scispace - formally typeset
Search or ask a question

Showing papers on "Variable-range hopping published in 1998"


Journal ArticleDOI
TL;DR: In this article, a pseudoband gap of 1.5 −2.5 eV was found for conjugated polymers, a form of carbon nanotubes, with a pseudometal conductivity of approximately 10 5 S/m.
Abstract: The discovery two decades ago that polyacetylene could be made extremely conductive on doping with iodine or AsF5 ~Ref. 1! was the initial step that indicated that conjugated polymers may be an important alternative to inorganic semiconductors as optoelectronic materials. However, conventional doping of polymers tends to quench their luminescent properties by introducing nonradiative trapping states such as solitons, polarons, and bipolarons. 2 The key feature of conjugated polymers is the extended p-electron system in their backbone, which, due to Peierls distortion, results in a pseudoband gap. In these polymers conduction proceeds by variable range hopping of polarons. The band gap is approximately 1.5‐2.5 eV, hence conductivity and mobility are generally low. Carbon nanotubes, another form of conjugated carbon system, consist of one or more sheets of graphene wrapped around each other in concentric cylinders. Individually, nanotubes may be metallic or semiconducting, but in bulk they form a pseudometal with conductivity 3 of approximately 10 5 S/m. It is well known that the soot produced in a

429 citations


Journal ArticleDOI
TL;DR: In this article, the authors measured poly(3n-decylpyrrole in the 80-330 K interval to characterize the charge transport behavior of the system and attributed the observed relaxation to the hopping charge transport, as further confirmed by the temperature behaviour of the relaxation strength.
Abstract: The d.c. conductivity and the electric a.c. response from 100 Hz up to 40 MHz of poly(3n-decylpyrrole) were measured in the 80-330 K interval to characterize the charge transport behaviour of the system. The d.c. conductivity well fitted the variable range hopping model, and the loss factor, after having deducted the d.c. contribution, showed a relaxation peak when the conductivity versus frequency started to rise. The strength of this relaxation increased with temperature and became too large to be related to a dipolar relaxation; moreover, the temperature dependence of the loss peak frequency and d.c. conductivity coincided. The observed relaxation was attributed to the hopping charge transport, as further confirmed by the temperature behaviour of the relaxation strength and by the frequency dependence of the exponents of the power law which locally approximate the conductivity behaviour. As the activation energy of the d.c. conductivity differed from the frequency of the loss peak, the theoretical prediction concerning the selfsimilarity of the a.c. conductivity was roughly verified.

121 citations


Journal ArticleDOI
TL;DR: In this article, the time and temperature dependences of conductivity of electrochemically prepared polyindole and polycarbazole perchlorates are analyzed to understand the aging process and mechanism of conduction.

78 citations




Journal ArticleDOI
TL;DR: In this article, the temperature dependence of resistivity for both PANI and PANI-MC has been analyzed by three-dimensional variable range hopping (VRH) type of conduction.
Abstract: Electrical resistance and magnetoresistance of the HCl doped polyaniline (PANI) in aqueous ethanol medium and polyaniline–methyl cellulose dispersion (PANI–MC) has been investigated at low temperature down to 1.8 K and in magnetic field up to 8 T. The weaker temperature dependence of resistivity characterized by the resistivity ratio ρr=ρ(1.8 K)/ρ(300 K) indicates that a better homogeneity and less disorder can be obtained by protonation with HCl in ethanol media. The temperature dependence of resistivity for both PANI and PANI–MC has been analyzed by three-dimensional variable range hopping (VRH) type of conduction. The samples with the resistivity ratio lying in the range of 102⩽ρr⩽103 exhibit a crossover from Mott to Efros–Shklovskii VRH conduction below 10 K. A large positive magnetoresistance at low temperature has been obtained.

50 citations


Journal ArticleDOI
TL;DR: In this paper, three electronic transport mechanisms, namely, transport in extended states, in band tails by hopping and variable range hopping near the Fermi level, were observed from the thermal activation measurements in the temperature range from 100 to 450 K.
Abstract: Highly tetrahedral amorphous carbon thin films were deposited by the filtered cathodic vacuum arc technique at room temperature. Nitrogen was found to be a good n-type dopant of the tetrahedral amorphous carbon thin films. The Fermi level shifts from 0.91 eV above the valence band to 0.65 eV below the conduction band with increasing nitrogen flow rate from null to 16 sccm (nitrogen partial pressure from 0 to ). At the same time, the optical band gap drops from 2.7 to 1.8 eV. Three electronic transport mechanisms, namely, transport in extended states, in band tails by hopping and variable range hopping (VRH) near the Fermi level, were observed from the thermal activation measurements in the temperature range from 100 to 450 K. The VRH transport parameters for ta-C films are studied, and the density of states near the Fermi level extracted from the hopping transport parameters was found in the range of . The dominant doping configuration is the substitution in the coordination at low N concentration and adoption of bonding at high N concentration.

45 citations


Journal ArticleDOI
TL;DR: In this article, the conduction mechanism in damaged and partially graphitized diamond is described by the variable range hopping mechanism, allowing the extraction of the number of hopping sites at different stages of thermal treatment from the temperature dependence of resistivity.

42 citations


Journal ArticleDOI
TL;DR: The role of the JT polaron in determining the electronic and magnetic properties above is discussed in this article, where the conductivity follows Mott's VRH model and provides evidence of the formation of spin clusters.
Abstract: Resistivity (15 - 710 K), magnetization (80 - 723 K), and calorimetric (200 - 623 K) measurements on have shown evidence for the formation of Jahn - Teller (JT) polarons below . Above 560 K, activated conduction with is observed and shows Curie - Weiss behaviour. Over the temperature range 270 - 340 K, the conductivity follows Mott's VRH model and provides evidence of the formation of spin clusters. Calorimetric study shows phase transitions at , 345 K and 545 K. The role of the JT polaron in determining the electronic and magnetic properties above is discussed.

38 citations


Journal ArticleDOI
TL;DR: In this article, an experimental study of electrical transport phenomena in magnetic-fields H up to 5 T in pressed samples of a porous system composed of multiwalled carbon nanotubes is reported.
Abstract: An experimental study of electrical transport phenomena in magnetic-fields H up to 5 T in pressed samples of a porous system composed of multiwalled carbon nanotubes is reported. The exp[(T0/T)1/4] temperature dependence in the zero-field dc electrical resistance R(0) is found at low temperatures. Below 50 K, transverse magnetoresistance (MR), [R(H)−R(0)]/R(0), is negative. In the weak-field region (H<0.1 T), the negative MR is quadratic in H, and becomes linear in the field range 0.1–0.3 T. The linear and negative MR shows T−0.65±0.06 temperature dependence. The temperature dependence of R(0) is described in terms of Mott’s law for three-dimensional variable range hopping (3D VRH) conduction. The low-field magnetotransport features support a predicted quantum interference effect on 3D VRH conduction.

36 citations


Journal Article
TL;DR: The role of the JT polaron in determining the electronic and magnetic properties above T c is discussed in this paper, where phase transitions at T c (=245 K), 345 K and 545 K are discussed.
Abstract: Resistivity (15-710 K), magnetization (80-723 K), and calorimetric (200-623 K) measurements on La 0.7 Ca 0.3 MnO 3 have shown evidence for the formation of Jahn-Teller (JT) polarons below ∼545 K. Above 560 K, activated conduction with E a = 0.05 eV is observed and X -1 (T) shows Curie-Weiss behaviour. Over the temperature range 270-340 K, the conductivity follows Mott's VRH model and X -1 (T) provides evidence of the formation of spin clusters. Calorimetric study shows phase transitions at T c (=245 K), 345 K and 545 K. The role of the JT polaron in determining the electronic and magnetic properties above T c is discussed.

Journal ArticleDOI
TL;DR: In this paper, the authors reported magnetotransport of (Ga, Mn)As below 1 K in the reentrant insulating phase, where the external magnetic field drove the samples from the strongly insulating regime to the variable range hopping one.
Abstract: We report magnetotransport of (Ga, Mn)As below 1 K in the reentrant insulating phase. The external magnetic field drove the samples from the strongly insulating regime to the variable range hopping one. Below 1 K, the resistivity was strongly anisotropic (by about two orders of magnitude). The conduction along the highly resistive direction ([11-0]) was well described by variable range hopping in the soft Coulomb gap regime while that along the lower resistive direction ([110]) seemed to undergo an insulator-to-metal transition by the external magnetic field. The result may be a key to solve the problem of reentrant metal-to-insulator transition in (Ga, Mn)As with increasing Mn content.

Journal ArticleDOI
TL;DR: In this article, the conduction mechanism is found to be due to variable range hopping near the Fermi level for temperatures below 150 K and tunneling of carriers in the localized states in the band edges.
Abstract: Thermoelectric power measurements have been made on macroporous (pore width >500 A°) porous silicon samples prepared by an anodic dissolution technique. The sign of thermopower is found to be negative indicating that conduction takes place due to electrons in the conduction band. The conduction mechanism is found to be due to variable range hopping near the Fermi level for temperatures below 150 K. At higher temperatures the conduction is due to the tunneling of carriers in the localized states in the band edges. It was concluded that these localized states are formed because the nanocrystallites in porous silicon are randomly distributed in size and orientation leading to fluctuations in band gap. This results in the constitution of a disordered system on a macroscopic scale.

Journal ArticleDOI
TL;DR: In this paper, electrical resistance and magnetoresistance of the HCl-doped polyaniline (PANI) in aqueous ethanol have been investigated at low temperature down to 1.8 K and in magnetic field up to 8 T.

Journal ArticleDOI
TL;DR: In this article, the authors measured the conductivities of HCl-doped polyaniline in the temperature range 77-300 K and found that at 77 K, the a.c. showed dispersion at 77 k, which decreases with an increase in the doping level.
Abstract: The d.c. and a.c. (100 Hz–1 MHz) conductivities of HCl-doped polyaniline have been measured in the temperature range 77–300 K. At 77 K, the a.c. conductivity data, σ(ω), can be described by the relation σ(ω)=Aωs, where the parameter s lies close to unity and decreases with increase in the doping level. The ratio of measured a.c. to d.c. conductivity shows dispersion at 77 K, which decreases with increase in the doping level. This decrease is found to be sharp around pH∼3.0. In the temperature range 77–150 K, the observed d.c. conductivity data can be described by Mott's three dimensional variable range hopping (VRH) model. Scanning electron microscopy studies reveal a sharp change in structural morphology of HCl-doped polyaniline at a pH∼3.0. A strikingly remarkable structural morphology has been observed in the formc of a channel at this pH value. This change is accompanied by a rapid increase in d.c. conductivity, dielectric constant, along with sharp changes in structural morphology, which indicates the existence of a doping-induced structural conductivity correlation in this system. © 1998 Chapman & Hall

Journal ArticleDOI
TL;DR: In this article, the power law dependence of conductivity σ(T)∝Tβ is observed for the sample close to metallic side of the M-I boundary with β=0.83 for 20
Abstract: Transport data for paratoluene sulfonate dope polypyrrole films in the insulating regime and near the metal-insulator (M-I) boundary are presented and analyzed. Samples in the insulating region show a crossover from Mott to Efros–Shklovskii variable range hopping conduction at T=5 K and magnetoconductance of these samples is also explained by variable range hopping theory. The power law dependence of conductivity σ(T)∝Tβ is observed for the sample close to metallic side of the M-I transition with β=0.83 for 20 K

Journal ArticleDOI
TL;DR: In this article, a non-Arrhenius dependence on 1/T was found, and actual activation energy Eact was determined by derivation of the log σ versus T−1.
Abstract: Temperature dependence of dc conductivity in hydrogenated amorphous carbon (a-C:H) samples, deposited in the range of −200 to −700 V self-bias voltages, was studied above room temperature from 300 K to 650 K. Because a non-Arrhenius dependence on 1/T was found, actual activation energy Eact was determined by derivation of the log σ versus T−1. This Eact increased gradually with temperature by a rate depending on self bias. Strong fluctuations of Eact, observed above 500 K, indicates a transition to another conduction mechanism. Below 500 K the plot of log σ versus T−1/4 could be fitted to linear functions, with an unrealistic density of states at Fermi energy, supporting, that underlying mechanism cannot be variable range hopping near EF rather it is related to conduction by hopping of carriers excited to band tails.

Journal ArticleDOI
TL;DR: In this article, the dc conductivity of polypyrrole films doped with p-toluene sulfonic acid was measured in the temperature range of 77 to 300 K, using a modified four-probe rig.

Journal ArticleDOI
TL;DR: In this paper, high electric field conduction properties of polypyrrole films prepared by anodic deposition as a function of temperature were measured and it was shown that the grain tunnelling model is still a better description than variable range hopping.

Journal ArticleDOI
TL;DR: In this article, the authors found that micro-cracks in thin Nd2/3Sr1/3MnO3 films create a series of intrinsic break junctions which are ideal for investigating tunneling phenomena in this system.
Abstract: We found that microcracks in thin Nd2/3Sr1/3MnO3 films create a series of intrinsic break junctions which are ideal for investigating tunneling phenomena in this system. A comparison of films with and without cracks, which have similar ferromagnetic Curie temperature (Tc) of 140–150 K, shows that the cracked film has a lower insulator to metal transition temperature TM (97 K vs 140 K), three orders of magnitude higher resistivity at TM, and two times larger magnetoresistance at 1 T near TM. At T>TM we observed that ln ραT−1/4 in the uncracked film while in the cracked film a ln ραT−1/2 dependence was found. This indicates that the conductivity in the first case is due to variable range hopping in three dimension, while in the second case it is dominated by thermally activated tunneling across the insulating barriers (the microcracks).

Journal ArticleDOI
TL;DR: In this article, the slave boson mean field studies of the ground state of the Hubbard model with correlated hopping were performed, and the results for the case of the hopping integral t equal to the correlated hopping integral X were recovered.
Abstract: The slave boson mean-field studies of the ground state of the Hubbard model with correlated hopping were performed. The approach qualitatively recovers the exact results for the case of the hopping integral t equal to the correlated hopping integral X. The phase diagram for the strongly correlated state with only singly occupied sites, the weakly correlated state, where single and double occupation is allowed, and for the superconducting state, was determined for any values of X and any electron concentration n. At the half-filled band (n=1) a direct transition from the superconductor to the Mott insulator was found. In the region of strong correlations the superconducting solution is stable for n close to 1, in contrast to the case of weak correlations, in which superconductivity occurs at n close to 0 and n close to 2. We found also that strong correlations change characteristics of the superconducting phase, e.g. the gap in the excitation spectrum has a nonexponential dependence close to the point of the phase transition.

Journal ArticleDOI
TL;DR: In this article, the Davydov-Scott model is extended to include distance dependent nonlocal hopping and/or a nonlinear lattice, which leads to a rich phase diagram for the corresponding minimum energy states.

Journal ArticleDOI
TL;DR: In this article, the electrical transport properties of polycrystalline semiconducting β-FeSi2 films have been evaluated by conductivity measurements over the temperature range 50-300 K.
Abstract: The electrical transport properties of polycrystalline semiconducting β-FeSi2 films have been evaluated by conductivity (σ) measurements over the temperature range 50–300 K. At low temperatures (T<200 K), a variable range hopping conduction was observed, from which the number of states near the Fermi level and the degree of disorder in the material were obtained. At moderate temperatures (200–300 K), the ln σ vs 103/T curves show anomalous features such as kinks or continuous bending. In this temperature range, the conductivity data satisfy the Meyer–Neldel rule, (MNR), which is of fundamental importance for the transport properties of the β-FeSi2. The results show that the MNR parameters are related with the degree of disorder in the material.

Journal ArticleDOI
TL;DR: In this article, the frequency-dependent conductivity of randomly packed aggregates of ligand-stabilized, monodisperse metal clusters from dc to the infrared over 12 decades in frequency was studied.
Abstract: We have studied the frequency-dependent conductivity of randomly packed aggregates of ligand-stabilized, monodisperse metal clusters from dc to the infrared over 12 decades in frequency. At low frequencies the ac conductivity of these compounds is dominated by phonon-assisted hopping between localized states, and shows scaling behavior as a function of temperature and frequency. Due to the unique structure of these cluster aggregates, the transition from phonon- to photon-mediated charge transport is clearly observed. The data are found to be in excellent quantitative agreement with the two-site tunneling model. @S0163-1829~98!50324-4# Charge transport in disordered systems, where conduction proceeds by hopping between localized states, has been a widely studied subject over the last decades. A powerful method to study hopping transport is to perform conductivity experiments where both temperature and applied frequency are varied. In this way, not only the various theoretical hopping models can be tested, 1 but also insight in the structural properties of the material at mesoscopic length scales may be gained. 2,3

Journal ArticleDOI
TL;DR: In this paper, the electrical transport properties of have been investigated using complex-plane impedance analysis and dielectric measurements with measurements of four-probe dc conductivities and magnetic susceptibilities.
Abstract: The electrical transport properties of have been investigated using complex-plane impedance analysis and dielectric measurements with measurements of four-probe dc conductivities and magnetic susceptibilities. The temperature dependencies of the bulk conductivities obtained by impedance analysis, and dielectric relaxation processes provide evidence of the hopping conduction below 60 K, which is remarkably different from the hopping conduction in , despite the degree of doping with Sr being very small. The features in the low-doping range are: (i) the emergence of the hopping conduction at markedly lower temperatures in comparison with the case for non-doped ; (ii) the abrupt reduction of the activation energy required for the conduction, 0.042 eV at x = 0.03 to 0.021 eV at x = 0.07, from 0.34 eV for ; and (iii) a progressive decrease in the activation energy with increasing x. The difference in conduction behaviour between and is due to the alternation of the spin state of caused by the Sr doping. The results have been discussed in terms of a process of hopping of small polarons localized by the electron-lattice interaction in tandem with the electron-magnon interaction.

Journal ArticleDOI
TL;DR: In this paper, the authors established that variable-range hopping conduction takes place between states localized near the Fermi level in layered TlGaS2 and TlInS2 single crystals both along and across their natural layers in a constant electric field at T⩽200 K.
Abstract: It is established that variable-range hopping conduction takes place between states localized near the Fermi level in layered TlGaS2 and TlInS2 single crystals both along and across their natural layers in a constant electric field at T⩽200 K. The densities of states near the Fermi level and the hopping distances at different temperature are estimated. The occurrence of activationless hopping conduction is established in TlGaS2 and TlInS2 single crystals in the temperature range 110–150 K.

Journal ArticleDOI
TL;DR: In this paper, a photochemical deposition method was proposed to make amorphous films of photochemically active precursors, which were then exposed to light and, in air, converted to metal oxides.
Abstract: This paper describes a new approach to making the active elements of gas sensors using a photochemical deposition method. The method involves making amorphous films of photochemically active precursors. These precursors are then exposed to light and, in air, convert to metal oxides. Amorphous films constructed of W(CO){sub 4}(Et{sub 2}-en) are deposited on interdigitated microelectrodes. Photolysis of these films, in air at room temperature, results in the deposition of amorphous films of tungsten oxide. This forms the sensing element of an NO{sub x} sensor. Films thus prepared were also annealed to yield sensors whose active elements were crystalline tungsten oxide. An investigation of the conduction mechanism in the sensor materials was performed. The conduction in the polycrystalline materials is controlled by grain boundaries through thermionic emission. Absorption of NO{sub 2} leads to a modification of the grain boundary resulting in the change in current. In contrast, the amorphous materials conduct via variable range hopping. In this case, absorption of NO{sub 2} leads to a reduction in the number of carriers and a change in conductivity of the material. The response of the amorphous materials has been fit to a model based on a Langmuir isotherm.

Journal ArticleDOI
TL;DR: In this article, the low-field mobility of a small concentration of charge carriers hopping among a random distribution of transport sites is studied, as a function of the mean interparticle spacing ρ and the temperature T, for model systems having different site-energy distribution functions.
Abstract: The low-field mobility μ of a small concentration of charge carriers hopping among a random distribution of transport sites is studied, as a function of the mean interparticle spacing ρ and the temperature T, for model systems having different site-energy distribution functions. For a uniform density of states our calculations show that the mobility obeys empirical scaling laws similar to those found in the theory of variable-range hopping. For a binary distribution of site energies we observe a crossover as a function of site density between trap-limited conduction and trap-mediated conduction. For a Gaussian density of states our results confirm the quadratic inverse temperature dependence of lnμ found in Monte Carlo studies, although quantitative characterization of this dependence is found to depend sensitively on the degree of spatial disorder in ways that could impact the extraction of microscopic parameters from experimental data.

Journal ArticleDOI
TL;DR: The 8−N rule, variable range hopping, the Austin-Mott ac conductivity, the mobility edge, and the minimum metallic conductivity of amorphous semiconductors are reviewed in this paper.
Abstract: We review some of the fundamental concepts which have been introduced into the field of amorphous semiconductors by Professor Sir Nevill Mott. These include the 8−N rule, variable range hopping, the Austin–Mott ac conductivity, the mobility edge, and the minimum metallic conductivity. We demonstrate that there are still severe problems, although there is no real alternative to Mott's concepts.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the magnetic and electrical properties at low temperature down to 1.8 K and found that magnetic susceptibility and magnetoresistance increase with the increase of x. The increase in magnetic susceptibility is due to reduction in Mn-O-Mn bond angle and the mobility of charge carriers.
Abstract: Electrical and magnetic properties of have been extensively investigated at low temperature down to 1.8 K. The irreversibility of magnetic susceptibility between zero field cooling and field cooling increases with increasing Y content. The metal-insulator transition temperature shifts towards lower temperature and the peak resistivity and magnetoresistance increase with the increase of x. Electrical conduction above is dominated by the variable range hopping mechanism. The increase in resistivity at low temperature has been interpreted in terms of electron-electron interaction. The enhancement of magnetoresistance with the increase of Y content is due to the reduction in Mn-O-Mn bond angle and the mobility of charge carriers.