Showing papers on "Variable-range hopping published in 1991"

Hopping transport in solids

Abstract: Preface. 1. Hopping conduction in the critical regime approaching the metal-insulator transition (T.G. Castner). 2. Hopping in band tails far from equilibrium (D. Monroe). 3. Non-Ohmic microwave hopping conductivity (Yu.M. Galperin, V.L. Gurevich and D.A. Parshin). 4. Thermal conduction due to hopping processes in amorphous solids (A. Jagannathan, R. Orbach and O. Entin-Wohlman). 5. The hopping thermopower (I.P. Zvyagin). 6. Slow processes in disordered solids (M. Pollak and A. Hunt). 7. Hopping conductivity in the intermediate frequency regime (A.R. Long). 8. Hopping in mesoscopic samples (A.B. Fowler, J.J. Wainer and R.A. Webb). 9. Scattering and interference effects in variable range hopping conduction (B.I. Shklovskii and B.Z. Spivak). 10. Hopping conduction in III-V compounds (R. Mansfield). 11. Hopping conduction in electrically conducting polymers (S. Roth). 12. Hopping conduction in heavily doped semiconductors (A.N. Ionov and I.S. Shlimak). Author index. Subject index. Cumulative index.

Vortex variable-range-hopping resistivity in superconducting films

Abstract: We study both classical and quantum vortex creep in disordered thin-film superconductors in an applied magnetic field. Quantum tunneling of vortices leads to a variable-range-hopping resistivity with a non-Arrhenius temperature dependence at low T. Mott's 1/3 law is modified by long-range vortex interactions, and a numerical analysis enables us to estimate a temperature exponent between roughly 2/3 and 4/5. At higher T, a classical hopping regime is expected with a current-density scale for nonlinearities varying (roughly) as ${\mathit{T}}^{3}$.

Variable range hopping in PrBa2Cu3O7−δ

Abstract: Measurements of the temperature dependence of the resistivity of PrBa 2 Cu 3 O 7−δ (δ ≤ .04) show that variable range hopping is the dominant conduction mechanism in this material below room temperature. This result was found for epitaxial as well as for ceramic samples of this material. The dimensionality of the process could not as yet be determined.

Negative magnetoresistance in variable-range-hopping conduction.

Abstract: The orbital mechanism of negative magnetoresistance of variable-range-hopping conductivity is reconsidered by numerical simulations and qualitative arguments. At small magnetic fields the positive magnetic-field corrections to the localization length in two-dimensional samples are universal in the sense that they are independent of the details of the scattering potential (impurity concentration, amplitude of scattering, and proximity to the metal-insulator transition). Implications of these results to transport properties on the dielectric side of the superconductor-insulator transition are discussed.

Novel concepts in electronic polymers: Polyaniline and its derivatives

Abstract: Polyanilines have been known for over one hundred years. Recent studies of this chemically flexible polymer have demonstrated unusual chemical, electrical, and optical phenomena, both in insulating forms and conducting forms. The polyanilines differ substantially from earlier studied polyacetylene, polythiophene, polypyrrole, polydiacetylene, and other polymers in that their electronic structure is based on the overlap of alternating nitrogen atoms and C6 rings. Two classes of the emeraldine form of polyaniline are distinguished by preparation route and crystal structure. For both classes, the Pauli susceptibility indicative of metallic state is correlated with the formation of a three-dimensionally crystalline region. Charge conduction studies of oriented films and fibers demonstrate that three-dimensional order between chains is critical for high conductivities. Within the crystalline regions the conduction electrons are substantially delocalized in three-dimensions while the dc transport is dominated by quasi-one-dimensional variable range hopping in the disordered regions. This delocalized charge leads to high absorbance (loss) at microwave frequencies, important for a broad range of applications. The ability to derivatize polyaniline at ring and nitrogen positions allows one to test concepts for the control of conductivity as well as improved processing. The monomethyl derivative, poly(orthotoluidine), has nearly identical electronic structure and crystal structure, yet its conductivity is three orders of magnitude lower than that of emeraldine hydrochloride. This is attributed to increased interchain separation and decreased interchain correlation causing 1-D localization. Similar 1-D localization is observed in the self-doped sulfonated polyaniline system. Extensive photoinduced optical studies of the insulating base forms of polyaniline show the importance of ring rotation in stabilizing photoexcited positive polarons. Long-lived photoinduced absorptions are observed which are suggested to be a possible basis for erasable optical information storage technology.

Ion implantation of polymers for electrical conductivity enhancement

Abstract: The polymers PET, PAN, PES and PEEK were implanted with ions of He, B, C, N, Ar and As at an energy of 50 keV. PEEK was implanted with I at 22.5 MeV, Xe at 24 MeV, and Ni at 47 MeV. Surface resistivity for electrical conduction indicated a plateau effect in the dose range 1016–1017 ions/cm2. The more aliphatic polymer, PET, indicated the lowest resistance of the low energy implants. The high energy ions produced much lower resistivities at much lower doses. The temperature dependence of the resistivities indicate a quasi-one-dimensional variable range hopping mechanism for electrical conduction.

Structure and Electronic Transport of Highly Conductive Langmuir-Blodgett Films of Tridecylmethylammonium-Au(dmit)2

Abstract: Electronic transport measurements are reported for Langmuir-Blodgett (LB) films of tridecylmethylammonium-Au(dmit)2 for 300-4 K. The conductivity after electrochemical oxidation is 30-50 S/cm at room temperature, increases with decreasing temperature, and begins decrease at about 200 K. The conductivity at lower temperatures is explained by two-dimensional variable range hopping (2D VRH). The thermoelectric power (S) is positive and well fitted by a linear combination of the temperature dependence of 2D VRH (Sv\proptT1/3) and that of metal (Sm\proptT). These results can be explained by a model in which highly conductive metallic regions are separated by thin weakly conductive regions. The UV/visible absorption spectra and the X-ray diffraction patterns indicate that a large structural change occurs accompanied by electrochemical oxidation.

Negative magnetoresistance in the variable-range-hopping regime in n-type CdSe

Abstract: La dependance de la magnetoresistance negative vis-a-vis de la temperature est observee pour tous les echantillons, independamment du fait que la resistivite a champ nul indique un saut de Mott ou un saut a distance variable en presence d'une bande interdite coulombienne. Implications phenomenologiques de ces observations

The a.c. conductivity of variable range hopping systems such as amorphous semiconductors

Abstract: A new theory for the frequency-dependent conductivity of variable range hopping (VRH) systems is presented The physical basis of the theory is that local relaxation processes dominate at high frequencies, non-local processes at low frequencies The loss peak in the imaginary part of the dielectric constant represents the onset of percolation, defining approximately the boundary between these two regimes At high frequencies, the pair approximation is modified to allow the density of contributing pairs to be frequency dependent At lower frequencies, a cluster expansion consistent with percolation theory is used to calculate σ(ω) The relaxation times of clusters of impedances are strongly enhanced through the necessity to transport a large amount of charge over large distances The theory gives the correct analytical expressions for the frequency and the temperature dependence of σ(ω) in every frequency regime, and the pair approximation joins smoothly on to the low-frequency results at the loss

Electronic charge transport and magnetism of the system (La1−xSrx)2NiO4+δ

Abstract: We report measurements of the electrical resistivity, the Seebeck-coefficient and the magnetic susceptibility of samples (La1−xSrx)2NiO4+δ in the concentration range 0 ≤ χ ≤ 0.5. All compounds exhibit semiconducting behaviour at low temperatures with thermally activated or variable range hopping type of conductivity. The comparison of the temperature dependence of the conductivity and the Seebeck-coefficient reveals the polaron character of the charge carriers. From the susceptibility measurements at high temperatures we derive an exchange interaction Jaf≈ 140 K for the NiO2-planes in La2NiO4. We develop a magnetic phase diagram for (La1−xSrx)2NiO4 with an antiferromagnetic range for x≤0.04 and a spin glass range for x > 0.04. The results are discussed in comparison with those of the high temperature superconducting system (La1−xSrx)2CuO4.

Electronic and vibrational hopping transport in boron carbides

Abstract: General concepts of hopping‐type transport and localization are reviewed Disorder, electronic correlations and atomic displacements, effects ignored in electronic band structure calculations, foster localization of electronic charge carriers Examples are given that illustrate the efficacy of these effects in producing localization This introduction is followed by a brief discussion of the relation between hopping‐type transport and localization The fundamentals of the formation, localization, and hopping transport of small polarons and/or bipolarons is then described Electronic transport in boron carbides is presented as an example of the adiabatic hopping of small bipolarons Finally, the notion of vibrational hopping is introduced The high‐temperature thermal diffusion in boron carbides is presented as a potential application of this idea

Semiclassical small-polaron hopping in a generalized molecular-crystal model.

Abstract: The thermally activated rates for high-temperature adiabatic and nonadiabatic small-(bi)polaron hopping are calculated for a generalization of Holstein's molecular-crystal model. In the expanded model a carrier occupying a molecule is coupled to many molecular vibrational modes rather than to just the single vibrational mode envisioned in the original model. This generalization of the molecular-crystal model does not significantly affect the semiclassical small-(bi)polaron jump rates. In particular, for the generalized model the hopping activation energy becomes a sum of contributions associated with each of the vibrational modes to which the carrier is coupled. The vibrational frequency that is the preexponential factor for adiabatic small-(bi)polaron hopping is the square root of the sum of the squares of the vibrational frequencies weighted by their relative contributions to the net hopping activation energy.

The effect of the density of states on the conductivity of the small-polaron hopping regime in disordered systems

Abstract: The authors investigate the effect of the energy-dependent model densities of states on the DC conductivity of the small-polaron hopping regime in disordered systems. The DC conductivity behaviour is obtained for the high- and low-temperature cases, ignoring or taking into account correlations between bonds due to the energy of the common site.

Electrical and optical properties of plasma-deposited amorphous hydrocarbon films

Abstract: Amorphous hydrocarbon layers ( a -C:H) prepared by plasma assisted chemical vapor deposition (PECVD) were investigated with respect to their possible semiconducting properties. Cluster calculations were performed to confirm that a true band gap may be formed in these materials. Values corresponding to those derived from optical investigations were calculated. Electrical conductivity is in accordance with a variable range hopping mechanism over a large temperature range. Conductivity and gap width vary with the preparation conditions, but for the same bandgap, the conductivities of the nitrogen containing films were always lower than that of the pure hydrocarbon films. From this we conclude that doping resulting in a shift of the fermi level does not occur in the case of nitrogen.

Resonant tunneling and hopping through a series of localized states in a two-dimensional electron gas.

Abstract: We report measurements of the temperature and length dependence of the conductance of a wide two-dimensional electron gas in a strongly localized regime. Our results indicate that the conduction occurs via resonant tunneling below about 0.1 K and, at higher temperatures, via hopping through channels that contain more than one localized state

Variable-range-hopping magnetoresistance

Abstract: The hopping magnetoresistance R of a two-dimensional insulator with metallic impurities is considered. In sufficiently weak magnetic fields it increases or decreases depending on the impurity density n: It decreases if n is low and increases if n is high. In high magnetic fields B, it always exponentially increases with \ensuremath{\surd}B . Such fields yield a one-dimensional temperature dependence: lnR\ensuremath{\propto}1/ \ensuremath{\surd}T . The calculation provides an accurate leading approximation for small impurities with one eigenstate in their potential well. In the limit of infinitesimally small impurities, an impurity potential is described by a generalized function. This function, similar to a \ensuremath{\delta} function, is localized at a point, but, contrary to a \ensuremath{\delta} function in the dimensionality above 1, it has finite eigenenergies. Such functions may be helpful in the study of scattering and localization of any waves.

Electrical conductivity of ion-irradiated carbon

Abstract: Amorphous carbon films have been irradiated with Cl-ions with energies between 1 and 40 MeV, and the electrical conductivity of the material has been measured as a function of the ion dose The room temperature conductivity is increased by nearly three orders of magnitude and saturates at a dose of about 10^(15)cm^(−2) The rate of conductivity change vs the ion energy can be explained by an ion track model The temperature dependence of the conductivity between 100 and 300 K at low doses is in accordance with variable range hopping with a temperature exponent of 1/2 Hopping sites are assumed to be graphite rings or microcrystallites At higher doses the variable range hopping is replaced by a metallic conductivity with a linear dependence on the temperature similar to that of polycrystalline graphite This is probably due to the formation of complete percolation paths through the material by interconnection of the graphite regions Estimates of the size of the crystallites, the hopping activation energy, and the cross section for producing crystallites in the material have been extracted

Variable-range hopping in YBa2(Cu1-xMx)3O6+γ[M=Fe, Zn]

Abstract: We report on resistivity and thermoelectric power measurements of YBaCuO doped and undoped compounds in the semiconducting state. The substitution of Cu by Fe and Zn increases the activation energy with increasing concentration while variable-range hopping conduction is maintained. In particular, it is found that Fe greatly affects the activation energy while Zn has a much smaller effect. Finally, our results suggest that the Cu(1) sites may be the hopping centers in YBa 2 Cu 3 O 6 .

Hall effect under hopping conduction conditions

Abstract: A theory of the Hall effect under hopping conduction conditions is developed. General expressions are obtained and these are used to derive the Hall mobility μ H in specific cases such as variable-range-hopping and e 3 conduction. An allowance is made of the effects of the electron-electron Coulomb correlations, leadidng to the appearance of a Coulomb gap. It is shown that under hopping conduction conditions the Hall effect has a correlation radius L H >>L, when L is the correlation radius of an infinite cluster governing the conductivity. In practice, the value of L H can exceed the dimensions of a sample, which gives rise to size effects

Non-Ohmic hopping conductivity in disordered systems

Abstract: The electrical conductivity of disordered systems in the non-ohmic regime has been calculated using the Green function formalism of Movaghar et al. The isotropic and directed hopping cases are treated. For the temperature-related behaviour, we find as particular cases the Mott ¼ and the ½ laws, and for the electrical-field-related behaviour, we find a general form that agrees with expressions in the literature.

Phonon-Assisted Ion Hopping in Superionic Conductors Based on the Lattice Gas Model

Abstract: The jump diffusion of interacting mobile ions in superionic conductors (SIC) is studied by the phonon-assisted hopping theory. The interacting mobile ions are described by the lattice gas model. In order to study the correlated hopping of mobile ions, we calculate the average jump rate taking account of the average distribution of mobile ions at the nearest neighboring sites around the hopping ion after and before the hopping. We find that the average jump rate is thermally activated and that its activation energy is low at high concentration of mobile ions. This suggests that a low activation energy of SIC is due to the mutual interaction of mobile ions.