Showing papers on "Variable-range hopping published in 1993"
TL;DR: In this paper, the electrical conductivity of polyaniline was investigated with a quasi-one-dimensional variable-range-hopping (VRH) mechanism and a linear dependence of the conductivity on temperature with T−1/2.
Abstract: The electrical conductivity of chemically synthesized and annealed polyaniline has been investigated. The room‐temperature conductivity varies with the time and the temperature of annealing, and exhibits a maximum peak on its time‐dependence curves for the polymer annealed above 120 °C. The time showing the maximum peak of the conductivity shifts to the shorter side with higher annealing temperature. The linear dependence of the conductivity on temperature with T−1/2 is observed in all samples as explained for this polymer with the quasi‐one‐dimensional variable‐range‐hopping processes. In the initial stage of annealing, the conductivity increases in the whole temperature range without varying the exponential factor of the variable range hopping. This indicates that the density of states remains unchanged in this stage. The results concerning the pre‐exponential factor of the quasi‐one‐dimensional variable‐range‐hopping mechanism demonstrate the increase of the mean free time, the transverse localization length, and the interchain transfer integral with annealing. X‐ray‐diffraction patterns exhibit the generation of regularized amorphous structure in annealed samples, which leads to the enlargement of the interchain interaction. The results show the increase of regularity of the polymer chain at first, and the phase transition to the loose‐packed and ordered structure in next stage of annealing.
95 citations
TL;DR: In this article, the electrical conductivity of Na and Li-implanted diamond was investigated and it was shown that the conduction may be understood in terms of variable range hopping between implant sites in the crystal rather than due to thermal activation.
Abstract: The electrical conductivity of Na‐ and Li‐implanted diamond is investigated. The highest conductivities are obtained for high dose implantations followed by thermal graphitization of the heavily damaged layer and chemical removal of the graphitized layer. The temperature dependence of the resistivity is measured, yielding activation energies of 0.2 eV for Li (400
70 citations
TL;DR: When polyimide (Kapton) is irradiated by a krypton fluoride (KrF) laser, an increase of the electrical conductivity of up to 16 orders of magnitude is observed.
Abstract: When polyimide (Kapton) is irradiated by a krypton fluoride (KrF) laser, an increase of the electrical conductivity of up to 16 orders of magnitude is observed In the high conduction regime, the resistivity is about 01 Ω cm, the current voltage characteristic is ohmic and the contacts of gold and silver with the irradiated conducting polymer are also ohmic The conduction mechanism is phonon-assisted variable range hopping, evident from the observed temperature and electric field dependence of the resistivity at low conductivities The laser-induced conductivity depends on the ambient atmosphere during irradiation Transmission spectroscopy in the visible region and infrared Fourier transform spectroscopy have been used to characterize the material A thermal mechanism is proposed for the formation of conducting polyimide, by excimer-laser irradiation
67 citations
TL;DR: It is shown that in the strong-scattering limit the magnetoresistance is universal in the sense that it does not depend on the scattering strength.
Abstract: An alternative approach to the calculation of the scattering-induced magnetoresistance in the regime of variable-range-hopping conductivity is developed. Under the assumption that the direct tunneling and the various single-scattering tunneling paths give the main contribution to the amplitude of a hop, the exact averaging over the positions and energies of scatterers of the logarithm of the resistance is performed. It is shown that in the strong-scattering limit the magnetoresistance is universal in the sense that it does not depend on the scattering strength. The transition from negative magnetoresistance in low magnetic fields, caused by the suppression of the destructive interference, to positive magnetoresistance in strong enough fields, caused by the orbital shrinkage of the impurity wave functions, is traced. The theory developed is applied to the system of two parallel impurity planes in a magnetic field parallel to the planes. The oscillations of the hopping resistance with magnetic field in such a system are studied quantitatively.
49 citations
TL;DR: In this paper, the authors present charge transport studies on recently developed molecular-based magnets V(TCNE)x⋅y(solvent) (TCNE=tetracyanoethylene) which show local magnetic ordering at temperatures, T, as high as 400 K.
Abstract: We present charge transport studies on recently developed molecular‐based magnets V(TCNE)x⋅y(solvent) (TCNE=tetracyanoethylene) which show local magnetic ordering at temperatures, T, as high as 400 K. V(TCNE)x⋅y(solvent) prepared from the solvent CH2Cl2 has σ(300 K)∼10−3 S/cm, and that prepared from the solvent CH3CN has σ(300 K)∼10−5 S/cm. The σ(T) of both materials follows the Mott 3D variable range hopping [log(T1/2σ)∝T−1/4] behavior. An anomalously strong T‐dependent ac conductivity is likely caused by the short‐range ferrimagnetic correlations. Cole–Cole analysis of the ac complex dielectric constant suggests there are two relaxation mechanisms, with an Arrhenius relation fitted to both. The microwave frequency response is consistent with audio frequency data and yields a localization length of ∼ 5 A, comparable with the dimensions of a [TCNE]−. group.
45 citations
TL;DR: In this paper, the temperature dependece of resistivity was reported for the PrBa 2 Cu 3− x Ga x O 7− y system with x = 0, 0.01, 1.5, 2.5 and 0.6.
Abstract: The temperature dependece of resistivity is reported for the PrBa 2 Cu 3− x Ga x O 7− y system with x =0, 0.01, 0.03, 0.05, 0.07, 0.09, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6. We find that the substitution of Cu by Ga increases the resistivity of PrBa 2 Cu 2.4 Ga 0.6 O 7− y to reach 10 10 Ωcm at 100 K. This is a 123-structural insulator and thus an ideal barrier material for fabrication of S-I-S junctions. The results are interpreted in terms of hopping conductivity (both nearest-neighbor hopping and variable-range hopping) for doped semiconductors.
35 citations
TL;DR: The analysis of primary hopping events shows that there might be a relationship between the distribution of the hopping centers and the biochemical activity of insulin, and that the disulfur bridges of insulin have particular relevance for the ac conduction.
Abstract: The ac conductivity for a native protein, pig insulin, was calculated in the ab initio scheme using Clementi's minimal basis set. The hopping centers and «main residue» of an orbital are defined. It is assumed that the hopping events of charge carriers happen among these centers of main residues. The analysis of primary hopping events shows that there might be a relationship between the distribution of the hopping centers and the biochemical activity of insulin, and that the disulfur bridges of insulin have particular relevance for the ac conduction. The formulas for calculation of the ac conductivity of proteins are given
28 citations
TL;DR: In this article, a small polaron hopping model is applied to the glass system and the physical properties such as density, number of ions per unit volume, hopping distance, polaron radius, d,c. conductivity and activation energy are examined.
Abstract: The physical and transport properties of semiconducting glasses are highly interesting, providing useful information regarding the structure and conduction mechanism, respectively. The physical and transport properties such as density, number of ions per unit volume, hopping distance, polaron radius, d,c. conductivity and activation energy are reported. Similarly the hopping conduction mechanism is examined. The physical property (density) is used to determine the probable structure of the glass sample. The small polaron hopping model is applied to the glass system. The methods of examining the hopping conduction mechanism are discussed in the light of the small polaron model. The methods suggested by Holstein and by Sayer and Mansingh are applied to the determination of adiabatic or non-adiabatic hopping conduction. It was observed that in this system adiabatic hopping conduction is present.
25 citations
15 Dec 1993
TL;DR: In this article, the authors present a model for the description of the temperature and concentration dependence of the mobility of photoinjected charge carriers in molecularly doped polymers on the basis of polaron transport in disordered media.
Abstract: We present a model for the description of the temperature and concentration dependence of the mobility of photoinjected charge carriers in molecularly doped polymers on the basis of polaron transport in disordered media. We develop an existing variable range hopping technique to incorporate excluded volume effects which can be of importance in molecularly doped polymers, use the technique in conjunction with a Gaussian distribution of site energies in the polymeric solid, apply the formalism to calculate the mobility and address recent observations on hole motion in TPD and TTA in polycarbonate matrices. We show that the competition between spatial and energetic disorder can provide a reasonable mechanism for the observed concentration dependence of the activation energy for charge transport. Our theory thus provides an alternative to a recently proposed explanation based on an adiabatic-diabatic polaron transition.
25 citations
TL;DR: The physical properties of the copper polyaniline vermiculite intercalation compound [Cu(PANI)VIC] have been studied by x-ray diffraction, electrical resistivity, and dc-magnetic-susceptibility measurements.
Abstract: The physical properties of the copper polyaniline vermiculite intercalation compound [Cu(PANI)VIC] have been studied by x-ray diffraction, electrical resistivity, and dc-magnetic-susceptibility measurements. Aniline is intercalated into the interlamellar space of Cu-VIC, where the Cu 2+ ions act as oxidation centers for the polymerization of aniline, forming an insulator of polyaniline. A conductor is achieved by protonation via exposure to HCI gas
17 citations
TL;DR: In this paper, the temperature dependence of the dc conductivity for polyamide-imide films, implanted with 50 keV As ions to different fluences, has been studied and high-resolution data reveal a two-component conductivity that depends on both one-dimensional variable range hopping (VRH) and three-dimensional VRH.
Abstract: The temperature dependence of the dc conductivity for polyamide‐imide films, implanted with 50 keV As ions to different fluences, has been studied. Our high‐resolution data reveal a two‐component conductivity that depends on both one‐dimensional variable range hopping (VRH) and three‐dimensional VRH. For low fluence levels (5×1015 ions/cm2), the one‐dimensional VRH is dominant while at higher fluences (1×1017 ions/cm2), the three‐dimensional VRH dominates. These materials become highly disordered and form hard carbon materials along the ion track. The conductivity may be explained by 1D VRH along the ion track at low fluences while at higher fluences, regions of carbon rich material form three‐dimensional structures where 3D VRH exists. This rather simple model and its composite conductivity can explain the exact curvature of the temperature dependent conductivity, while single VRH models and percolation models cannot.
TL;DR: Magnetic polarons in magnetically disordered systems (for brevity, spin-glass polarons) are in one respect very unlike other polarons: Usual polaron hopping at low T proceeds via tunneling, and these polarons do not affect the T dependence or the hopping rate.
Abstract: Magnetic polarons in magnetically disordered systems (for brevity, spin-glass polarons) are in one respect very unlike other polarons: Usual polaron hopping at low T proceeds via tunneling, and these polarons do not affect the T dependence or the hopping rate, while the spin-glass polaron tunneling is suppressed by the disorder, and their hopping proceeds via activation for all T. It gives rise to the reentrance or the simple activated T dependence or variable range hopping conductivity at very low T. Recent experiments in CdMnTe are discussed
TL;DR: In this paper, the electrical properties of polypyrrole films fabricated on a ferric stearate template have been studied and it was shown that they exhibit a highly organized two-dimensional linear array of polymer granules sandwiched between insulating fatty acid layers.
TL;DR: In this article, the authors performed light ion irradiation experiments on high quality YBCO thin films, and studied the metal insulator transition induced by disorder, showing that at small damage level the observed decrease of Tc is compatible with a depairing process.
TL;DR: Temperature-dependent measurements show that VRH conductivity at low energies is in agreement with the existence of a soft Coulomb gap at the Fermi level, and the so-called crossover phenomenon in VRHonductivity which reduces the effect of the Coulombgap at higher magnetic fields and temperatures is discussed.
Abstract: The variable-range-hopping (VRH) resistance of n- and p-type neutron-transmutation-doped (NTD) Ge was measured at temperatures down to 30 mK and in magnetic fields up to 7 T. It is shown that the temperature dependence of the VRH resistance can be reduced to a universal curve for all samples and fields, from which one can determine the density of states (DOS) near the Fermi level. Temperature-dependent measurements show that VRH conductivity at low energies is in agreement with the existence of a soft Coulomb gap at the Fermi level. The tendency to a constant DOS at higher energies is shown: the so-called crossover phenomenon in VRH conductivity which reduces the effect of the Coulomb gap at higher magnetic fields and temperatures is discussed.
TL;DR: In this article, the magnetoresistance in the metallic region is interpreted in terms of weak localization, while that in the nonmetallic region is explained by field effect to variable range hopping conduction among Anderson localization states.
Abstract: Highly conductive heavily iodine-doped polyacetylene changes from metallic to nonmetallic by room-temperature aging. For this change, thermoelectric power varies little, suggesting that the transition is not ascribed to change in carrier concentration but to increase of the degrees of disorder. Corresponding to the transition, low temperature magnetoresistance changes from negative to positive on crossing the transition. The magnetoresistance in the metallic region is interpreted in terms of weak localization, while that in the nonmetallic state is explained by field effect to variable range hopping conduction among Anderson localization states. The transition to the nonmetallic state proceeds inhomogeneously.
TL;DR: In this article, Mott's variable range hopping in one dimension was studied in detail, and it was shown that the large conductance fluctuations in disordered insulators result from a subtle interplay between purely quantum phenomena and geometrical fluctuations arising from the energies and locations of the impurities.
Abstract: We anajyse in detail Mott's variable range hopping in one dimension, expanding on earlier work by Raikh and Ruzin. We show that the large conductance fluctuations in disordered insulators result from a subtle interplay between purely quantum phenomena and geometrical fluctuations arising from the energies and locations of the impurities. Our results compare very well with both experiments and numerical simulations.
TL;DR: In this paper, the electrical conductivity of the semiconducting glassy silicon vanadates in the temperature range 80-400 K was measured and the experimental results have been analyzed with reference to the existing theoretical models of hopping conduction.
Abstract: Measurements are reported for the electrical conductivity of the semiconducting glassy silicon vanadates in the temperature range 80–400 K The experimental results have been analyzed with reference to the existing theoretical models of hopping conduction The analysis shows that at high temperatures the conductivity data are consistent with Mott’s model [J Non‐Cryst Solids 1, 1 (1968)] of phonon‐assisted polaronic hopping, while Mott’s variable range hopping model [Philos Mag 19, 835 (1969)] is valid at low temperatures The small polaron model of Schnakenberg [Phys Status Solidi 28, 623 (1968)] is also found to be consistent with the temperature dependence of the conductivity The temperature dependence of the conductivity can also be interpreted in the framework of the percolation model proposed by Triberis and Friedman [J Phys C 18, 2281 (1985)] The analysis of the conductivity data also indicates that the hopping in these vanadate glasses occurs in the nonadiabatic regime
TL;DR: In this paper, the authors showed that the electrical resistivities of pentacene thin films doped with iodine in the direction parallel to the substrate plane fit the T−1/3 dependence well.
Abstract: The logarithmic electrical resistivities of pentacene (PEN) thin films doped with iodine in the direction parallel to the substrate plane fit the T−1/3 dependence well. This indicates that the lateral transport of the film was governed by two‐dimensional variable range hopping. In contrast, the logarithmic resistivities of the films perpendicular to the substrate were observed to be almost independent of temperature and to be weakly T−1/2 dependent. This shows that the tunneling mechanism is predominant in perpendicular transport. Two‐dimensional conduction of the film was confirmed by these transport properties, which was expected from the intercalated structure of PEN film doped with iodine.
TL;DR: A new mechanism for the positive magnetoconductance is suggested: the Zeeman effect increases the number of high-spin states, in which the orbitals at the Fermi level are more extended than in the low- spin states owing to the weaker short-range correlation effect.
Abstract: Both the correlation effect and the interference effect are taken into account in hopping conduction in the strongly Anderson-localized regime. A new mechanism for the positive magnetoconductance is suggested: the Zeeman effect increases the number of high-spin states, in which the orbitals at the Fermi level are more extended than in the low-spin states owing to the weaker short-range correlation effect. The magnetoconductance is independent of the direction of the magnetic field and can be observed in nearest-neighbor hopping conduction.
TL;DR: In this article, Mott's variable range hopping in one dimension was analyzed in detail, and it was shown that the large conductance fluctuations in disordered insulators result from a subtle interplay between purely quantum phenomena and geometrical fluctuations arising from the energies and locations of the impurities.
Abstract: We analyse in detail Mott's variable range hopping in one dimension, expanding on earlier work by Raikh and Ruzin. We show that the large conductance fluctuations in disordered insulators result from a subtle interplay between purely quantum phenomena and geometrical fluctuations arising from the energies and locations of the impurities. Our results compare very well with both experiments and numerical simulations.
TL;DR: In this paper, the temperature dependence of the dark electrical conductivity and of the thermopower were measured for bulk Ge30Sb10S60 alloyed with CoS down to 10 K.
Abstract: The temperature dependence of the dark electrical conductivity and of the thermopower were measured for bulk Ge30Sb10S60 alloyed with CoS down to 10 K. Mott's variable-range hopping was found to be dominant in the whole temperature range from 300 K to 10 K. Co-induced gap states in the middle of the mobility gap are supposed to be responsible for these observations.
TL;DR: In this article, the conduction mechanism is found to be phonon assisted variable range hopping between small (~10 nm) carbon rich clusters that form 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 KrF (248 nm) excimer laser irradiation. The conduction mechanism is found to be phonon assisted variable range hopping between small (~10 nm) carbon rich clusters that form a macroscopic percolation cluster. Using a holographic technique, periodic line structures with periods ranging from 166 nm to 950 nm have been produced in polyimide by direct ablation with a KrF excimer laser. Taking advantage of the large nonlinearity in the laser ablation process, linewidths ranging from 30 nm to several hundred nm could be obtained. This technique was combined with the ability to induce electrical conductivity in polyimide to produce an array of 500 nm wide electrically conducting wires. The conductivity of these wires was similar to that found in macroscopic regions of laser induced conductivity.
TL;DR: The value of the Hall coefficient remains approximately classical, R H =1/nec, for diverging longitudinal resistivity over a broad range of loalization strength, and the results show the behavior of a recently proposed Hall insulator.
Abstract: We have studied the Hall coefficient for a two-dimensional electron gas in a GaAs/Al x Ga 1-x As heterojunction characterized by variable range hopping. We find the value of the Hall coefficient remains approximately classical, R H =1/nec, for diverging longitudinal resistivity over a broad range of loalization strength. The results show the behavior of a recently proposed Hall insulator in which σ xx →0, σ xy →0, and ρ xx →∞ but ρ xy →const for T→0
TL;DR: In this article, the temperature dependence of variable range hopping resistivity ρ in NTD isotopically enriched70Ge:Ga samples was investigated and the results were compared with theoretical predictions for variable-range hopping conduction.
Abstract: The temperature dependence of variable range hopping resistivity ρ in neutron transmutation doped (NTD) isotopically enriched70Ge:Ga samples is reported. Five samples with compensation ratios K less than 0.001 and Ga concentrations between 3×1016 and 1.77×1017 cm−3 were studied. All samples investigated show the ln ρ∝T−1/2 dependence in the temperature range below 1.5K. As thermistor materials NTD70Ge:Ga samples are found to have more than factor of two higher sensitivity than commonly used natural NTD Ge in the temperature range between 0.2K and 1K. Our results are compared with theoretical predictions for variable range hopping conduction.
TL;DR: In this paper, the electron assisted hopping rate of an atom is calculated for an assymmetric double well potential and it is shown that the intuitive results of Vladar and Zawadowski based on barrier fluctuation agree with the matrix elements obtained by direct evaluation of the electron scattering amplitude due to the atom.
TL;DR: In this paper, the authors suggest that the most plausible explanation for the tunnel probability for one of the hops on the best-conductive chain of defects which are caused by a spontaneous rearrangement of the atomic structure of the material.
TL;DR: It is found that substantial increase in the density of states at the Coulomb gap occurs only when the ratio between the hopping integral t and the gap width B exceeds a critical value.
Abstract: In this paper, we use the coherent potential approximation to study the role quantum hopping plays on the Coulomb gap of insulating strongly correlated d=2 and d=3 dimensional systems. We find that substantial increase in the density of states at the Coulomb gap occurs only when the ratio between the hopping integral t and the gap width B exceeds a critical value. We estimate that the hopping integral corresponding to the experimental condition n${\mathit{n}}_{\mathit{c}}$/3 satisfies t/B0.05. For such values of t/B, quantum hopping brings about little change in the single particle density of states. The classical Coulomb gap therefore remains intact in both three and two dimensional systems, in contrast to a previous claim that the gap disappears for d=2 systems. The implication of these results on experiments on doped semiconductors is discussed.
TL;DR: In this paper, the authors analyzed the localized density-of-states distribution in a system that exhibits variable-range hopping transport at the Fermi energy and showed that the overall effect of LDOS non-uniformity and intra-site electron correlation is a significant reduction of the inter-site tunnelling distances.
Abstract: Photothermal deflection spectroscopy measurements have been performed on amorphous silicon (a-Si) films produced by ion bombardment of crystalline silicon (c-Si). The analysis of the sub-bandgap optical absorption data provides, for the first time, detailed information about the localized density-of-states (LDOS) distribution in a system that exhibits variable-range hopping transport at the Fermi energy. We find that in a-Si the Fermi energy is pinned within a relatively non-uniform distribution of positive-correlation-energy states. Qualitatively, our findings support previous suggestions that the pre-exponential or [sgrave]0 problem in variable-range hopping might be related to LDOS non-uniformities around (EF). Comparing electronic transport data on a-Si produced by ion bombardment, to expectations based on the Mott theory, we show that the overall effect of LDOS non-uniformity and intra-site electron correlation is a significant reduction of the inter-site tunnelling distances. Possible reaso...
TL;DR: In this paper, the results of resistivity measurements performed on Li-doped Y-Ba-Cu-O (1:2:3) polycrystalline samples are presented.
Abstract: Presented are the results of resistivity measurements performed on Li-doped Y-Ba-Cu-O (1:2:3) polycrystalline samples. In the insulating range, the data are found to follow the form exp [(T0/T)1/4]. The results provide experimental evidence for a variable-range hopping mechanism, as observed in a variety of high-T c materials. A monotonic relation between T0 and Li concentration is observed.