Abstract: Films of ZnO doped with magnetic ions Mn and Co and, in some cases, with Al have been fabricated with a very wide range of carrier densities. Ferromagnetic behavior is observed in both insulating and metallic films, but not when the carrier density is intermediate. Insulating films exhibit variable range hopping at low temperatures and are ferromagnetic at room temperature due to the interaction of the localized spins with static localized states. The magnetism is quenched when carriers in the localized states become mobile. In the metallic (degenerate semiconductor) range, robust ferromagnetism reappears together with very strong magneto-optic signals and room temperature anomalous Hall data. This demonstrates the polarization of the conduction bands and indicates that, when ZnO is doped into the metallic regime, it behaves as a genuine magnetic semiconductor.

Abstract: We report on the influence of structural disorder on the electrical properties of multilayer graphene (MLG). Exponential decreases in the conductance and transconductance with increase of defects in the MLG were observed, which could be explained by the percolation and the variable range hopping conduction. An enhancement of p-type nature with increasing disorders was considered to be the result of oxygen doping in the graphene sheets introduced by oxygen plasma. The rapid increase of low-frequency noise was attributed to the formation of conductive network through the continuum percolation, as the low-frequency noise could be increased by the enhanced carrier scattering at the defect sites. We hope that our result should suggest a simple method of tuning the electrical properties of graphene.

Abstract: We derive the theory of the quantum (zero temperature) superconductor to metal transition in disordered materials when the resistance of the normal metal near criticality is small compared to the quantum of resistivity. This can occur most readily in situations in which 'Anderson's theorem' does not apply. We explicitly study the transition in superconductor-metal composites, in an swave superconducting film in the presence of a magnetic field, and in a low temperature disordered d-wave superconductor. Near the point of the transition, the distribution of the superconducting order parameter is highly inhomogeneous. To describe this situation we employ a procedure which is similar to that introduced by Mott for description of the temperature dependence of the variable range hopping conduction. As the system approaches the point of the transition from the metal to the superconductor, the conductivity of the system diverges, and the Wiedemann-Franz law is violated. In the case of d-wave (or other exotic) superconductors we predict the existence of (at least) two sequential transitions as a function of increasing disorder: a d-wave to s-wave, and then an s-wave to metal transition.

Abstract: The temperature dependence of the electrical conductivity of assemblies of ZnO nanocrystals, studied with an electrochemically gated transistor is very accurately described by the relation ln σ = ln σ0 − (T0/T)x with x = 2/3 over the entire temperature range from 7 to 200 K, independent of charge concentration and dielectric environment These results cannot be explained by existing models but are supported by results on Au nanocrystals where an identical temperature dependence was observed (Zabet-Khosousi et al, Phys Rev Lett 2006, 96 (15), 156403) We propose an adaptation of the Efros−Shklovskii variable-range hopping model by introducing an expression for nonresonant tunneling based on local energy fluctuations, which yields exactly the temperature dependence that is observed experimentally

Abstract: Mn1.56Co0.96Ni0.48O4 films with spinel structure were prepared on Al2O3 substrate by chemical solution deposition method. The microstructure of the films was studied by atomic force microscope and field-emission scanning electron microscope. The current-voltage characteristics showed Ohmic conductivity in the temperature range of 245–295K. The conduction was described by a variable range hopping model for a parabolic density of states. The advantages of high characteristic temperature, as well as high transition temperature (201K) between ferromagnetic and paramagnetic phases make the Mn1.56Co0.96Ni0.48O4 films very promising for infrared detection, especially for functional devices by integrating magnetic and electronic properties of the materials.

Abstract: A traditional constant voltage conductivity test method was used to measure how the conductivity of highly insulating low-density polyethylene (LDPE) polymer films depends on applied electric field, repeated and prolonged electric field exposure, and sample temperature. The strength of the applied voltage was varied to determine the electric field dependence. At low electric field, the resistivity was measured from cryogenic temperatures to well above the glass transition temperature. Comparisons were made with a variety of models of the conduction mechanisms common in insulators, including transient polarization and diffusion and steady-state thermally activated hopping conductivity and variable range hopping conductivity, to determine which mechanisms were active for LDPE and to provide a better picture of its electrical behavior.

Abstract: We show how the main features of the electronic transport properties of single-wall carbon nanotube (SWNT) networks can be understood in terms of a simple model involving metallic conduction interrupted by thin tunnelling barriers, backscattering by zone-boundary phonons, and variable range hopping. Within this framework we examine the effect of reducing the thickness of the SWNT networks, chemical treatments, and ion irradiation. The conduction mechanism can be tuned from variable-range hopping between localized states (for the thinnest networks), to metallic conduction interrupted by thin barriers through which conduction is by tunnelling (for thick freestanding films). Chemical treatment of the thick films by different molecules leads to retention of metallic character but changes (increases or decreases) the charge carrier density. For ion-irradiated thick films we find two competing effects: a change to hopping-type conduction in the direct impact layer that lowers conductivity, and annealing effects extending deeper than the ion penetration depth that increase conductivity and lead to a peak in conductivity as a function of irradiation dose. We briefly discuss current–voltage characteristics and possible Luttinger liquid effects.

Abstract: The existence of conducting islands in polyaniline films has long been proposed in the literature, which would be consistent with conducting mechanisms based on hopping. Obtaining direct evidence of conducting islands, however, is not straightforward. In this paper, conducting islands were visualized in poly(o-ethoxyaniline) (POEA) films prepared at low pH, using Transmission Electron Microscopy (TEM) and atomic force spectroscopy (AFS). The size of the islands varied between 67 and 470 A for a pH=3.0, with a larger average being obtained with AFS, probably due to the finite size effect of the atomic force microscopy tip. In AFS, the conducting islands were denoted by regions with repulsive forces due to the double-layer forces. On the basis of X-ray diffraction (XRD) patterns for POEA in the powder form, we infer that the conducting islands are crystalline, and therefore a POEA film is believed to consist of conducting islands dispersed in an insulating, amorphous matrix. From conductivity measurements we inferred the charge transport to be governed by a typical quasi-one dimensional variable range hopping (VRH) mechanism.

Abstract: For hopping transport in disordered materials, the mobility of charge carriers is strongly dependent on temperature and the electric field. Our numerical study shows that both the energy distribution and the mobility of charge carriers in systems with a Gaussian density of states, such as organic disordered semiconductors, can be described by a single parameter - effective temperature, dependent on the magnitude of the electric field. Furthermore, this effective temperature does not depend on the concentration of charge carriers, while the mobility does depend on the charge carrier concentration. The concept of the effective temperature is shown to be valid for systems with and without space-energy correlations in the distribution of localized states.

Abstract: Flexible boron nanowires have been synthesized via thermoreduction in boron-oxygen compounds with magnesium. These as-prepared nanowires, which are structurally uniform and single crystalline, represent good semiconductor at high temperature. Tensile stress measurements demonstrate excellent mechanical property of boron nanowires as well as resistance to mechanical fracture even under a strain of 3%. Importantly, simultaneous electrical measurement reveals that the corresponding electrical conductance is very robust and remains constant under mechanical strain. Our results can be briefly explained by Mott’s variable range hopping model.

Abstract: Electrical and magnetotransport properties of single walled carbon nanotube (SWCNT) fibers are reported. The dependencies of resistance on temperature can be approximated by the Mott law for three-dimensional variable range hopping (VRH) below 80 K and by typical law for fluctuation induced tunneling model within the range of 80–300 K. Both negative and positive magnetoresistances (MRs) were observed. At low fields, MR is negative. Positive upturn was observed on the MR curves, which shifted to the high field’s values with temperature increase. The upturn field of the MR effect was shifted from 1.5 T at 2 K to a value of about 20 T at 40 K. The value of positive MR varies as exp(B2), which changes to B1/3 at sufficiently high fields as expected for the VRH transport. The model of VRH transport is illustrated by the influence of strong microwave field and terahertz radiation induced photocurrent manifestation at low temperatures.

Abstract: Two sets of tellurium-based glasses doped with vanadium and vanadium-cobalt oxides have been prepared by melt quench method and investigated for density, glass-transition temperature, specific-heat capacity and dc electrical conductivity in the temperature range 300–500 K. Thermal stability, fragility and glass-forming tendency in these systems have been estimated. Thermal stability of these glasses was found to be larger than for similar systems. The dc electrical conductivity has been analyzed in the light of Mott's small polaron hopping (SPH) and Mott's and Greave's variable range hopping (VRH) models. The electrical conduction was confirmed to be due to non-adiabatic SPH. The conductivity behavior at temperatures above TD (=θD/2) (where θD is Debye's temperature) was found to be in agreement with Mott's SPH model and at low temperatures behaved in accordance with Mott's VRH model. The temperature regions in which Mott's SPH and VRH models hold for the present systems are found to be different from similar systems already reported. Various physical parameters in these models such as, activation energy, polaron hopping energy, etc., were determined. It is for the first time that the above-mentioned properties have been investigated for the present mixed transition-metal ions-doped glasses.

Abstract: We examine the crystal structure and electrical and optical properties of ZnO epitaxial films grown by pulsed laser deposition in a H2 or D2 ambient. n-type electrical conductivity is enhanced by three orders of magnitude as a result of growing in H2 (D2) compared to ZnO films grown in O2. Hall effect measurements reveal very small carrier activation energies and carrier concentrations in the mid-1018 cm−3 range. Optical absorption measurements show that the enhanced conductivity is not a result of ZnO reduction and interstitial Zn formation. Photoluminescence spectra suggest excitonic emission associated with exciton-hydrogen donor complex formation and show no evidence for midgap emission resulting from defects. We have modeled the transport properties of H (D) doped ZnO films using variable range hopping and surface layer conductivity models, but our data do not fit well with these models. Rather, it appears that growth in H2 (D2) promotes the formation of an exceedingly shallow donor state not seen in...

Abstract: The charge transport in organic semiconductors has been investigated theoretically. An analytical model describing the effect of partially filled localized states on the concept of transport energy is presented, based on variable range hopping theory. The results illustrate that at low enough temperature the partially filled localized states in organic semiconductor systems play an important role on the transport energy. Moreover, the charge concentration will change the transport energy dramatically at higher carrier concentration. The paper also discusses the carrier-concentration-dependent mobility in organic semiconductors.

Abstract: It is shown that the dominant conduction mechanism in single-wall carbon nanotube networks at room temperature is variable range hopping. Thin networks follow a variable range hopping law throughout the temperature range 4 K < T < 300 K, thick networks display variable range hopping characteristics in the range 50 K < T < 300 K, with deviation to Bloch–Gruneisen behaviour at lower temperatures. The adsorption state of these materials is shown to determine the sign of change in conductance upon subsequent exposure to oxygen. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Abstract: In this paper, we have studied the electrical conductivity for different compositions of the zinc vanadate semiconducting glasses within a wide temperature range. We have analyzed the temperature dependence of the electrical conductivity within the framework of various models for the hopping conduction. We have observed that Mott’s phonon-assisted nearest neighbor hopping model of small polarons is appropriate for describing the conductivity data at high temperatures. We have further observed that at lower temperatures, Mott’s variable range hopping model is operative, while Greaves’ variable range hopping model is valid within the intermediate temperature range. The values of the density of states at the Fermi level, which are obtained from the analysis, are consistent with those for localized states.

Abstract: This work describes the preparation and electrical characterization of conducting polypyrrole (PPy) and silica nanocomposites. Four samples were investigated: (i) pure PPy, (ii) PPy-covered SiO2 spherical nanoparticles, (iii) PPy-covered SiO2 spherical nanoparticles modified with 3-aminopropyltriethoxysilane (APS), and (iv) PPy-covered SiO2 fibers. Structural characterization was made by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The electrical conductivity was measured from 80 K to 300 K and three-dimensional variable range hopping conduction was observed. From the hopping parameter the mean hopping distance was obtained as well as the density of active center and the energy associated with it.

Abstract: La0.67Sr0.33Mn1−xTixO3 (0≤x≤0.20) polycrystalline materials are prepared by employing lower annealing temperature compared to the temperatures reported for the materials of the same composition. The transport and magnetic properties of these materials are significantly different from those compounds prepared at higher annealing temperature. Samples with x<0.10 show metal–insulator transition and those with x≥0.10 exhibit insulating behavior over the entire temperature range investigated. A gradual transition occurs from the ferromagnetic-metallic state to the ferromagnetic-insulator state with increasing Ti substitution. Lattice parameters and bond lengths of Mn and its near neighbors however do not change appreciably with the dopant content x in these materials. It is shown that Ti4+ doping in the low-temperature annealed samples is inhomogeneous, resulting in isolated Mn rich regions that are connected by a variable range hopping polaron.

Abstract: We have investigated the systematic transport properties of the layered 112-type cobaltite LaBaCo2O5.5 by means of electrical resistivity, magnetoresistance, electroresistance and thermoelectric measurements in various conditions. In order to understand the complex conduction mechanism of LaBaCo2O5.5, the transport data have been analyzed using different theoretical models. The system shows semiconductor-semiconductor like transition (TSC) around 326K, corresponding to ferromagnetic transition and in the low temperature region resistivity data follows the Motts variable range hopping model. Interestingly, near and below the room temperature this compound depicts significant change in electro- and magnetoresistance behavior, the latter one is noteworthy near the magnetic phase boundary. The temperature dependence of thermopower, S(T), exhibits p-type polaronic conductivity in the temperature range of 60-320K and reaches a maximum value of 303 uV/K (at 120K). In the low temperature AFM region, the unusual S(T) behavior, generally observed for the cobaltite series LnBaCo2O5.5 (Ln = Rare Earth), is explained by the electron magnon scattering mechanism as previously described for perovskite manganites.

Abstract: The enhancement in ferromagnetism and ferroelectricity at room temperature for Pb07Sr03(Fe0012Ti0988)O3 (PSFT) nanoparticles is proved by magnetization and polarization hysteresis loop The x-ray diffraction and micrograph show that the PSFT nanoparticles have distorted tetragonal single phase, and their average particle’s size is 8 nm The effect of Sr content reduces the particle size, and hence the multiferroic system becomes more resistive, which dominates the superparamagnetic/paraelectric relaxation The variable-range-hopping conduction mechanism explained the high resistivity of PSFT nanoparticles, which suggests that the room temperature movement of electrons involves short-range order through defect states