Showing papers on "Variable-range hopping published in 1997"

Investigation of Structure and Electrical Conductivity in Doped Polyaniline

Abstract: Polyaniline (PAn) was synthesized chemically and doped with various dopants, such as HCI, HCOOH, I 2 and methylene blue (C 16 H 18 ClN 3 S), by an immersion method. The structure of these samples was investigated by infrared (IR) spectroscopy and wide-angle X-ray diffraction (WAXD) analysis. Remarkable changes have been observed in the IR spectra of doped PAn, indicating that doping is affecting the chemical structure. The percentage crystallinity was also found to increase after doping. The electrical conductivity (σ) of these samples was measured at various temperatures (T = 308K to 423K). Plots of log σ versus T y , where y = - 1/2, - 1/3, - 1/4, were obtained and used to identity the conduction mechanism. Undoped PAn shows semiconducting behaviour, while doped samples show a variable range hopping mechanism. A primary cell was constructed with HCl-doped PAn as one of the electrodes and a copper plate as the other electrode. It gave an open circuit voltage of 0.38 V and a short circuit current of about 5.4 mA.

Electrical stress-induced variable range hopping conduction in ultrathin silicon dioxides

Abstract: Electrical stress-induced variable range hopping conduction is reported in the ultrathin silicon dioxides. The conduction is mediated by the localized states, including various trap sites and interface states induced by the electrical stressing. Based on the results, a model for the conduction mechanism of the “B-mode” stress-induced leakage current (B-SILC) is proposed. This model well explains the temperature dependence and large fluctuation of the B-SILC, which cannot be explained by the previous models for the B-SILC. Furthermore, an empirical expression for the current–voltage characteristics of the B-SILC is also proposed.

Colossal magnetoresistance of the variable range hopping regime in the manganites

Abstract: Evidence for variable range hopping (VRH) is presented in resistivity measurements in thin films and bulk samples of different substituted x=0.3 manganese perovskites. The necessity to modify the Mott VRH model is shown and we have developed a model based on magnetic localization hopping, which is used to reproduce temperature and field dependence of resistivity in these mixed-valence compounds.

Screening of the Coulomb interaction in two-dimensional variable-range hopping

Abstract: A Coulomb gap in the density of states is inferred from variable-range-hopping measurements in two dimensions in a gated GaAs/Al${}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ heterostructure. For large hopping lengths the Coulomb interaction is screened by a metallic gate, and a universal crossover to Mott hopping at low temperatures is observed. Excellent agreement with theory is achieved by adjusting a single combination of constants associated with variable-range hopping. This change in constants is explained by correlated hopping. The Coulomb interaction is not screened by the two-dimensional electron layer.

A useful Mott - Efros - Shklovskii resistivity crossover formulation for three-dimensional films

Abstract: Useful and simple 3D crossover expressions are presented for the resistance versus temperature behaviour in highly insulating 3D films. At high temperatures, this theory extrapolates to the Mott variable-range hopping law, and at low temperatures to the Efros - Shklovskii variable-range hopping law. Good agreement is found between the crossover theory and resistance measurements.

Mechanism of conduction in doped polyaniline

Abstract: The conductivity of -doped emeraldine polymers is studied as a function of temperature in the range . The dopant concentration of the samples varies between 0.1 M and 1.0 M. The temperature dependence of the dc electrical conductivity gives evidence for a transport mechanism based on variable-range hopping in three dimensions. Using Mott's formula for the dc conductivity, physically meaningful values of the density of states at the Fermi energy, the hopping energy and hopping distance are calculated.

Correlation between electron spin resonance and dc conductivity data of polypyrrole, poly(N-methyl pyrrole-pyrrole), and poly(N-methyl pyrrole)

Abstract: The dc conductivity, dielectric constant, and electron spin resonance measurements have been performed on samples of polypyrrole, poly(N-methyl pyrrole-pyrrole), and poly(N-methyl pyrrole) having different BF4− concentrations in the temperature range 77–350 K. Different parameters evaluated from these measurements support the existence of Mott’s variable range hopping mechanism in these systems.

Low-frequency noise probe of interacting charge dynamics in variable-range hopping boron-doped silicon

Abstract: Low-frequency voltage noise is used to probe stochastic charge dynamics in nonmetallic boron-doped silicon. A `` $1/f$'' noise spectrum is observed down to 0.1 Hz. The noise magnitude is suppressed and the frequency dependence strengthens at low temperature. The data are inconsistent with single-particle hopping fluctuations, but are compatible with thermally activated rearrangements of configurations involving many charges. Such configurational fluctuations indicate that many-electron excitations are important to charge dynamics in the interacting regime.

Electrical resistivity of AgInSe2 films

Abstract: Single-phase AgInSe 2 ingot material, having the tetragonal chalcopyrite structure, was prepared by direct fusion of the constituent elements in vacuum-sealed silica tubes. Nearly stoichiometric films were prepared by thermal evaporation of the bulk material using a single source in 10 −3 Pa vacuum on glass substrates. The chemical composition of the films was determined by energy-dispersive X-ray spectrometry, where the chemical formula could be represented by Ag (1− x ) In (1− x /2) Se 2(1+ x ) with x ⩽ 0.02. All the deposited films exhibited n-type conduction. The temperature dependence of the dark electrical resistivity in the range 100–400 K revealed the dominance of the shallow donors below 150 K. A variable range hopping conduction mechanism due to localized state dominated in the range 150–200 K. At higher temperatures (200–300 K) grain boundaries effects dominated, whereas above 300 K extrinsic conduction due to impurities dominated.

DC Conductivity in Barium Hexaferrites Doped with Bismuth Oxide

Abstract: Barium hexaferrites of compositions BaO(6- x)Fe2O3 xBi2O3 with 0 \leqslantx \leqslant0.4 have been synthesized by the ceramic calcination route. The bismuth substituted hexaferrites exhibit dc electrical conductivities more than two orders of magnitude higher than the pure ones. This is believed to arise due to an increase in the concentration of Fe2+ ions as a result of the presence of Bi5+ ions in these compounds. The dc conductivity at temperatures higher than 250 K can be explained on the basis of a non-adiabatic hopping model of small polarons, the variable valence iron ions forming the polaronic sites. At temperatures lower than 220 K a variable range hopping mechanism appears to control the conductivity.

Negative magnetoresistance in p-type beta -fesi2 single crystals in two regimes of variable-range hopping

Abstract: Single crystals of p-type b-FeSi2 demonstrate the negative magnetoresistance below 40 K that increases with decreasing temperature and does not reach either a maximum or a saturation up to at least 12 kOe. This effect is observed in both the Mott and the Shklovskii-Efros variable-range hopping regimes. At fields below approximately 3–5 kOe ~depending on the temperature! the magnetoresistance displays a quadratic dependence on the magnetic field, while at higher fields the dependence is linear. The experimental data are analyzed using two different approaches to the interference effects in hopping conductivity in a magnetic field. @S0163-1829~97!01427-6#

Low temperature electronic transport in RuO2-Based cermet resistors

Abstract: We have studied the temperature (T) dependence of resistance (R) of RuO2-based thick films down to 1.2 K. Samples were prepared from inks containing conductive RuO2 powders (<-10% wt.), high lead-silicate glass particles and Mn (≤1.4% wt.). We found that the resistance fits the exponential lawR=R0 exp (T0/T)x with x=1/4 and the most resistive samples show a cross-over to the x=1/2 regime as the temperature decreases. Both the fitting parametersR0 andT0 scale down as the RuO2 fraction increases and they are affected in a similar way by a change of the Mn content. The presence of the two regimes is similar to that observed inn-type GaAs [Phys. Rev. B39, 8059 (1989)] andn-type CdSe [Phys. Rev. Lett.64, 2687 (1990)] whilst it disagrees with the behaviour predicted for grain to grain hopping [Phys. Rev. B27, 2583 (1983)] and with that expected for resonant tunneling between metallic particles [J. Appl. Phys.48, 5152 (1977)]. We conclude that in our systems the driving charge transport mechanism is electron hopping within the glassy matrix. Since in our case the hopping length is of the same order of the localization length, the puzzling questions arising from our experiments are whether and how the variable range hopping model can be extended beyond its conventional limits.

Ions implanted into TiO2 rutile single crystals: Lattice disorder, lattice site occupation and conductivity

Abstract: Ions (Au, W, Sb, Hg, Sn, In, Hf, La) with different size mismatch-energies have been implanted at 77 K and 293 K into TiO2. The lattice disorder and lattice site occupation were measured by RBS-C in the 〈001〉 and 〈100〉 crystalline directions. The conductivity was measured as a function of temperature. For the partially damaged rutile phase recovery was observed below room temperature, in contrast to the amorphous phase. The lattice site occupation is discussed within the solubility rules for equilibrium solid solutions using the electronegativity and the atomic size as coordinates. A large increase of the conductivity σ was observed with increasing Sb and Sn dose, indicating a saturation behaviour at about 30 Ω −1 cm−1. Between 40 K and 293 K ln σ was proportional to −1 2 for low doses, and proportional to T −1 4 for doses of about 1 × 1016 / cm2 and above, indicating that the transport mechanism is due to variable range hopping.

Electronic transport in RuO2-based thick film resistors at low temperatures

Abstract: The electrical conductivity of various RuO2-based thick film paste resistors was investigated in the temperature range between 50 mK and 20K. It is shown that models based on variable range hopping and simple models based on tunnelling of charge carriers between conductive grains do not provide a satisfactory explanation for the temperature dependence of the electrical conductivity in these materials at very low temperatures. We suggest a new mechanism based on tunnelling of electrons through graded barriers between conductive particles.

Low Temperature Electronic Transport in RuO 2 -Based Cermet Resistors

Abstract: We have studied the temperature (T) dependence of resistance (R) of RuO2-based thick films down to 1.2 K. Samples were prepared from inks containing conductive RuO2 powders (⩽ 10% wt.), high lead-silicate glass particles and Mn (⩽ 1.4% wt.). We found that the resistance fits the exponential law R = R 0 exp(T 0/T)x with x = 1/4 and the most resistive samples show a cross-over to the x = 1/2 regime as the temperature decreases. Both the fitting parameters R 0 and T 0 scale down as the RuO2 fraction increases and they are affected in a similar way by a change of the Mn content. The presence of the two regimes is similar to that observed in n-type GaAs [Phys. Rev. B 39, 8059 (1989)] and n-type CdSe [Phys. Rev. Lett. 64, 2687 (1990)] whilst it disagrees with the behaviour predicted for grain to grain hopping [Phys. Rev. B 27, 2583 (1983)] and with that expected for resonant tunneling between metallic particles [J. Appl. Phys. 48, 5152 (1977)]. We conclude that in our systems the driving charge transport mechanism is electron hopping within the glassy matrix. Since in our case the hopping length is of the same order of the localization length, the puzzling questions arising from our experiments are whether and how the variable range hopping model can be extended beyond its conventional limits.

Electron conduction within Landau-level tails of medium-mobility GaAs/AlGaAs heterostructures

Abstract: The temperature dependence of both components, and , of the resistivity tensor has been studied at within integer quantum Hall effect plateaux around filling factors and of medium-mobility GaAs/AlGaAs heterostructures. In the middle of the mobility gap standard activated conductivity has been found with activation energies scaling well with . At filling factors slightly below another contribution adds to the activated conductivity at . This additional contribution can be further enhanced at higher measuring direct currents. We suggest that it arises due to enhanced electric-field-assisted tunnelling across potential barriers separating localized states within the bulk of the sample. This effect contributes to the backscattering across the sample, leading to an enhanced longitudinal conductivity. The additional contribution to can be reasonably well fitted to the formula for the variable range hopping in strong magnetic fields, indicating that the hopping can persist even at temperatures well above 4.2 K.

General crossovers from two-dimensional Mott T -1/3 to soft-gap T -ν variable-range hopping

Abstract: General expressions are presented for describing crossovers from the two-dimensional Mott ${T}^{\ensuremath{-}1/3}$ to the soft-gap ${T}^{\ensuremath{-}\ensuremath{ u}}$ variable-range hopping with any \ensuremath{ u} between $1/3$ and 1. Theoretical expressions fit quite well experimental resistance data of $\ensuremath{ u}\ensuremath{\approx}0.77,$ observed in amorphous nickel-silicon films.

Electrical transport properties of thin films of a-Se80 – x Ga20 Bix

Abstract: Dark conductivity measurements on thin films of a-Se80 – x Ga20 Bix (where x = 0, 5, 10, 15, 20) are reported in the temperature range 148–318 K. The results indicate that at higher temperatures (287–318 K), the conduction occurs in the band tails of localised states and at lower temperatures (148–287 K) the conduction is due to variable range hopping, which is in reasonable agreement with Mott's condition of variable range hopping conduction.

Negative magnetoresistance in the nearest-neighbor hopping conduction

Abstract: We propose a size effect which leads to the negative magnetoresistance in granular metal-insulator materials in which the hopping between two nearest-neighbor clusters is the main transport mechanism. We show that the hopping probability increases with magnetic field. This is originated from the level crossing in a few-electron cluster. Thus, the overlap of electronic states of two neighboring clusters increases, and negative magnetoresistance results.