Showing papers on "Variable-range hopping published in 1992"
TL;DR: In this article, the interdependence of electron hopping between redox centers and the physical motion of redox centres is investigated systematically, and Monte-Carlo simulations are employed to study the transition between static percolation and mean-field behaviors as a function of the relative rates of electron hop and physical motion.
Abstract: The interdependence of electron hopping between redox centers and the physical motion of redox centers is investigated systematically. When physical motion is either nonexistent or much slower than electron hopping, charge propagation is fundamentally a percolation process. In the opposite extreme, rapid molecular motion thoroughly rearranges the molecular distribution between successive electron hops, thereby leading to mean-field behavior. Monte-Carlo simulations are employed to study the transition between static percolation and mean-field behaviors as a function of the relative rates of electron hopping and physical motion
289 citations
TL;DR: In this article, the electronic transport mechanism in polypyrrole is discussed in terms of Mott variable range hopping (VRH) in samples with a wide range of conductivities and which have been formed using different doping techniques.
Abstract: The electronic transport mechanism in polypyrrole is discussed in terms of Mott variable range hopping (VRH) in samples with a wide range of conductivities and which have been formed using different doping techniques. Samples were synthesized in both aqueous and organic media and samples were either doped during polymerization or dedoped afterwards giving a three order of magnitude range of conductivities at 300 K and a range of sample morphologies. No difference in behavior is observed for materials with different morphologies, suggesting that transport predominantly involves monomer units and occurs independent of structure. The various transport parameters obtained appear reasonable with the exception of the apparent ‘‘hopping attempt frequency,’’ related to the phonon frequency in VRH, some values of which are anomalously high. The density of states at the Fermi level was found to be between 5×1018 and 1×1022 eV−1 cm−3 for a range of samples and the mean hopping distances ranged between 2 and 34 monomer units. The minimum hopping distance of 2 monomer units is consistent with electron delocalization on individual monomer units. An upper limit of conductivity in polypyrrole of no more than 400 S cm−1 is suggested in the limit of the VRH regime in which hopping occurs between adjacent monomer units.
81 citations
TL;DR: In this paper, a multilayered polypyrrole-fluorohectorite hybrid is characterized by X-ray diffraction, UV-visible, infrared and Raman spectroscopies and electrical conductivity measurements.
77 citations
TL;DR: In this paper, a series of dielectric compounds having perovskite-related structures Ca 3 Ti 2 O 7, Ca 4 Ti 3 O 10, and CaTiO 3 with the general formulaCa n +1 Ti n O 3 n + 1 (n = 2, 3, and∞) showed a metallic conductivity when electron carriers were doped by reduction under H 2 atmosphere.
75 citations
TL;DR: The temperature and magnetic-field dependencies of hopping transport in dilutely doped \ensuremath{\delta} layers have been measured and a distinct nonmonotonic variation of the resistance with magnetic field in the perpendicular orientation is noted.
Abstract: The temperature and magnetic-field dependencies of hopping transport in dilutely doped \ensuremath{\delta} layers have been measured under the conditions for which the variable-range mechanism applies. We trace the transition from negative magnetoresistance in low fields to positive magnetoresistance in high fields. In the range of intermediate fields, the resistance in the perpendicular orientation appears to be several times less than that in the parallel one. It is shown that this ``inverted'' relation cannot be accounted for in terms of the interference of alternative hopping paths alone. We note also a distinct nonmonotonic variation of the resistance with magnetic field in the perpendicular orientation. The results are explained by considering both the interference of alternative hopping paths and the influence of a magnetic field on the density of states at the Fermi level, which defines the magnitude of Mott's ${\mathit{T}}_{0}$ parameter in variable-range hopping.
49 citations
TL;DR: It is found the transition from negative to positive magnetoresistance is temperature independent and occurs at a magnetic field corresponding to the lowest Landau-level filling factor of v=2.
Abstract: We present an experimental study of magnetoresistance of a degenerate, two-dimensional electron gas with a broad range of localization length in the variable-range-hopping regime. At low magnetic fields, a giant negative magnetoresistance of R(0)/R(H)∼7 is observed and is confirmed to be an orbital mechanism in origin. At high fields, the two-dimensional electron gas shows a contrasting transport behavior of a positive magnetoresistance. We found the transition from negative to positive magnetoresistance is temperature independent and occurs at a magnetic field corresponding to the lowest Landau-level filling factor of v=2
43 citations
TL;DR: In this paper, the intrinsic competition between superconductivity and localization in an essentially localized system is discussed in terms of a theoretical model which determines the intrinsic competitiveness between supercondivity and localized system.
Abstract: Electrical resistivity measurements have been performed on the Bi 2 Sr 2 Ca 1−x M x Cu 2 O 8+y (M=Eu, Dy, Tm) system with various heat treatments. A gradual change in normal state resistivity ( ϱ n ) and T c has been observed with increasing dopant concentration. The results provide experimental evidence for a transition from a metallic behaviour to a variable range hopping regime at low temperatures. In the insulator regime (highly semiconducting-like) the resistivity behaviour is found to turn from 2D to 3D variable range hopping for x ⩾0.6. The results are discussed in terms of a theoretical model which determines the intrinsic competition between superconductivity and localization in an essentially localized system. T c data are compared with the earlier reported findings.
41 citations
TL;DR: Magnetization and resistivity of La 0.8 Sr 0.2 Mn 1− x Cu x O 3 compounds are measured for 10 T B Mn 3+ / Mn 4+ valences as discussed by the authors.
40 citations
TL;DR: In this paper, the electrical properties of as-grown and heat-treated copper indium diselenide (CuInSe2) thin films electrochemically deposited were measured in the temperature range 30 to about 300 K.
Abstract: The electrical properties of as‐grown and heat‐treated copper indium diselenide (CuInSe2) thin films electrochemically deposited were measured in the temperature range 30 to about 300 K. In general the films showed variable range hopping conduction mechanism, i.e., σ= (σ0 T−1/2) exp[ − (T0/T)1/4] with T0 between 4 × 102 and 6 × 103 K, at the lowest temperatures, and thermally activated conductivity, i.e., σ = σ0 exp( − Ea/kT) with Ea between 240 and 80 meV, over the temperature range close to the room temperature. The activation energy, Ea, decreased and the conductivity, σ, increased with the annealing treatment. The data of these annealed films are in agreement with the Meyer–Neldel rule. Grain boundary trapping models were considered in the analysis of the results.
38 citations
TL;DR: The electrical properties of triclinic PbPc have been studied for thin films exposed to oxygen and dry air at atmospheric pressure for periods up to 25 weeks as mentioned in this paper.
35 citations
TL;DR: Here the low-temperature ac conductivity for adiabatic small-polaronic hopping between close pairs of sites is calculated presuming a distribution of local site energies, and is in accord with observations in many systems with hopping conduction.
Abstract: Electronic hopping is commonly treated as occurring between localized states that are so widely separated that the motion is limited by the electronic transfer energy linking the sites. Then, the jump rate is usually assumed to fall exponentially with increasing intersite separation. However, this approach is inappropriate in many situations where the separation between the hopping sites is small enough that electronic carriers {ital adiabatically} follow the atomic motion. For adiabatic motion, the jump rates are essentially independent of intersite separation. Here the low-temperature ac conductivity for adiabatic small-polaronic hopping between close pairs of sites is calculated presuming a distribution of local site energies. Low-temperature relaxation of each such carrier is assumed to occur primarily through the emission of a very-low-energy acoustic phonon. For small-polaronic hops, low-temperature one-phonon emission rates are extremely slow. Dispersion of the transition rates arises from the dependence of the relaxation rates on the energy separations between the sites. In the low-temperature limit, the polarization conductivity is proportional to both temperature and frequency. Above this low-temperature limit, the severity of this temperature dependence increases with increasing temperature. In this higher-temperature regime, the temperature dependence of the conductivity also decreases as the frequency is increased. These resultsmore » are in accord with observations in many systems with hopping conduction, including those for which there is explicit evidence of adiabatic small-polaronic hopping (e.g., {ital p}-type MnO, boron carbides, and many transition-metal-oxide glasses).« less
TL;DR: Calculating the linear response of the momentum distribution function, it is found that one-dimensional Luttinger liquid phases are unstable with respect to arbitrarily small transverse hopping, even if the interaction is strong.
Abstract: We analyze interacting lattice fermions with anisotropic hopping amplitudes. Calculating the linear response of the momentum distribution function, we find that one-dimensional Luttinger liquid phases are unstable with respect to arbitrarily small transverse hopping, even if the interaction is strong. This instability is expected to signal the formation of states with broken symmetry in the strongly anisotropic regime
TL;DR: In this article, anion dependent processes of electrical conduction in chemically prepared polyaniline compressed pellets doped with alkylbenzenesulfonate anion were investigated by the measurement of the temperature dependence of the conductivity.
Abstract: Anion dependent processes of the electrical conduction in chemically prepared polyaniline compressed pellets doped with alkylbenzenesulfonate anion of alkyl chain length 1, 8, and 12 have been investigated by the measurement of the temperature dependence of the conductivity. The conductivity of the samples changes with the temperature as T−1/2, which shows the conduction followed a one dimensional variable range hopping process. With the increase of the alkyl chain length of the dopant anion, the conductivity of the heavily doped 50 mol % sample decreases in whole temperature range, but an exponential factor of the variable range hopping remains unchanged. The results concerning the alkyl chain length of the dopant anion have been discussed from the ‘‘quasi‐one dimensional’’ variable range hopping mechanism and demonstrated the decrease of the transverse localization length and the interchain transfer integral of electron.
TL;DR: The data support the recent theory of Glazman and Matveev pertaining to correlated resonant tunneling and infer correlated inelastic hopping along chains consisting of two localized sites.
Abstract: We have observed Coulomb correlations between the two resonant tunneling conduction channels, one per spin orientation, associated with each localized site in the barrier of a metal-insulator-metal tunnel junction. Our data support the recent theory of Glazman and Matveev pertaining to correlated resonant tunneling. We also infer correlated inelastic hopping along chains consisting of two localized sites.
TL;DR: In this paper, the authors measured the pressure dependence of poly(3-octylthiophene) (POT), poly (3-decylthyphene)(PDT), and poly( 3-dodecylthio-thiophenes) (PDDT) as a function of FeCl 3 doping and applied Kivelson's theory for intersoliton hopping.
TL;DR: In this paper, the results of the numerical scaling approach for localization are used to discuss the statistical behavior of the zero-temperature conductance of disordered systems of finite size, and it is shown that the temperature dependence of the fluctuations of the logarithm of the conductance/resistance does not depend on the assumptions concerning the statistics of the hopping processes.
Abstract: The results of the numerical scaling approach for localization are used to discuss the statistical behaviour of the zero-temperature conductance of disordered systems of finite size. In the asymptotic regime of strong localization, where transport is dominated by hopping processes, explicit expressions for the temperature dependence of the fluctuations of the conductance and the resistance are obtained by assuming that the phase coherence length is given by the Mott hopping law. It is shown that the temperature dependence of the fluctuations of the logarithm of the conductance/resistance does not depend on the assumptions concerning the statistics of the hopping processes. The results are compared with other theoretical and experimental approaches.
TL;DR: In this paper, conductivity data for a range of amorphous indium oxide samples and analyse the region of activated conductivity in an attempt to identify hopping processes were presented.
Abstract: We present conductivity data for a range of amorphous indium oxide samples and analyse the region of activated conductivity in an attempt to identify hopping processes. The results suggest that, in highly resistive samples, conduction is by variable-range hopping in a Coulomb gap, the localized states being deep in the gap and related to the disorder. In more conductive material, there appears to be a trend towards Mott hopping as the Fermi level rises towards the conduction band.
TL;DR: In this article, electrical transport properties of pellets of oxidized poly(N -vinylpyrrole), a new conducting ladder polymer, are studied. And the conductivity data are coherent with a three-dimensional variable range hopping transport model.
TL;DR: In this article, fractionated precipitation of poly(3-octylthiophene) was used to prepare polymers with different molecular mass and molecular mass distributions, and the molecular mass dependent processes of charge transport in these samples have been investigated.
Abstract: Fractionated precipitation of poly(3-octylthiophene) was used to prepare polymers with different molecular mass and molecular mass distributions. The molecular mass dependent processes of charge transport in these samples have been investigated. The temperature dependence of the electrical conductivity corresponds to the three-dimensional variable range hopping mechanism. The conductivity and the absorbance at interband transition increase with molecular mass, although the absorption edge is unchanged. These facts are interpreted in terms of an increase in the radius of a localized wavefunction with the molecular mass.
TL;DR: In this article, high-field positive and negative magnetoresistance in semiconducting materials in which variable-range hopping occurs at low temperatures was reported. But the experimental results were limited to two regimes: the Mott regime and the Shklovskii-Efros regime.
Abstract: Experimental results are reported on high-field positive and negative magnetoresistance in semiconducting materials in which variable-range hopping occurs at low temperatures. In the case of positive magnetoresistance, both the Mott (T −1/4) and the Shklovskii-Efros (T −1/2) regimes, are observed. We discuss the transition between these two regimes, the shape of the Coulomb gap, and the scaling behaviour of the parameter T o. At low magnetic fields and for different semiconducting materials, we have observed a negative magnetoresistance associated with variable-range-hopping conduction. Experimental data are tentatively compared with available models for orbital magnetoresistance mechanisms, based on quantum interference between tunnelling paths in the insulating regime.
TL;DR: In this article, the structure of the grain boundary depends on the preferential orientation (P.O.) and affects the electrical properties of polycrystalline Si (poly-Si) films.
Abstract: Undoped polycrystalline Si (poly‐Si) films were prepared on a fused quartz substrate as a function of the rf power (0–30 W) and deposition temperature (620–770 °C) by a plasma‐enhanced chemical vapor deposition method. The preferential orientation to a random, 〈100〉 or 〈110〉 crystal axis can be selected by changing some of these preparation conditions. Electron spin resonance (ESR) and the dependence of the dark conductivity σd on the measurement temperature T have been investigated to examine how the structure of the grain boundary depends on the preferential orientation (P.O.) and affects the electrical properties. An enhancement in the degree of P.O. is found to lead to a reduction in the local ESR spin density within the boundary region and g factor. Also, for the films with a random or weak P.O., σd as a function of 1/T exhibits a kink around room temperature, and the conduction in a low‐T range is interpreted in terms of the model of variable range hopping within the boundary region. These results a...
TL;DR: In this article, a consistent interpretation is obtained if it is assumed that the wavefunctions of the electrons contributing to conduction are extended through clusters of Ni atoms and only localized by longer-range disorder.
Abstract: Data are presented for hopping conductivity in amorphous hydrogenated 1-yNiy alloys. The behaviour is well described by the form σ = σo [(T o/T)0.5] corresponding to variable-range hopping in a parabolic density of states. However, there is strong quantitative disagreement with the model of Efros and Shklovskii if it is assumed that the hopping is between orbits localized on the impurity atoms. However, a consistent interpretation is obtained if it is assumed that the wavefunctions of the electrons contributing to conduction are extended through clusters of Ni atoms and only localized by longer-range disorder.
TL;DR: In this article, a set of eigenvalues and eigenstates for relaxational fractons is derived for the relaxation mode theory of hopping conduction on fractal lattices, and the obtained conductivity formula gives us full information over all the frequency and temperature range.
Abstract: On the basis of the relaxation mode theory, the hopping conduction on fractal lattices is investigated by the direct use of a set of eigenvalues and eigenstates for the relaxational fractons, where the relaxational fracton is a hopping version of the vibrational fracton. The obtained conductivity formula gives us full information over all the frequency and temperature range. Especially at a frequency and temperature range, its frequency-dependence is identical to the result derived by Alenxander and Orbach for fractal and percolation lattices. This scheme of dealing with the relaxational fractons is quite useful for the study of dynamical properties in the hopping conduction: the quasi-elastic light scattering and ultrasonic attenuation are also discussed on the same ground.
TL;DR: In this paper, the conductivities of 1-benzothiophene (1BT) thin film were measured in the temperature range from 80 to 290 K in order to investigate the conduction mechanism.
TL;DR: In this article, the conduction mechanism was found to be phonon assisted variable range hopping between small (~10 nm) carbon clusters that formed a macroscopic percolation cluster.
Abstract: The electrical conductivity of high temperature polymers (i.e. polyimide) has been changed permanently from 10−17 Ω−1 cm−1 to 10 Ω−1 cm−1 by excimer laser irradiation. The conduction mechanism is found to be phonon assisted variable range hopping between small (~10 nm) carbon clusters that form a macroscopic percolation cluster. The critical concentration of carbon clusters is found to be pc = 0.24 ± 0.02 and the critical exponent of the insulator-conductor transition is t = 1.97 ± 0.25.
TL;DR: In this paper, the resistivity of Pr1−zCazBa2Cu3O7−δ (for z ≲ 1 2 ) as a function of temperature was measured and it was shown that thermally activated hopping is the dominant conduction mechanism in this material.
TL;DR: In this article, the resistivity temperature dependence of diamond-like films containing no hydrogen has been investigated in terms of variable range hopping between localized states at temperature T below 400 K and the exponent of the Mott's law changes from 1/4 to 1/2 with the decreasing thickness of films, due to a stronger contribution of states localized on impurities in the vicinity of interfaces.
Abstract: The resistivity temperature dependence of diamondlike films containing no hydrogen has been investigated for the first time It is discussed in terms of variable range hopping between localized states at temperature T below 400 K The exponent of the Mott's law changes from 1/4 to 1/2 with the decreasing thickness of films, due to a stronger contribution of states localized on impurities in the vicinity of interfaces At higher temperatures, the transport proceeds from the direct excitation of localized electrons up to the mobility edge But in any case a metallic conduction through percolating sp2 sites has been observed
TL;DR: In this article, the immitance spectroscopy method in very low frequency range (0.001Hz ÷ 0.8Hz) has been applied to the cleaved single crystal of LiNbO3.
Abstract: The immitance spectroscopy method in very low frequency range (0.001Hz ÷ 0.8Hz) has been applied to the cleaved single crystal of LiNbO3. The results of measurements has been treated using complex plane analysis techniques. The best representation of the data in the impedance plane is obtained. The experiments performed in the temperature range 300K ÷ 950K have defined relaxation processes with the relaxation time of order 102s. Important role of variable range hopping in the charge transport have been confirmed.
07 Sep 1992
TL;DR: In this article, the authors show that the non-ohmic hopping conductivity of disordered dielectrics is due to a change in charge transport paths and electronic density redistribution, and is inherent in systems with strong electron-phonon interaction.
Abstract: It has been stated in a number of studies that the field dependence of differential conductivity in disordered dielectrics with hopping conduction has a minimum, an exponential rise in gamma (E) being preceded by a region of conductivity decline with increasing field strength Bottger and Bryksin (1985). The present pair shows that such a behaviour of the nonohmic hopping conductivity of disordered dielectrics is due to a change in charge transport paths and electronic density redistribution, and is inherent in systems with strong electron-phonon interaction. >
TL;DR: In this article, the authors studied the low temperature conductance of small silicon metal-oxide-semiconductor field effect transistors operating in the localized regime and found that at the lowest temperatures the fluctuations are caused by variable range hopping via paths containing only a few (≲ 3) localized states.