Showing papers on "Variable-range hopping published in 2001"
TL;DR: In this paper, the dc and ac conductivities of polyaniline and polyvinyl alcohol (PVA) blends have been reported from room temperature to 80 K in the frequency range of 1.8 kHz to 5.5 kHz.
192 citations
TL;DR: In this article, a thin film magnetic system consisting of nanoscale Mn11Ge8 ferromagnetic clusters embedded in a MnxGe1−x dilute Ferromagnetic semiconductor matrix was fabricated.
Abstract: We have fabricated a thin film magnetic system consisting of nanoscale Mn11Ge8 ferromagnetic clusters embedded in a MnxGe1−x dilute ferromagnetic semiconductor matrix. The clusters form for growth temperatures of ∼300 °C with an average diameter and spacing of 100 and 150 nm, respectively. While the clusters dominate the magnetic properties, the matrix plays a subtle but interesting role in determining the transport properties. Variable range hopping at low temperatures involves both nanoclusters and MnGe sites, and is accompanied by a negative magnetoresistance attributed in part to spin-dependent scattering analogous to metallic granular systems.
186 citations
TL;DR: A model to describe electrical conductivity along the DNA double helix quantitatively explains the temperature dependence of the conductivity observed in the lambda phage DNA (lambda-DNA).
Abstract: We present a model to describe electrical conductivity along the DNA double helix In this model, DNA is considered as a one-dimensional disordered system, and electrons are transported via variable range hopping between localized states Thermal structural fluctuations in DNA further localize electronic wave functions, giving rise to a temperature-dependent localization length The model quantitatively explains the temperature dependence of the conductivity observed in the lambda phage DNA (lambda-DNA)
183 citations
TL;DR: In this paper, an analytic expression for the effective transport energy in a positionally random and energetically disordered hopping system is obtained, and it is shown that multiple carrier jumps within pairs of occasionally close localized states strongly affect the position of the effective transportation level on the energy scale.
Abstract: An analytic expression for the effective transport energy in a positionally random and energetically disordered hopping system is obtained. It is shown that multiple carrier jumps within pairs of occasionally close localized states strongly affect the position of the effective transport level on the energy scale and lead to a noticeable difference between the effective transport energy and the energy of most probable jumps. In a hopping system with a Gaussian density-of-states energy distribution, the equilibrium carrier mobility is found to be an almost factorized function of temperature and concentration of localized states.
130 citations
TL;DR: In this paper, the structure and electronic properties of polymer-derived silicoboron-carbonitride ceramics are reported, and the local structure is comprised of Si tetrahedra with B, C, and N at the corners.
Abstract: The structure and electronic properties of polymer-derived silicoboron–carbonitride ceramics are reported. Structural analysis using radial-distribution-function formalism showed that the local structure is comprised of Si tetrahedra with B, C, and N at the corners. Boron doping of SiCN leads to enhanced p-type conductivity (0.1 Ω−1 cm−1 at room temperature). The conductivity variation with temperature for both SiCN and SiBCN ceramics shows Mott’s variable range hopping behavior in these materials, characteristic of a highly defective semiconductor. The SiBCN ceramic has a low, positive value of thermopower, which is probably due to a compensation mechanism.
125 citations
TL;DR: In this paper, the authors argue that high-dielectric polymers such as polyaniline and polypyrrole have unusually strong temperature dependence due to the low dimensionality of charge transport in the polymers.
100 citations
TL;DR: In this paper, the anomalous Hall effect in the inelastic hopping regime was identified, which is an extension of Holstein's model for the ordinary Hall effect, arising from the combined effects of the double exchange-induced quantal phase in triads of Mn ions and spin-orbit interactions.
Abstract: The low-temperature Hall resistivity r xy of La2/3A1/3MnO3 single crystals ~where A stands for Ca, Pb, and Ca, or Sr! can be separated into ordinary and anomalous contributions, giving rise to ordinary and anomalous Hall effects, respectively. However, no such decomposition is possible near the Curie temperature which, in these systems, is close to metal-to-insulator transition. Rather, for all of these compounds and to a good approximation, the r xy data at various temperatures and magnetic fields collapse~up to an overall scale! ,o n to a single function of the reduced magnetization m[M/ M sat , the extremum of this function lying at m’0.4. A mechanism for the anomalous Hall effect in the inelastic hopping regime, which reproduces these scaling curves, is identified. This mechanism, which is an extension of Holstein’s model for the ordinary Hall effect in the hopping regime, arises from the combined effects of the double-exchange-induced quantal phase in triads of Mn ions and spin-orbit interactions. We identify processes that lead to the anomalous Hall effect for localized carriers and, along the way, analyze issues of quantum interference in the presence of phononassisted hopping. Our results suggest that, near the ferromagnet-to-paramagnet transition, it is appropriate to describe transport in manganites in terms of carrier hopping between states that are localized due to the combined effect of magnetic and nonmagnetic disorder. We attribute the qualitative variations in resistivity characteristics across manganite compounds to the differing strengths of their carrier self-trapping, and conclude that both disorder-induced localization and self-trapping effects are important for transport.
94 citations
TL;DR: In this paper, the authors describe localized states in the conduction band of solid insulators based on the Mott's formalism, where the mobility edge Ec between localized and extended states is located in the Conduction band and is a function of the degree of disorder.
93 citations
TL;DR: In this paper, small polaron hopping models of Mott, Schnakenberg, and Emin are found to fit the conductivity data of all the samples above the corresponding metal-insulator transition (MIT) temperature.
Abstract: Two different phases of LaMnO3+δ [one showing a metal–insulator transition (MIT), referred to as LaMn–C, and the other not showing a MIT, referred to as LaMn-S] have been clearly observed to follow two different conduction mechanisms. Interestingly, small polaron hopping models of Mott, Schnakenberg, and Emin are found to fit the conductivity data of all the samples above the corresponding MIT temperature. The conductivity data of the insulating (semiconducting) LaMn–S followed a nonadiabatic hopping conduction mechanism while LaMn–C and the Pb doped samples viz. La1−xPbxMnO3 (x=0.05–0.5) showed a similar type of MIT and followed an adiabatic small polaron hopping conduction mechanism in the high temperature paramagnetic phase (above the respective MIT temperature). Activation energy (W), density of states at the Fermi level N(EF), Debye temperature (θD), electron–phonon interaction constant (γP), etc. of LaMn–S showed appreciable differences from those of LaMn–C and La1−xPbxMnO3, which show a MIT. Polaro...
91 citations
TL;DR: In this paper, the temperature dependence of the local activation energy, Eloc (T) = dln ρ(T)/d(kT)-1 gives evidence for a variable-range hopping conductivity over states of the Coulomb gap.
Abstract: In ceramic La1-xCaxMnO3 samples with 0≤x≤0.15 and the concentration of holes c = 0.18-0.22 between T = 77 and 340 K the resistivity, ρ, is shown to have an activated character both above and below the ferromagnetic Curie temperature, TC. The temperature dependence of the local activation energy, Eloc (T) = dln ρ(T)/d(kT)-1 gives evidence for a variable-range hopping conductivity over states of the Coulomb gap. The width of the gap Δ≈0.24 eV, the effective dielectric permeability κ≈3.6 and the carrier localization radius ξ≈5.1 A in the paramagnetic phase are determined. The value of ξ suggests hopping of small lattice polarons at T>TC. With decreasing temperature two successive transitions, at temperatures TC' and TC'', associated with divergence of the correlation length, are observed in the vicinity of TC. They can be interpreted in terms of critical increase (as T approaches TC' from higher temperatures) and decrease (when T shifts from TC'' towards lower temperatures) of the radius of magnetic polarons, generated by a small fraction of the lattice polarons as T→TC. The difference between TC and the temperature of the first divergence of ξ, TC'
64 citations
TL;DR: Kohary et al. as discussed by the authors analyzed the temperature and field dependences of the hopping drift mobility of charge carriers in strongly anisotropic disordered solids where transport can be treated as one dimensional.
Abstract: Analytic calculations are carried out for temperature and field dependences of the hopping drift mobility of charge carriers in strongly anisotropic disordered solids where transport can be treated as one dimensional. The solutions obtained are exact for hopping processes with noninteracting carriers and carrier transitions to the nearest sites in a one-dimensional chain. Only such transitions are essential in systems with strongly correlated space-energy distributions of localized states [see K. Kohary et al., following paper, Phys. Rev. B 63, 094202 (2001)]. Comparison is given for results obtained with symmetrical and asymmetrical transition rates. It is shown that mesoscopic effects play an essential role even for rather long chains with hundreds of localization sites. Moreover, not only the magnitude of the drift mobility, but also its temperature dependence is influenced by the chain length. A suggested theoretical description provides a general basis for the treatment of transport processes in one-dimensional disordered organic solids, such as columnar discotic liquid-crystalline glasses.
TL;DR: In this paper, a small-polaron hopping conduction mechanism for mixed valence oxides with Tp between 230 and 275 K (depending on x) has been thoroughly examined.
Abstract: Above the semiconductor-to-metallic transition (SMT) temperature (Tp), transport properties of the La1−xPbxMnO3+δ (0 ≤ x ≤ 0.5)-type mixed valence oxides with Tp between 230 and 275 K (depending on x) have been thoroughly examined for a small-polaron hopping conduction mechanism of the carriers. Although the variable range hopping (VRH) model was used earlier to fit the entire conductivity data above SMT, we noticed two distinct regions (above and below θD/2; θD is the Debye temperature) where different types of conduction mechanisms are followed. The high temperature (T > θD/2) conductivity data of all the Pb-doped samples follow the adiabatic hopping conduction mechanism, while those of LaMnO3 (x = 0) showing no SMT follow the non-adiabatic hopping conduction mechanism of Mott or Emin with reasonable values of polaron radius, hopping distance, polaron binding energy, activation energy, etc being different for different systems. The VRH model, however, fits the corresponding low temperature (T < θD/2) data of all the samples. Both resistivity ρ(T) and thermoelectric power S(T) follow a similar microscopic theory above Tp supporting the small-polaron hopping mechanism. Thermoelectric power also showed appreciable magnetic field dependence around SMT.
TL;DR: In this article, the electrical conductivity of several phosphate glasses was studied over a wide temperature range and it was shown that electrons are the dominant charge carriers in the phosphate glasses and that the dc conductivity is typically 10 −12 −10 − 10 −10 Ω −1 cm −1 with an activation energy of 0.55 eV.
Abstract: The electrical conductivity of several phosphate glasses was studied over a wide temperature range. Contents of several metals (Fe, Na, Cs, K) were changed in an attempt to study the influence of composition on the electrical properties. Electrons are shown to be the dominant charge carriers. At room temperature, the dc conductivity is typically 10 −12 –10 −10 Ω −1 cm −1 with an activation energy of 0.55 eV. Over a temperature range from 150 to 400 K the conductivity is described by the variable range hopping mechanism. Several common problems and ambiguities in the analysis of the dc electrical conductivity are discussed briefly. The data and interpretation are compared to other published results.
TL;DR: In this article, it was shown that the charge carrier mobility in a positionally and energetically disordered hopping system can be evaluated by averaging either the hopping rates or the hopping times over the thermal equilibrium energy distribution of localized carriers.
Abstract: It is shown that the charge carrier mobility in a positionally and energetically disordered hopping system can be evaluated by averaging either the hopping rates or the hopping times over the thermal equilibrium energy distribution of localized carriers. However, at variance with averaging the hopping rates, averaging the hopping times can be correct only if the energy dependence of the carrier energy relaxation time is also taken into consideration. The temperature and concentration dependences of the equilibrium carrier mobility were calculated by averaging the hopping rates. The consideration was based on the variable-range hopping approach with full account for the interplay between jump distance and energy difference. However, the obtained results prove that, in good quantitative agreement with both Monte Carlo simulations and experimental data, the mobility can be approximated as a product of two functions. The first function depends almost solely upon the temperature and reveals only a wea...
TL;DR: In this paper, thin films of NiMn 2 O 4 NTC thermistor material were produced by electron-beam-evaporation from phase pure NiNiO 4 source material and the films were characterised using SEM, EDAX, XRD and an Alpha-Step stylus profileometer.
TL;DR: In this paper, temperature-dependent transport and magnetic measurements indicate that ferromagnetism appears along with a crossover between two forms of variable-range-hopping (VRH) conductivity on cooling.
Abstract: Temperature-dependent transport and magnetic measurements on Sr{sub 2}Y{sub 0.5}Ca{sub 0.5}Co{sub 2}O{sub 7} indicate that ferromagnetism appears along with a crossover between two forms of variable-range-hopping (VRH) conductivity on cooling. Efros-Shklovskii (ES)-type VRH conduction was found below approximately 30 K, transformed from Mott-type VRH at higher temperature. The magnitude of the Coulomb gap and the Mott-ES VRH crossover temperature are {approx}57 K and {approx}170 K, respectively. These are unusually large compared to those of nonmagnetic disordered materials. The peculiar electronic state for the ferromagnetic Coulomb gap is probably due to Coulomb correlations among d electrons in the disordered system.
TL;DR: In this article, the dielectric, magnetic, and transport properties of Gd0.33Sr0.67FeO3 have been analyzed in the light of the small polaron hopping conduction mechanism.
Abstract: In this study, the dielectric, magnetic, and transport properties of Gd0.33Sr0.67FeO3 have been analyzed. The temperature dependence of resistivity exhibits the adiabatic small polaron hopping conduction; i.e., ρ/T=A exp(E/kBT), E being the small polaron hopping energy. The dielectric loss anomaly was found to be around 170 K. The activation energy corresponding to the relaxation process of this dielectric anomaly was 0.16 eV. A dielectric relaxation process shows up with an activation energy nearly equal to that of dc conduction. The dielectric anomaly and resistivity of Gd0.33Sr0.67FeO3, which arises due to the presence of Fe4+ and Fe3+ ions, has been analyzed in the light of the small polaron hopping conduction mechanism.
TL;DR: Two new compounds synthesized from transition metal dioxides and alkaline earth metal carbonates, showing nonlinear reciprocal susceptibility at all temperatures and described by a variable-spin cluster model, and the importance of metal-metal bonding and the relationships to other related compounds are discussed.
Abstract: Ca3MnRu2O9 and Ba3MnRu2O9 were synthesized from transition metal dioxides and alkaline earth metal carbonates at 1100−1300 °C. Ca3MnRu2O9 adopts the prototypical GdFeO3-type perovskite structure with Mn and Ru statistically disordered over the single metal atom site. The susceptibility shows Curie−Weiss behavior above 240 K with μeff = 3.14 μB/metal atom, which is in excellent agreement with the expected spin-only moment of 3.20 μB. Below 150 K, the compound shows spin-glass-like short-range ferrimagnetic correlations. The high-temperature region of the electrical resistivity reveals a small activation energy of 17(1) meV whereas the low-temperature region is nonlinear and does not fit a variable range hopping model. Ba3MnRu2O9 crystallizes in the 9-layer BaRuO3-type structure containing M3O12 face-shared trioctahedral clusters in which Mn and Ru are statistically disordered. Ba3MnRu2O9 shows nonlinear reciprocal susceptibility at all temperatures and is described by a variable-spin cluster model with an ...
TL;DR: In this paper, the DC conductivity of semiconducting Gd1/3Sr2/3FeO3 has been measured in the temperature range 77-300 K.
Abstract: The DC conductivity of semiconducting Gd1/3Sr2/3FeO3 has been measured in the temperature range 77– 300 K . The semiconducting phase is found to obey Mott's variable range hopping mechanism of the conduction. The parameters related to hopping conduction: viz., the characteristic temperature (T0), localization length (α), hopping range (R), and hop energy (W) have been calculated.
TL;DR: In this article, the electric resistance of single-walled nanotube mats has been studied systematically under both ambient and high hydrostatic pressures up to 1.5 GPa.
TL;DR: In this paper, a combination of junction capacitance, electron spin resonance and electrical conductivity measurements are used to investigate the electronic properties of two different types of a-C:H films grown in a dual ECR-RF glow discharge system at substrate bias equal to −30 and −600 V, respectively.
TL;DR: In this article, a model based on displacement spikes was developed to describe the formation of graphite sp2 bonds in diamond by means of high dose ion implantation, and the model fits the latter data well, and is also consistent with recent results on interstitial-vacancy generation and the interactions of these defects, in electron-irradiated diamonds.
Abstract: Variable-range-hopping (VRH) conduction that has been ascribed to the formation of graphite sp2 bonds, can be generated in diamond by means of high dose ion implantation. With increasing implantation temperature, the ion doses required to generate this conduction increase owing to some form of self-annealing. Similar behaviour has been reported for the formation of amorphous material in silicon. Morehead and Crowder (1970 Radiat. Eff. 6 27) developed a model based on displacement spikes to describe this process in silicon. Prawer and Kalish (1995 Phys. Rev. B 51 15711) adapted this model to describe the onset of VRH conduction in diamonds that they had implanted with 100 keV C+ and 320 keV Xe+ at different temperatures. In this study it is assumed that the latter ions could not have formed displacement spikes in diamond. Equations, based on interstitial-vacancy generation and recombination, are then derived to describe the processes of damage formation and annealing in diamond during ion implantation, and applied to the data of Prawer and Kalish. The model fits the latter data well, and is also consistent with recent results on interstitial-vacancy generation, and the interactions of these defects, in electron-irradiated diamonds.
TL;DR: In this article, temperature dependent conductivity measurements were carried out in the temperature range 300-77 K. Transmission electron microscopy analysis has shown that carbon thin films were grown by magnetron sputtering at room temperature on silicon substrates, with the substrate bias voltage varying from +10 to −200 V. With respect to conductivity, the investigated carbon films are classified in three groups: (i) in α-C films deposited at Vb=+10 V (sp2 rich bonds), the variable range hopping (VRH) conduction dominates below 300 K.
Abstract: Carbon thin films were grown by magnetron sputtering at room temperature on silicon substrates, with the substrate bias voltage varying from +10 to −200 V. Transmission electron microscopy analysis has shown that films deposited at Vb=+10 and −40 V are amorphous (α-C), while films deposited at Vb=−200 V are nanocrystalline (nc-C). Temperature dependent conductivity measurements were carried out in the temperature range 300–77 K. With respect to conductivity, the results indicate that the investigated carbon films are classified in three groups: (i) In α-C films deposited at Vb=+10 V (sp2 rich bonds), the variable range hopping (VRH) conduction dominates below 300 K. (ii) In α-C films deposited at negative Vb up to −100 V (sp3 rich bonds), VRH conduction dominates at low temperatures (T 150 K). (iii) In nc-C film deposited at Vb=−200 V, the conductivity is explained by a heteroquantum-dots model based on a t...
TL;DR: In this article, the resistance behavior of C+-damaged diamond layers was studied as a function of the C+ ion dose and it was found that the postulated graphitisation threshold of 1022 cm−3, as simulated by the TRIM computer program, could be exceeded by a large margin before such a layer became graphite when annealed at temperatures higher than 500°C.
TL;DR: A temperature dependent dc conductivity consistent with variable range hopping in a Coulomb glass is found and it is found that this conductivity is not consistent with extant theories for the hopping transport.
Abstract: We have conducted temperature and frequency dependent transport measurements in amorphous NbxSi1-x samples in the insulating regime. We find a temperature dependent dc conductivity consistent with variable range hopping in a Coulomb glass. The frequency dependent response in the millimeter-wave frequency range can be described by the expression sigma(omega) varies with (-iota omega)(alpha) with the exponent somewhat smaller than 1. Our ac results are not consistent with extant theories for the hopping transport.
TL;DR: In this paper, the ceramic samples of Nb doped hexagonal barium titanate h-Ba Nb Ti O were characterized by X-ray diffraction XRD, electric and magnetic measurements.
TL;DR: In this paper, the electrical and magnetoresistive features systematically change around the percolation threshold of ferrimagnetic Fe3O4 metal, and the electrical conduction can be described on the basis of small-polaron hopping and variable range hopping with coulomb gap.
Abstract: Magnetotransport nature of Fe3O4/γFe2O3 granular thin films is examined in the transformation process from Fe3O4 to γFe2O3. The electrical and magnetoresistive features systematically change around the percolation threshold of ferrimagnetic Fe3O4 metal. The electrical conduction can be described on the basis of small-polaron hopping and variable range hopping with coulomb gap in the Fe3O4/γFe2O3 thin films. Possible causes for the change in the magnetoresistance are also addressed.