 at.%. Taking into account the incorporated argon atoms no large (empty) void fraction has been detected. The films appear composed of Si-Si4-nOn (n= 0,...,4) building blocks, with an occurrence of the five individual building blocks in the different compounds generally in agreement with the statistical distribution of randomly dispersed O atoms in a bridging con- figuration between Si atoms. Furthermore, a large density of neutral paramagnetic defects (~ 1021 cm) has been observed. These defects are predominantly associated with silicon dangling bonds (DBs) in a silicon rich environment ( Si-Si3), although in the as deposited compounds with higher O/Si ratio x also Si DBs in more oxygenated environments ( Si-Si2O, Si-SiO2) are observed. Simultaneously, the electrical conductivity of the films indicates a dominant variable range hopping (vrh) mechanism in the compounds, at least up to room temperature. This conduction mechanism is described by a charge transport via thermally assisted tunneling events through localized (defect) states. Since the host network of the defect states in our SiOx films appears rather homogeneous on both macroscopic (mu-m) and microscopic (nm) scale the localized hopping states are expected to be more or less randomly distributed throughout the material. The energy distribution of the corresponding hopping sites is expected to extend over several tenths of an eV, as indicated by measurements on the conductivity around room temperature and supported by a theoretical Density Of States (DOS) model based on a defect-poole model. As a result the vrh conduction process in these SiOx films is adequately described by a system of localized states, randomly distributed in both space and energy. Analytically, it is possible to express the conductivity in such a system qualitatively in terms of the density and localization of the states around the position of the chemical potential. The conductivity appears to depend on the external parameters temperature (T) and electric field (F) and is described differently in different regimes of T and F. In this thesis a more quantitative description of the vrh process is derived numerically using percolation theory. To this purpose calculations on the percolation threshold of the site-to-site impedance in a system of randomly distributed hopping sites were performed. The results appear in qualitative agreement with the analytically derived results. Moreover, the calculations result in a clear quantified description of 1. the temperature dependence of the vrh conduction at low field strengths and 2. the field dependence of the conduction at low field strengths and/or low temperatures. Furthermore, a numerical study on the vrh conduction in a system of non-uniform localized states shows that the conductivity is effectively dominated by the charge transport through a subset of more weakly localized states. Measurements on the temperature and field dependence of the electrical conductivity in the deposited SiOx films appear in good agreement with the theoretical vrh models. Furthermore, quantitative information on the density and localization of the electronic states dominating the macroscopic charge transport is obtained by fitting the measured data with the numerically derived relations. The observed differences in resistivity of the investigated SiOx films appear well explained by a stronger localization of the hopping sites in the more oxygenated environments. Furthermore, it appears that only a small fraction of the neutral paramagnetic states, viz. the more weakly localized states, contributes significantly to the macroscopic charge transport in the vrh conduction process. Both observations are supported by measurements on annealed samples, showing a preferential annihilation of paramagnetic spins in the more oxygenated environments

Abstract: In its pure state rutile is an insulator with a 3 eV band gap. Its electrical conductivity, less than 10 −13 Ω −1 cm−1, can be enhanced up to more than 12 orders of magnitude by reduction or doping. In this work we study the doping of [0 0 1] TiO2 single crystals with chromium ions implanted with 140 keV at room temperature and fluences of the order of 1017–1018 ions/cm2. Implantation damage and damage recovery were accessed using the Rutherford backscattering technique in the channeling mode. Electrical resistivity measurements were performed as a function of temperature between 5 and 300 K. Upon implantation the implanted region becomes completely disordered and displays an enhanced electrical conductivity with a variable range hopping behaviour. After annealings at temperatures up to 1300 K, the lattice recovers and the implanted Cr moves out of the implanted region. The electrical conductivity behaviour changes from variable to fixed range hopping and after the 1300 K annealing becomes typical of a thermally activated semiconductor with 0.36 eV activation energy.

Abstract: The density of states (DOS) near the Fermi level is calculated using DC conductivity (Mott parameters) and Space Charge Limited Conduction (SCLC) measurement data. DC conductivity measurements on thin films of a-Se 100− x Bi x ( x =0, 0.5, 2.5 and 5) are reported in the temperature range (219–375 K). At high temperature 249–375 K, the conduction occurs in the extended states while at lower temperature (219–249 K) the conduction is due to variable range hopping. I – V measurements have also been done on a-Se 100− x Bi x at different electric fields. SCLC is observed in a-Se 100− x Bi x .

Abstract: This paper reports on the magnetotransport behavior of Fe–Al2O3 granular thin films when the injected dc current is varied. The electric resistance as a function of temperature, magnetoresistance, and the current vs applied bias potential measurements were used to characterize the samples. It was found that the transport mechanism which best describes the electronic properties of these samples is variable range hopping. Non-Ohmic behavior was observed and is claimed as responsible for the great modification of the electronic characteristics of the system as a function of the applied bias potential. Inversion of the tunneling magnetoresistance is observed for applied bias potential greater than 3 V. Such inverted magnetoresistance comes from the activation of low resistivity tunneling paths that are promoted by increasing the bias potential. An expression is proposed to describe the magnetoresistance behavior.

Abstract: Magnetic and transport properties of two-dimensional (2D) Mn layers in GaAs/Mn digital alloys have been determined as a function of Mn layer coverage. Ferromagnetism was observed in samples having more than 10% Mn in each Mn-containing layer. The Curie temperature showed a strong correlation with the effective spin density in the digital alloy samples. Magnetotransport studies revealed activated behavior hole conductivity consistent with variable range hopping in two dimensions. The general features of magnetoresistance and the anomalous Hall effect are similar to those seen in metallic ferromagnetic GaMnAs. However, the temperature dependence of the observed negative magnetoresistance follows the pattern seen in insulating ferromagnetic GaMnAs.

Abstract: The dark conductivity, σd(T), and the time-resolved microwave conductivity (TRMC) of hydrogenated microcrystalline silicon (μc-Si:H) films prepared by hot-wire chemical vapor deposition (HWCVD) and very-high-frequency plasma-enhanced CVD have been investigated The σd(T) simulations were carried out based on a simplified energy band model Fitting the σd(T) data indicates that the electrical transport mechanism in μc-Si:H film depends on the nanocrystallite volume fraction (Xc) Around room temperature, thermionic emission dominates for samples with low Xc, however, for high Xc, tunneling is the major conduction mechanism In the low temperature range, the behavior of films with low-Xc is fairly explained by variable range hopping conduction An activation energy of 015 eV related to oxygen was found in HWCVD films TRMC measurements show that the mobility increases with Xc

Abstract: A new analysis of electronic transport in chalcogenide glasses, based on bipolaron hopping in the extended pair approximation, is presented. It is assumed that the relaxation time of the carrier when hopping has a Meyer-Neldel type of temperature dependence instead of a simple activated form. In this way, the experimental data for both dc conductivity and ac conductivity can be fitted over a wide range of temperatures, and for a number of glasses using the same set of parameters.

Abstract: The dc electrical conduction in xV 2 O 5 20SnO (80 - x)TeO 2 (18 mol% < x < 50 mol%) glasses has been studied in the temperature range from 100 to 480 K. Glasses with x < 20 exhibited p-type conduction whereas the rest show n-type conduction. All the glass compositions exhibited a crossover from variable range hopping (VRH) to small polaron hopping (SPH) conduction at a characteristic temperature well below room temperature. The low temperature dc conduction mechanism in these glasses has been analysed using Mott's approach. Mott parameter analysis gave values for the density of states at the Fermi level N(E F ) between 1.42 × 10 26 m -3 eV -1 and 15.0 x 10 26 m -3 eV -1 at 230 K. The disorder energy W d was found to be varying between 0.02 and 0.03 eV. N(E F ), the average hopping distance R VRH and the VRH-SPH transition temperature T R exhibit an interesting composition dependence, which is interpreted in terms of the majority charge carrier reversal (MCCR) phenomenon occurring in these glasses.

Abstract: Conducting polymers of polysiloxane-polypyrrole were synthesized by electropolymerization of the pyrrole monomer through pyrrole moieties in N-pyrrole-terminated polysiloxanes Sodium paratoluene sulfonate was used as the electrolyte Scanning electron microscopy (SEM) was used to determine the surface morphology of the films The room-temperature conductivity values of the films were found to be in the range of 19–44 × 10−4 (Ω cm)−1, depending on the supporting electrolyte concentration The temperature dependence of the dc conductivities of the copolymers having different dopant concentrations was investigated within the temperature range of 100–320 K The evaluated parameters showed that the electrical transport is dominated by variable range hopping © 2002 Wiley Periodicals, Inc J Appl Polym Sci 85: 52–56, 2002

Abstract: Temperature-dependent transport properties of Ge nanocrystalline films (nanofilms) prepared by the cluster beam evaporation technique have been studied. The nanofilms of thicknesses about 20 nm , deposited on the substrates at room temperature, exhibit non-linear current–voltage characteristics in the low bias range with decreasing temperature. In order to understand the conduction via the grain boundaries between two adjacent Ge nanocrystals, the temperature-dependent conductivity of the nanofilms has also been investigated, which could be explained by Mott's variable range hopping mechanism between 100 and 300 K . Below 100 K , the conductivity is limited by ordinary tunneling of carriers giving rise to temperature-independent behavior.

Abstract: In this paper, the magnetic and transport properties of ${\mathrm{FeCr}}_{2\ensuremath{-}x}{\mathrm{Ga}}_{x}{\mathrm{S}}_{4}$ $(0lxl~0.4)$ are studied. M\"ossbauer spectra reveal that this system belongs to an inverse spinel type, where the Ga ion has ``preferes'' to occupy the tetrahedral site. With increasing Ga content, the transport behavior is found to transit into variable-range hopping process gradually both at temperatures far below ${T}_{c}$ and at $Tg{T}_{c}.$ It is believed that the random distribution of cations in inverse spinel material may induce a large Coulomb potential fluctuation, which causes carrier localization and transforms the conduction from narrow band semiconducting behavior into a variable-range hopping process at temperatures far below ${T}_{c}.$ When $Tg{T}_{c},$ since the carriers are further localized by the presence of magnetic disorder, variable-range hopping again dominates the conduction behavior. Accordingly, by taking account of the relative weight of the magnetic and chemical disorder, we give an explanation for the variation of the colossal magnetoresistance effect.

Abstract: The X-ray diffraction has revealed that the polycrystalline hexagonal structured α-In 2 Se 3 thin films grown at substrate temperature of 200°C with the unit cell parameters a=4.03 A and c=19.23 A becomes polycrystalline hexagonal structured InSe with a unit cell parameters of a=4.00 A and c=16.63 A by Cd-doping. The analysis of the conductivity temperature dependence in the range 300-40 K revealed that the thermionic emission of charged carriers and the variable range hopping are the predominant conduction mechanism above and below 100 K, respectively. Hall measurements revealed that the mobility is limited by the scattering of charged carriers through the grain boundaries above 200 K and 120 K for the undoped and Cd-doped samples, respectively. The photocurrent (I ph ) increases with increasing illumination intensity (F) and decreasing temperature up to a maximum temperature of ∼100 K, below which I ph is temperature invariant. It is found to have the monomolecular and bimolecular recombination characters at low and high illumination intensities, respectively. The Cd-doping increases the density of trapping states that changes the position of the dark Fermi level leading to the deviation from linearity in the dependence of I ph on F at low illumination intensities.