Showing papers on "Variable-range hopping published in 1989"
TL;DR: In this article, the authors present a model for conduction in disordered systems by variable-range tunnelling in a Coulomb gap, and show that the model cannot apply to these systems.
Abstract: The author sets out the model for conduction in disordered systems by variable-range tunnelling in a Coulomb gap. Detailed analysis of experimental data for three different types of granular metal shows that the model cannot apply to these systems. He concludes that the problem of the mechanism of conduction in granular metals remains unsolved.
115 citations
TL;DR: The onset of superconductivity as x is increased is found to be correlated with a substantial drop in the magnitude of the normal-state resistivity, and the low-temperature resistivities of the least concentrated samples can be described by variable range hopping.
Abstract: We have measured the temperature-dependent resistivities of a series of samples of La_(2-x)Sr_xCuO_4 with 0.02≤x≤0.1 over the temperature range 0.05 K≤T≤300 K. We find the onset of superconductivity as x is increased to be correlated with a substantial drop in the magnitude of the normal-state resistivity. We observe no change, however, in the qualitative shape of the resistivity as the superconducting threshold is crossed. We also find that the low-temperature (T≤8.0 K) resistivities of the least concentrated samples can be described by variable range hopping, with a crossover between Coulomb gap and single-particle behavior occurring as x is increased.
78 citations
TL;DR: In this paper, experimental data on the electrical conductivity of conducting polymers is reviewed and the conductivity is studied as a function of doping level, temperature, frequency and conjugation length.
Abstract: Experimental data on the electrical conductivity of conducting polymers is reviewed. Polyacetylene is regarded as a prototype and the conductivity is studied as a function of doping level, temperature, frequency and conjugation length. The consistency of the data with a model of anisotropic variable range hopping is pointed out, with the exception of very highly doped samples, where in addition to hopping also temperature-independent tunnelling between large conducting domains has to be assumed.
73 citations
TL;DR: The present authors have shown that the temperature dependence of the dc conductivity in sputtered a-C is empirically described by o.oT, which may suggest that the nonpolaronic multiphonon tunneling of localized m electrons (weak coupling with lattice) dominates charge transport.
Abstract: The temperature dependence of the dc and ac conductivities has been studied in sputtered arnorphous carbon films. The dc conductivity is proportional to T" with n =15— 17 below room temperature. The mechanism is discussed in terms of the mobility-edge conduction, variable-range hopping, and small-polaron hopping. Quantitatively, these models could not interpret the observed behaviors. Instead, the multiphonon tunneling of localized electrons with weak electron-lattice coupling is suggested to be the dominant transport mechanism. This may be attributed to the ~-bonded nature of sputtered amorphous carbon. The ac conductivity is strongly correlated to the dc conductivity, which has never been explained by the current theories based on the pair approximation. Alternatively, the continuous-time random-walk approximation is shown to be a useful approach. I. INTRODUCTION It is believed that amorphous carbon (a-C) prepared by evaporation or sputtering is predominantly sp bonded with an optical gap of 0.4— 0.7 eV. ' At a carbon sp site, there are three strong o. bonds and one weak m bond lying normal to the o. bonding plane. The ~ states will form both the valence- and conduction-band states. ' The sp sites must be clustered together in aromatic units. The large size of such clusters could produce localized states near the Fermi level E+. The eFect of disorder in a ~-electron system is therefore of particular interest. The present authors have shown that the temperature dependence of the dc conductivity in sputtered a-C is empirically described by o. d, =o.oT", with n =15— 17, which may suggest that the nonpolaronic multiphonon tunneling of localized m electrons (weak coupling with lattice) dominates charge transport. Measurement of ac conductivity could provide direct information about the hopping rate of localized electrons. In the present study, the temperature dependences of both dc and ac conductivities are reported. Possible transport mechanisms, mobility-edge conduction, variable-range hopping (single-phonon process), and multiphonon hopping with strongly (small-polaron) and weakly coupled states, are discussed. It is suggested that the conventional mobility-edge conduction or small-polaron conduction could not explain the overall features of transport in a-C. It is also found that the ac conductivity is strongly correlated with the dc conductivity. This feature, which has never been explained by the current theory based on the pair approximation, is interpreted well in terms of the continuous-time random-walk (CTRW) approximation.
71 citations
TL;DR: In this paper, it was shown that phase coherence may be lost after successive hops assisted by long-wavelength acoustic phonons or as a result of electron-electron interactions.
Abstract: Low-field magnetoresistance measurements for compensated, n-type three-dimensional GaAs with net donor concentration just below the metal-insulator transition show a quadratic field dependence for values of B less than 750 G. Temperature-dependent measurements in zero field show that transport is by variable-range hopping, and are consistent with the presence of a Coulomb gap which narrows close to the transition. It is found that the temperature dependence of the effective area in which the flux is enclosed is not related to the temperature dependence of the optimum hopping length. The relevant dephasing time appears not to be the hopping time. Rather, phase coherence may be lost after successive hops assisted by long-wavelength acoustic phonons or as a result of electron-electron interactions.
54 citations
TL;DR: In this paper, the influence of the lengths of the conjugated chains on the conductivity of polyacetylene polymers is investigated. But this question is of twofold interest.
42 citations
TL;DR: In this article, the temperature dependence of the electrical properties of amorphous copper-tellurite glasses is presented in a range of compositions and the theory of small polaron hopping in the adiabatic approximation may be most appropriate in the present system.
Abstract: The temperature dependence of the electrical properties of the semiconducting amorphous copper-tellurite glasses is presented in a range of compositions. A polaronic model is shown to be generally applicable, and the variation of activation energy with the glass composition is found to dominate the conductivity. In particular, a strong pre-exponential factor containing a term exp(-2 alpha R), arising from electron tunnelling, is not observed. The results suggest that the theory of small polaron hopping in the adiabatic approximation may be most appropriate in the present system. In the intermediate temperature range, Greaves' variable range hopping analysis seems applicable. The glass forming oxide is shown to affect the activation energy for hopping conduction.
36 citations
TL;DR: In this paper, the electrical resistivity of (Gd 1− x Ca x )MnO 2.98 (0.5 ≦ x ≦ 0.9) was measured in the temperature range 80 to 900 K, indicating the possible occurrence of variable range hopping of electrons due to Anderson localization.
29 citations
TL;DR: In this article, the optical, magnetic and electric properties of the ladder polymer BBL have been investigated and a gap of approximately 1.8eV in the neutral undoped state and the presence of charged defects which increase in number upon doping were reported.
28 citations
TL;DR: In this paper, the polaron hopping is shown to be nonadiabatic in character unlike that in the vanadate glasses V 2 O 5 −Bi 2 O 3 where it is adiabatic.
28 citations
TL;DR: A high density of microvoids could cause inhomogeneous distribution of deep localized states in amorphous germanium, and the electronic transport in such media is no longer dominated by Mott variable-range hopping.
Abstract: A high density of microvoids could cause inhomogeneous distribution of deep localized states in amorphous germanium. The electronic transport in such media is no longer dominated by Mott variable-range hopping. The dc conductivity is proportional to ${T}^{n}$ with n\ensuremath{\simeq}6, suggesting that the transport is by a multiphonon process with weak electron-lattice coupling. The ac conductivity is explained by the Scher-Lax continuous-time random-walk approximation, producing information on the hopping rate.
TL;DR: In this paper, the temperature variation of the conductivity is measured over a large temperature range in four different solids where the carriers are suspected to occupy localized polaronic states, and all the results are interpreted within a single model which accounts for disorder at different scales and where hopping is dominated by multiphonon processes.
TL;DR: In this paper, electrical conductivity and optical absorption coefficient were measured on thin films of gallium sulphide obtained by the evaporation of bulk single crystals onto unheated glass substrates.
TL;DR: In this paper, electrical conductivity data was presented for two types of polypyrrole film prepared using toluene sulphonate and sulphate as the counter-ions or dopant.
Abstract: Electrical conductivity data is presented for two types of polypyrrole film prepared using toluene sulphonate and sulphate as the counter-ions or dopant. The films based on polypyrrole and toluene sulphonate exhibit an ordered molecular organisation in which the planes of the aromatic units lie preferentially parallel to the film surface. For such films the conductivity versus temperature data fit to a 2D variable range hopping model. In contrast, films using SO42- as the dopant show an isotropic highly disordered molecular organisation and a temperature dependence of the electrical conductivity which fits a 3D variable range hopping model. Although such models present a rather simple picture of conductivity in these complex molecular composites, they do demonstrate the strong and direct influence of the molecular organisation on their electrical transport properties.
TL;DR: In this article, the atomic structure of amorphous Si1−xCx(a-Si1−cx) films was studied by transmission electron microscopy, and with the measurements of IR absorption, electron spin resonance and electrical conductivity.
TL;DR: In this paper, the effects of morphology and optical degradation in each type of conductivities were investigated to elucidate the nature of electronic conduction in Si-H and µc-Si:H films.
Abstract: Ac conductivity of a-Si:H and µc-Si:H films is studied experimentally with theoretical consideration. By separating ac conductivity into band, variable range hopping (VRH), band tail multiple hopping and intimate pair hopping conductivities, the effects of the morphology and optical degradation in each type of the conductivities were investigated to elucidate the nature of electronic conduction. Experimental facts on the pair-hopping conductivity seem to be explained in terms of a two-electron correlated barrier hopping (CBH) model between oppositely charged dangling bond pairs located at low-density Si–H alloy regions (voids). Long-range transport (band, multiple hopping, VRH) seems to be related to the percolation through Si clusters and microcrystalline grains, being sensitive to the morphology and Fermi level position.
TL;DR: The pyrochlore compounds Bi 2 Pt 2 O 7 (insulator) and Pb 2 Ru 2 O 6.5 (metallic conductor) form a continuous series of solid solutions and increasing substitution of the platinum by ruthenium results in a gradual insulator-metal transition as discussed by the authors.
TL;DR: In this paper, a low temperature conductivitu in plasma-polymerized hexamethyldisilazane films was investigated and a variable range hopping is responsible for this conductivity.
TL;DR: In this article, the temperature and electric field dependence of the resistivity of thin layers of As-doped, not intentionally compensated, Si with concentrations just below that of the metal-insulator transition are reported.
TL;DR: In this paper, a sufficient degree of damage induced by electron irradiation of 1.5 MeV was shown to allow the observation of conductance via nearest-neighbour hopping or variable-range hopping.
Abstract: Hopping conduction is studied in boron-doped Si at low temperatures. A sufficient degree of damage induced by electron irradiation of 1.5 MeV allows observation of conductance via nearest-neighbour hopping or variable-range hopping. Evidence for electron-electron interaction in the hopping conductance is presented.
TL;DR: In this article, the simple temperature dependence of carbon and germanium resistors in the variable-range hopping regime is applied for thermometry below 1 K. The linear dependence of ln(R ) on T −x ( 1 4 ⩽ x ⊽ 1 2 ) does not require the usual fitting procedure and makes calibration and thermometry more convenient.
TL;DR: In this paper, a large photoconduction was observed in amorphous In2Se3 films with an extremely slow decay rate after stopping of a steady illumination by band gap light.
Abstract: A large photoconduction has been observed in amorphous In2Se3 films. The films exhibit residual photocurrent with an extremely slow decay rate after stopping of a steady illumination by band gap light. They also exhibit photocurrent with an extremely slow rising rate after initiation of the illumination. The extremely slow decay and rising are explained by a model in which the illumination creates neutral defects through any structural change, the photocurrent is derived from the variable range hopping between the photocreated defect levels, and the photocreated defects decay according to the monomolecular reaction with a thermally activated decay rate.
TL;DR: In this paper, temperature and excitation intensity dependence of photoluminescence (PL) in hydrogenated amorphous silicon (α-Si:H) was studied.
TL;DR: In this article, an alternative mapping of the hopping problem onto a random resistor network, which takes into account correlations among the occupation probabilities of different sites, is introduced. And the effect of correlations on the low-temperature conductivity of one-dimensional chains in the nearest-neighbor and variable-range-hopping cases is investigated by approximation of the network conductances by the critical conductance at the percolation threshold.
Abstract: We introduce an alternative mapping of the hopping problem onto a random resistor network, which takes into account correlations among the occupation probabilities of different sites. The effect of correlations on the low-temperature conductivity of one-dimensional chains in the nearest-neighbor and variable-range-hopping cases is investigated by approximation of the network conductances by the critical conductance at the percolation threshold. Correlation effects are found to lead to increased activation energies and a mesoscopic behavior which is much closer to experimental data than that predicted by mean-field theory.
TL;DR: In this article, the dc resistivity of n-ZnSe in the region of the metal-dielectric transition induced by compensation is investigated in the temperature range from 1.6 to 300 K.
Abstract: In the temperature range from 1.6 to 300 K the dcresistivity of n-ZnSe in the region of the metal–dielectric transition induced by compensation is investigated. Over the dielectric part of the transition the power dependence ϱ(T) with an exponent smaller than zero in the critical region is replaced at sufficiently low temperatures by a dependence of the form ϱ(T) = ϱ0 exp (T0/T)1/2. The observable exponential dependence of ϱ on T is treated in the frames of a model of hopping conductivity with a variable range hopping over the states of the Coulomb gap near by the Fermi level. Such parameters as the screening radius Rs, which determines the size of large-scale fluctuations, the radius of localization of the electron wave function a, the dielectric constant x, and the coefficient g0 which determines the density of the localized states near the Fermi level are calculated. It is established that g0, χ, and a increase with the decrease of K = NA/ND, that is with the approach to the metal–dielectric transition. It is assumed that this is connected with the decrease of the width of the Coulomb gap and its possible disappearance in vicinity of the transition.
[Russian Text Ignored].
IBM1
TL;DR: In this article, a transition from activated band conduction to variable-range hopping was observed in the accumulation layer of hydrogenated amorphous silicon (a-Si:H) thin film transistors, for temperatures below approximately 240°K.
TL;DR: In this paper, the distribution function of hopping conduction mesoscopic fluctuations in the short channel GaAs MESFET was investigated and it was shown that the shape of short-channel log-conductance distribution function is asymmetrical and differs strongly from that for 1D hopping system.
TL;DR: In this article, a free standing polythiophene free standing film has been ion implanted with 25 keV F + ions and backscattering measurements showed that the surface of the film was modified to a depth of 0.4 μm.
TL;DR: In this article, electrical resistivity versus temperature from 50 K down to 1.5 K of Pt ǫAl2O3 and AuAl 2O3 cermets is reported.
Abstract: We report on electrical resistivity versus temperature from 50 K down to 1.5 K of PtAl2O3 and AuAl2O3 cermets. When the metal volume fraction q is below the percolation threshold qc, the dc conductivity shows a temperature dependence of the form ln σ ∝ T−n which is usually related to variable range hopping conduction. When T ∗ (T ∗ ≌4.6 K) , the exponent value is n = 1 4 and above T∗ the temperature dependence is modified with n = 1 2 . This behaviour is not clearly understood and is discussed in view of recent theoretical works. For q≌q c a metallic behaviour is observed at high temperature while at liquid helium temperature a two-dimensional electron-electron interaction effect is invoked to describe the anomalous non-metallic behaviour.
TL;DR: In this paper, the electrical characteristics of PELPCVD SIPOS-silicon heterojunctions are presented and the d.c. current density-voltage characteristics of all the samples over the entire measured temperature range are accurately predicted by the use of a variation of Mott's model for conduction in a non-crystalline medium.
Abstract: The electrical characteristics of PELPCVD SIPOS-silicon heterojunctions are presented. The d.c. current-voltage characteristics as a function of temperature for SIPOS-silicon heterojunctions, for SIPOS having silicon contents of from 31 to 79 atomic percent and film thickness of from 1400 to 6800 A are reported. At high temperatures, carrier transport occurs by thermal excitation over a mobility edge, giving an activation energy of half the bandgap. At lower temperatures, the tunneling mechanism of Variable Range Hopping (VRH) dominates the conduction process. The current density-voltage characteristics of all the samples over the entire measured temperature range are accurately predicted by the use of a variation of Mott's model for conduction in a non-crystalline medium.