Showing papers on "Variable-range hopping published in 1977"
TL;DR: In this article, the absolute sign of the Hall effect for several situations charac teristic of hopping in covalent materials was determined for two types of hopping: electron hopping between antibonding orbitals and hole hopping between the bonding orbitals of an even-membered (four-site) ring.
Abstract: The absolute sign of the Hall effect is determined for several situations charac teristic of hopping in covalent materials. In particular, it is shown that the hopping of an excess electron between the antibonding orbitals of an odd-membered (three-site) ring will yield an ‘anomalously’ signed Hall effect, namely, p-type. In addition, the Hall effect for holes moving between bonding orbitals in a similar odd-membered structure will yield an n-type Hall effect. However, electron hopping between the antibonding orbitals and hole hopping between the bonding orbitals of an even-membered (four-site) ring yields the conventional result : n-type and p-type Hall effects, respectively. Other physically relevant situations are also discussed. These examples all serve to illustrate that the sign of the Hall effect for both electrons and holes depends not only on the local geometry but on the nature and relative orientations of the local orbitals between which the carrier moves. Furthermore, these results pr...
134 citations
TL;DR: In this article, the authors evaluate the temperature dependence of phonon-assisted dc hopping conductivity of a model appropriate to a class of anisotropic quasi-one-dimensional conductors in which the electronic states in the vicinity of the Fermi level are localized because of intrinsic and extrinsic static disorder.
Abstract: Using a percolation construction we evaluate the temperature dependence of the phonon-assisted dc hopping conductivity of a model appropriate to a class of anisotropic quasi-one-dimensional conductors in which the electronic states in the vicinity of the Fermi level are localized because of intrinsic and or extrinsic static disorder. We find temperature dependences of the general form $\mathrm{ln}[\frac{\ensuremath{\sigma} (T)}{{\ensuremath{\sigma}}_{0}}]=\ensuremath{-}{[\frac{{T}_{0}(m)}{T}]}^{\frac{1}{m}}$ where $m$ is weakly temperature dependent and has the value 4 for asymptotically low temperatures, $T\ensuremath{\rightarrow}0$ K. With increasing temperature the interchain hopping distances become smaller and $m$ decreases gradually. In a model in which the two transverse directions are equivalent, $m$ decreases to 2.91 when all allowed interchain hops are to the nearest chain only. The percolation channel is three-dimensional. For a model in which the two transverse directions are inequivalent, the percolation channel becomes two-dimensional at high temperatures when all interchain hops along the more difficult direction become more difficult than the critical percolation hop. In this case $m$ decreases to 2.70. With further increase in temperature the percolation construction breaks down in both cases. The observed conductivity of NMP-TCNQ (N-methylphenazinium tetracyanoquinodimethane) is found to be in good agreement with our results.
66 citations
TL;DR: In this paper, a semiclassical random walk hopping model is proposed to explain anomalous magnetoresistance in amorphous germanium and silicon in the hopping regime, and a general expression is derived for the magnetoreduction.
Abstract: A semiclassical random walk hopping model is proposed in order to explain the anomalous magnetoresistance in amorphous germanium and silicon in the hopping regime. A general expression is derived for the magnetoresistance. The magnetoresistance is deduced to be the result of the modification of the spin-flip relaxation time by the external magnetic field. Explicit formulae are derived within the variable range hopping theory of Mott and the spin relaxation model described previously by the present authors. The model can then account for the variety of experimental observations in these materials.
58 citations
TL;DR: In this paper, electrical conductivity measurements show that Ln1-x Sr x CoO3, (Ln = Pr or Nd) undergoes a non-metal-metal transition when x ≥ 0 3.
Abstract: Electrical conductivity measurements show that Ln1-x Sr x CoO3, (Ln = Pr or Nd) undergoes a non-metal-metal transition when x≈0 3. The d.c. conductivity of compositions with 0
33 citations
TL;DR: The presence or absence of localized singly occupied gap states depends on the state of disorder of the amorphous solid (quenched from the melt, deposited at 77 K or annealed) and on the type of chemical bond present in the compound as mentioned in this paper.
Abstract: Localized singly occupied gap states as evidenced by variable range hopping (resistivity proportional to exp T −1/4) have been observed in several amorphous semiconducting compounds deposited at 77 K. The presence of these states has been confirmed in several cases (GaAs, As2Te3 and As4Te3Se3) by susceptibility and E.S.R. experiments. The number of localized singly occupied states is appreciably reduced by annealing as shown by resistivity, susceptibility and E.S.R. experiments. The presence or absence of localized singly occupied gap states depends on the state of disorder of the amorphous solid (quenched from the melt, deposited at 77 K or annealed) and on the type of chemical bond present in the compound.
27 citations
TL;DR: In this article, the importance of correlated multi-carrier transitions in hopping transport is discussed, and the regime where such transitions contribute significantly to the electrical conductivity of the system.
Abstract: Simple qualitative arguments are presented to show the importance of correlated multi-carrier transitions in hopping transport, and to outline the regime where such transitions contribute significantly to the electrical conductivity Some general features to be expected when such a mechanism operates are presented
23 citations
TL;DR: In this paper, the authors present evidence that correlated multi-electron hopping makes important contributions to conductivity in impurity conduction in n- and p-Ge. The evidence is based on the behavior of the pre-exponential with concentration and with magnetic field.
22 citations
TL;DR: In this paper, the Hall effect in the hopping region of a random resistor network with extrinsic currents is averaged by means of methods of percolation theory, and the dependence of the Hall mobility in disordered systems on temperature and concentration of sites (centres of localization) are discussed in detail for strong and weak interaction with phonons.
17 citations
TL;DR: In this paper, an exact asymptotic formula for the d.c. phonon-assisted hopping conductivity at low values of the electron-phonon coupling constant is derived.
13 citations
TL;DR: In this article, the rate equation for electron hopping between localized states is generalized to take account of the electron-electron interaction in the Hubbard model, and the effect of electron correlations, both repulsive and attractive, on the temperature dependence of hopping conductivity and thermopower is discussed.
Abstract: The rate equation for electron hopping between localized states is generalized to take account of the electron-electron interaction in the Hubbard model. The generalized rate equation is used to discuss the effect of electron correlations, both repulsive and attractive, on the temperature dependence of hopping conductivity and thermopower.
[Russian Text Ignored.]
13 citations
TL;DR: In this article, the dispersion of the variable range hopping contribution to the dielectric constant was investigated in amorphous Si100-xAux system and it was shown that at sufficiently high frequency or at sufficiently low temperature, the nonmetal-metal transition was predicted.
TL;DR: In this article, a hopping conduction mechanism was proposed for the mobile ions in evaporated silicon oxide films, which can also be combined with surface trapping, giving rise to Cole-Cole diagrams similar to those found experimentally.
Abstract: A hopping conduction mechanism is proposed for the mobile ions in evaporated silicon oxide films. The Cole-Cole diagram, calculated numerically, is identical to that of a simple Debye relaxation mechanism. The hopping conduction mechanism can also be combined with surface trapping, which gives rise to Cole-Cole diagrams similar to those found experimentally. Lastly it is shown that the hopping conduction model is closely related to a drift-diffusion mechanism, which has also been used to describe ion transport phenomena in thin insulating films.
TL;DR: In this paper, the hopping conductivity in a one-dimensional (1D) system with a finite density of impurities is considered and the nonlinear hopping equations are solved for steady-current conditions in an applied electric field.
Abstract: Hopping conductivity is considered in a one-dimensional (1D) system with a finite density of impurities which present a finite barrier to hopping. The nonlinear hopping equations, which exclude jumps to occupied sites, are solved for steady-current conditions in an applied electric field $E$. The current saturates at a value independent of $E$ and of $c$ for $c\ensuremath{\lesssim}\frac{\mathrm{eaE}}{{k}_{B}T}\ensuremath{\ll}1$ where $c$ is the impurity concentration and $a$ the lattice spacing.