Electrical conductance from the Fowler-Nordheim tunneling

Electrical conductance, including its normalized version, is discussed quantitatively in the context of the Fowler-Nordheim tunneling by considering ballistic electron transport through a generic insulating layer. This discussion is applicable to several nanostructures as, for example, nanowires as well as to specific problems in electron optics.     

Percolation modeling of conductance of self-healing composites

We explore the conductance of self-healing materials as a measure of the material integrity in the regime of the onset of the initial fatigue. Continuum effective-field modeling and lattice numerical simulations are reported. Our results illustrate the general features of the self-healing process; the onset of the material fatigue is delayed, by developing a plateau-like time-dependence of the material quality. We demonstrate that in this low-damage regime, the changes in the conductance and similar transport/response properties of the material can be used as measures of the material quality degradation.     

Electrical conductivity-microstructure correlation in AIN-CuO composites

Water vapor is an important constituent of any gas and in many applications is regarded as a contaminant that needs to be monitored and controlled. AIN-CuO composites (2 % ≤ CuO ≤ 50 % by weight) have been studied to exploit them as novel humidity sensors over wide ranges of moisture levels and temperature. Development of benign microstructure with open porosity has been attempted by varying the composition and firing conditions. The impedance data acquired on the composites over the frequency range 5 Hz to 13 MHz, revealed a bulk response in terms of a single semicircular relaxation in the complex Z*-plane. A systematic variation of electrical conductivity with CuO content in the composites has been explained in the light of percolation theory.     

Electrical conductance decay of arc columns in some common gases

Experimental measurements are reported of the conductance decay following current interruption of wall stabilised arcs in air, nitrogen, argon, oxygen and helium within the current range 6–42 A. The results are compared with the predictions of several analytical models so that the influence of various gas properties and processes may be estimated. Measurements in the molecular gases show clearly how the arc structure affects the recovery. The results in the inert gases differ to some extent from the predictions of the simple models.     

Spin conductance in three-terminal rings subject to Rashba and Dresselhaus spin-orbit coupling

Based on the Green's function formalism, we investigated spin transport properties in one-dimensional three-terminal rings in the presence of the Rashba spin-orbit coupling (RSOC) and Dresselhaus spin-orbit coupling (DSOC). The conductance as a function of the Fermi energy shows typical resonance and antiresonance (conductance zero) characteristics in the absence of spin-orbit coupling (SOC). When one type of SOC (RSOC or DSOC) is introduced, the original conductance zeros are lifted, but new conductance zeros emerge. It is found that all the conductance zeros depend sensitively on the disorder, and the fluctuate weakens and smoothens the oscillations of the conductance. In the presence of both types of SOCs, the interplay between the RSOC and the DSOC opens a gap in the energy spectrum and breaks the cylindrical symmetry of the ring. Consequently, symmetrically coupled three-terminal rings show anisotropic conductances, which are robust against weak disorders.     

Effect of charge-spin separation on the conductance through interacting low-dimensional rings

We calculate the conductance through Aharonov-Bohm chain and ladder rings pierced by a magnetic flux which couples with the charge degrees of freedom. The system is weakly coupled to two leads and contains strongly interacting electrons modeled by the prototypical t-J and Hubbard models. For a wide range of parameters we observe characteristic dips in the conductance as a function of magnetic flux which are a signature of spin and charge separation. We also show how the dips evolve when the parameters of the models depart from the ideal case of total spin-charge separation. The ladder ring can be mapped onto an effective model for large anisotropy which can be easily analyzed. These results open the possibility of observing this peculiar many-body phenomenon in anisotropic ladder systems and in real nanoscopic devices.     

Electrical transport in rings of single-wall nanotubes: one-dimensional localization

We report low-temperature magnetoresistance (MR) measurements on rings of single-wall carbon nanotubes. Negative MR characteristic of weak one-dimensional localization is clearly observed from 3.0 to 60 K, and the coherence length L(varphi) is obtained as a function of temperature. The dominant dephasing mechanism is identified as electron-electron scattering. Below 1 K, we observe a transition from weak to strong localization, and below 0.7 K a weak antilocalization is induced by spin-orbit scattering.     

Electrical conductance time constants for freely decaying arcs

Electrical conductance time constants for the early stages of free decay after current modulation have been calculated from experimental measurements on a 5 mm diameter cascade arc at atmospheric pressure. The time constants were found by measuring the electric field response of the asymptotic portion of the arc column immediately after a sudden step decrease of arc current. The electric field strength was monitored by means of the copper cooling disks of the cascade, whose probe characteristics were studied thoroughly. The initial high current was supplied by a capacitor discharge circuit which was inductively compensated to produce a square wave pulse of ～ 2 msec duration. Time constants for initial decay were measured in both argon and nitrogen for initial currents ranging from 100 to 400 amperes. The initial free decay time constants of nitrogen were found to increase weakly from approximately 25 to 35 usec over the initial current range considered. The time constants of argon decreased from approximately 100 to 60 Μsec over the same initial current range.     

Electrical conduction in copper-poly(vinyl-alcohol) composites

The temperature dependence of d.c. conductivity was investigated in composites of copper (Cu) and poly(vinyl-alcohol) (PVA) prepared by compression of a mixture. A minimum appeared in the conductivity vs reciprocal temperature curves, which is discussed as a result of the competition between hopping and thermally activated modes of conduction. It was also found that the conduction of Cu-PVA composites obeys a percolation model. Dependences of reverse current vs voltage were investigated and discussed in terms of a clean-up effect of charge carriers. Quantitative analysis was carried out to determine the activation energy, drift mobility and carrier concentration.     

Electrical conductance properties for magnetic tunnel junctions with MgO barriers

We measured inelastic electron tunneling (IET) spectra and conductance for MgO tunneling magnetoresistance (TMR) films to obtain information on the ferromagnetic/barrier layer interface. The IET spectra showed the difference between amorphous and crystalline structures in the barrier. In the magnetic tunnel junction (MTJ) with a crystalline barrier the IET spectra indicated an Mg-O phonon peak at a low bias voltage by measurement with a parallel magnetization configuration. On the other hand, no peak was observed in the MTJ with an amorphous barrier.     

Electrical and galvanomagnetic effects in new sodium-doped fullerene composites

The Hall effect, magnetoresistance, the temperature dependence of the electrical resistance, and the current-voltage characteristics of new C₆₀-based carbon-carbon composites were studied in magnetic fields of up to 28 kOe at temperatures T = 1.8–336 K. Sodium-doped samples were used. It is shown that there is weak localization of charge carriers and strong electron-electron interactions. Superconductivity occurs in the range T ≤ 15 K. A mixed current may exist at higher temperatures. Cooper pairs are assumed to form due to the interaction of the conduction electrons with the π-electron system of a C₆₀ molecule.     

Nonadiabatic noncyclic geometric phase and ensemble average spectrum of conductance in disordered mesoscopic rings with spin-orbit coupling

We generalize Yang's theory from the U(1) gauge field to the non-Abelian U(1)xSU(2)(spin) gauge field. Based on this generalization and taking into account the geometric Pancharatnam phase as well as an effective Aharonov-Bohm (AB) phase in nonadiabatic noncyclic transport, we calculate the ensemble average Fourier spectrum of the conductance in disordered mesoscopic rings connected to two leads. Our theory can explain the experimental results reported by Morpurgo et al. [Phys. Rev. Lett. 80, 1050 (1998)] more satisfactorily. We indicate that the observed splitting stems from the nonadiabatic noncyclic Pancharatnam phase and the effective AB phase, both being dependent on spin-orbit coupling.     

Electrical conductance of double layers composed of two metal thin films with different disorder

Thin films of Al, Cu, Ag and Au are coated at low temperatures with a highly disordered film of the same or one of the other metals, respectively. The total conductance of the thin film sandwich first decreases, then passes through a minimum, and finally increases with increasing thickness of the disordered overlayer. These total conductance versus overlayer thickness curves are interpreted within the Fuchs-Sondheimer-model of the geometrical size effect. Using this model the calculated fraction of specularly scattered conduction electrons at the surface of the base film is very much reduced by lowering the condensation temperature of the base film. Moreover the decrease in conductance is dependent on the choice of coating metal. This observation is an indication of fissures in the surface of the base film.     

Electrical and infrared dielectrical properties of silica aerogels and of silica-aerogel-based composites

In this contribution we have studied the key electrical parameters of silica aerogels and of silica-aerogel-based composites, namely the dielectric constants , the dielectric losses tan (at 1 kHz), and the breakdown fields E _b (at 50 Hz). For low-density bulk silica aerogels we find =1.25 and tan =0.0005. E _b is about 500 kV/cm in quasi-homogeneous fields, and of the order of MV/cm in strongly inhomogeneous fields. The dielectric constants of partially densified aerogels increase linearly with density; their dielectric losses are relatively large and their breakdown fields are comparativiely low. The same results are found for aerogels in the form of settled materials, i.e. aerogel granules and powders in air. Acrylate-based aerogel composites with volume fractions larger than 70% have low dielectric constants but their losses are at least 10 times higher than those of low-density aerogels. These materials sustain high local fields in the MV/cm region, while in quasihomogeneous fields, breakdown occurs at about 100 kV/cm. Based on the present results and the interplay with other physical properties (low mechanical resistance, low thermal conductivity, adsorption of water, etc.), silica aerogels and silica aerogel-acrylate-based composites are predicted to have a low potential for electrical insulation.     

Electrical properties of composites based on polystyrene with hybrid silicon dioxide particles

Composites based on polysterene and hybrid core-shell nanoparticles are studied; the nanoparticle core consists of silicon dioxide, and ethylphenylic groups (organic shell) are grafted to the core surface. It is shown that the permittivity, the volume resistivity, the thermostimulated depolarization current spectra, and the glass transition temperature of these materials depend on the nanofiller content and, what is more important, the nanofiller distribution over the polymer volume in the form of particles or their aggregates.     

Electrical properties of nickel - YSZ composites prepared by a sol gel method

Composite materials containing metallic nickel and yttria stabilised zirconia (YSZ) have been prepared by a sol gel method. Microstructural investigations show that metal particles with a diameter of less than 300 nm still remain present after sintering. In the present work, the conductivity behaviour of such composites was investigated. Furthermore, ageing phenomena, in particular the influence of the nickel content, were studied. A possible application of such materials as anodes in solid oxide fuel cells will be discussed. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11 – 18 Sept. 1994     

Electrical properties of expanded graphite/poly(styrene-co-acrylonitrile) composites

A four probe method was used to measure the electrical conductivities of poly(styrene-co-acrylonitrile)/expanded graphite composites. The composites showed excellent electrical conductivities in both a- and c-axes. The different effects of temperature and the applied current on the conductivities in a-axis and in c-axis were observed. It was found that the conductive behaviors of the composite were different from those of semiconductor and carbon black loaded polymer composites. All these differences and excellent conductivity were considered to be related to the structure of the composites.     

Electrical and optical properties of composites based on carbazole derivatives and silicon particles

The electrical and optical properties of polymer-silicon composites with particles incorporated by different means have been studied. It is shown that both when silicon particles are embedded in a carbazole-containing polymer matrix and in the case of a pure polymer, the I–V characteristics are nonlinear and asymmetric (the I–V characteristics of the carbazole-containing polyorganosiloxane, which has silicon atoms in the monomer link, behave in a more symmetric pattern). In all cases, the I-V characteristics can be fitted with power laws, I(V) ～ V ^p , with three different slopes for different voltage intervals, which remainds one of the pattern typical of the mechanism of space-charge-limited currents. It is shown that, in its luminescent properties, the carbazole-containing polyorganosiloxane is similar to a carbazole-containing polymer matrix with embedded silicon particles. The results obtained argue for charge transfer between the polymer and silicon nanoparticles if they are embedded in the matrix and for an formation of an interchain charge-transfer complex in the case of chemically bound silicon.     

Electrical bistable devices using composites of zinc sulfide nanoparticles and poly-(N-vinylcarbazole)

N-dodecanethiol capped zinc sulfide(Zn S) nanocrystals were synthesized by the one-pot approach and blended with poly(N-vinylcarbazole)(PVK) to fabricate electrical bistable devices. The corresponding devices did exhibit electrical bistability and negative differential resistance(NDR) effects. A large ON/OFF current ratio of 104 at negative voltages was obtained by applying different amplitudes of sweeping voltage. The observed conductance switching and the negative differential resistance are attributed to the electric-field-induced charge transfer between the nanocrystals and the polymer,and the charge trapping/detrapping in the nanocrystals.