Showing papers on "Conductivity published in 1978"

Fluctuation-Induced Tunneling Conduction in Carbon-Polyvinylchloride Composites

Abstract: We present evidence that in carbon-polyvinylchloride composites, consisting of aggregates of carbon spheres (100-400 \AA{}) dispersed in the insulating matrix, the electrical conductivity can be ascribed to a novel mechanism of tunneling with potential-barrier modulation by thermal fluctuations. Theoretical consideration of the tunneling-probability modification by thermal fluctuating electric field across tunnel junctions yields expressions for the temperature and the field dependences of the conductivity in excellent accord with experimental results.

Temperature dependence of a.c. conductivity of chalcogenide glasses

Abstract: A theory based on two electrons hopping between defect sites recently proposed (Elliott 1977, 1978) to account for the frequency dependence of the conductivity of chalcogenide glasses is analysed h...

Conductivity of quasi-one-dimensional metal systems

Abstract: A new technique is constructed for treatment of one-dimensional and quasione-dimensional metal systems with random impurities. The technique is applied for calculation of conductivity in several systems (see contents). The influence of impurity scattering on electron pairing is discussed (§ 3).

Two-Dimensional Electrical Conductivity in Quench-Condensed Metal Films

Abstract: The resistance of ultrathin films condensed on low-temperature substrates has been measured. All of the materials studied (Au, Cu, Al, Sn, and Pd) show a transition from activated to metallic behavior as the resistance is reduced below 30000 ..cap omega../D/sup 7/Alembertian. This result gives strong evidence is support of the concept of a minimum two-dimensional conductivity. In Pb films with resistance > 30000 ..cap omega../D/sup 7/Alembertian, the onset of superconductivity is marked by an increase in activation energy.

Conductivity of inhomogeneous materials: Effective-medium theory with dipole-dipole interaction

Abstract: We extend the old effective-medium theory, originally due to Bruggeman, by incorporating dipole-dipole interactions to account for local-field effects. The interactions are represented approximately by use of effective depolarization factors. For close-packed clusters of conducting regions, a description which is appropriate at the percolation threshold, we predict an onset of percolation processes when 15.6 vol% is conducting. This is in very good agreement with numerical simulations as well as with experimental data for the metal-insulator transitions in metal-ammonia solutions and in alkali-tungsten bronzes.

A perturbation analysis of the attenuation and dispersion of surface waves

Abstract: A perturbation formulation of the equations of linear piezoelectricity is obtained using a Green’s function approach. Although the resulting equation for the first perturbation of the eigenvalue strictly holds for real perturbations of real eigenvalues only, it is formally extended to the case of purely imaginary perturbations of real eigenvalues. The extended equation is applied in the calculation of the attenuation of surface waves due to the finite electrical conductivity of thin metal films plated on the surface and air loading. The influence of the viscosity of the air is included in the air‐loading analysis, and the calculated attenuation increases accordingly. Since the metal films are thin compared with a wavelength, an approximate thin‐plate conductivity equation is employed in the determination of the attenuation due to the electrical conductivity of the films. The resulting attenuation is obtained over a very large range of values of sheet conductivity. This is accomplished by using the equatio...

The effect of charged additives on the carrier concentrations in lone-pair semiconductors

Abstract: The Street—Mott model of gap states, interpreted in terms of valence alternation defects, is used to calculate the temperature dependence of the carrier concentrations and of the conductivity as a function of the concentration of charged additives in chalcogenide semiconductors. When the charged additives are allowed to equilibrate with the valence alternation defects at elevated temperatures we find that the conductivity activation energy remains essentially unchanged although the conductivity may be increased by a few orders of magnitude for large concentrations of additives. Very much larger increases in conductivity result when the charged additives are prevented from equilibrating with the valence alternation defects. At high additive concentrations the chalcogenide material is expected to behave like a partially compensated and nearly degenerate semiconductor in agreement with observations of Ovshinsky (1977). The gradual unpinning of the Fermi level with increasing additive concentration i...

Isolation of integrated circuits utilizing selective etching and diffusion

Abstract: Disclosed is a method of isolating portions of integrated circuits which permits closely packed structures. A semiconductor wafer is provided with a substrate of one conductivity type, a first layer of opposite conductivity type and high impurity concentration formed thereon, and a second layer of either conductivity type but lower concentration formed over the first layer. The major surfaces of the semiconductor layers are parallel to the (110) plane. Narrow grooves with sidewalls in the (111) plane are etched into the first layer. A shallow diffusion of impurities of the same conductivity type as the first layer is performed in the sidewalls and bottom of the grooves which permits the first layer to be contacted from the surface of the second layer. The groove is then etched further until it extends into the underlying substrate. Impurities of the same conductivity type as the substrate are diffused into the bottom and sidewalls of the grooves. The concentration of these impurities is chosen so that a chanstop region is formed in the substrate without appreciably affecting electrical conductivity between the first layer and the regions formed by the previous diffusion.

Conductivity anisotropy and transverse magnetoresistance of Nb Se 3

Abstract: The first-transverse de conductivity measurements on a member of the transiton-metal trichalcogenides (Nb${\mathrm{Se}}_{3}$) are presented The conductivity anisotropy $\frac{{\ensuremath{\sigma}}_{b}}{{\ensuremath{\sigma}}_{c}}$ (where $b$ is the chain axis) was obtained using the Montgomery technique, and varies from 10 to 20 as a function of temperature The transverse resistivity ${\ensuremath{\rho}}_{c}$ also shows anomalies associated with the charge-density-wave transitions at ${T}_{1}$ (142 K) and ${T}_{2}$ (58 K) However, these anomalies are smaller than the corresponding ones in ${\ensuremath{\rho}}_{b}$ The quantity $\frac{{R}_{H}(0)}{({\ensuremath{\rho}}_{b}{\ensuremath{\rho}}_{c})}$, where ${R}_{H}$ is the Hall constant, demonstrates explicitly the freeze-out of thermally excited quasiparticles below each transition We show that at both transition gaps appears on the hole surface, and the hole concentration decreases rapidly The transverse magnetoresistance at low fields has also been measured Its monotonic power-law decrease with increasing temperature shows explicitly that the resistivity anomalies are due to changes in the carrier concentration, and not in the lifetimes

Conduction mechanism in plasma‐polymerized tetrafluoroethylene films

Abstract: The ac conductivity and capacitance of plasma‐polymerized tetrafluorethylene were measured over the frequency range 10−3–105 Hz. Experimental evidence is given that the measured conductivity is a bulk phenomenon over the entire frequency range. These measurements show that there are three different conduction processes, each of them dominating in certain frequency and temperature regions. At intermediate frequencies and temperature, a dipole orientation process dominates which gives rise to a large dispersion of the ac conductivity and charge storage in the films. Both ir observations and ESR data as well as the electrical measurements confirmed that ‐C—O groups formed by oxidation of the films after preparation are responsible for this effect. Concentrations of the order of N×1020 cm−3 were determined from electrical as well as from ir data. Hopping conductivity dominates at high frequencies and low temperatures, whereas at very low frequencies and high temperatures, a frequency‐independent dc conductivi...

Optical properties of Cr O 2 , Mo O 2 , and W O 2 in the range 0.2-6 eV

Abstract: The dielectric functions and optical conductivities of Cr${\mathrm{O}}_{2}$, Mo${\mathrm{O}}_{2}$, and W${\mathrm{O}}_{2}$ have been obtained from reflectivity measurements, and the results are interpreted on the basis of qualitative models proposed for the band structures of the transition-metal dioxide series. Cr${\mathrm{O}}_{2}$ has been studied as a function of temperature above and below its ferromagnetic transition at ${T}_{c}=392\ifmmode^\circ\else\textdegree\fi{}$K. Three conductivity peaks are observed to shift substantially near ${T}_{c}$, and a band near 0.8 eV in the ferromagnetic phase is assigned to transitions between exchange-split Cr $d$ bands. An increase in $3d$ bandwidth and $2p\ensuremath{-}3d$ separation is observed in the sequence Cr${\mathrm{O}}_{2}$, V${\mathrm{O}}_{2}$, Mo${\mathrm{O}}_{2}$, and W${\mathrm{O}}_{2}$, and these effects are correlated with an increasing cation radius. No evidence is found for greatly differing bandwidths for $3d$ orbitals of different symmetries, as has been proposed on empirical grounds.

Electrical conductivity and relaxation in ice crystals with known impurity content

Abstract: Three-terminal dielectric bridge measurements (in the range 20 Hz to 100 kHz between — 5°C and —90 to — 120°C) have been made of ice doped with (a) conductivity-enhancing ionic impurities (HCl, HF, NaCl, KF, NH4F) and (b) conductivity-depressing solutes (NH4OH, NH4Cl, NH5CO3, NaHCO3). Blocking electrodes were used for the first group. The true ice parameters were extracted from linearized plots of the Debye equations. Chlorides and fluorides showed very similar characteristics in their spectra and static conductivity. The results suggest that static conductivity is controlled by extrinsic protons. On the other hand, bases, or solutes that impart a positive freezing potential to the ice, suppress extrinsic protons. In this case, the static conductivity was not, or only weakly, temperature dependent and lower than in the first group. A conductivity cross-over was observed in neither case. The dielectric conductivity contribution is strongly dependent on impurity concentration but apparently less affected than the static conductivity by the nature of the solute. The principal relaxation time is reduced by most solutes, exceptions are pure (bicarbonate-free) bases, sodium bicarbonate, and carbon dioxide.

Forced convection in liquid metals with variable thermal conductivity and capacity

Abstract: An analysis of the steady state laminar thermal boundary layer of liquid metals is carried out for the case of the simultaneous linear variation in thermal capacity and conductivity. Explicit closed form analytical solutions are given for temperature distributions using perturbation technique, for a general class of potential flows specified by the power law distribution.