Showing papers on "Charge density published in 1988"

Nonlinear diffusion limit for a system with nearest neighbor interactions

Abstract: We consider a system of interacting diffusions. The variables are to be thought of as charges at sites indexed by a periodic one-dimensional lattice. The diffusion preserves the total charge and the interaction is of nearest neighbor type. With the appropriate scaling of lattice spacing and time, a nonlinear diffusion equation is derived for the time evolution of the macroscopic charge density.

Measurement of spatial charge distribution in thick dielectrics using the pulsed electroacoustic method

Abstract: Measurement techniques of volume charge distribution within insulating materials are developed using a pulsed electroacoustic method The fundamental equation of the relation between the charge distribution in the insulating material and the signal voltage from the electroacoustic transducer is presented along with the typical measured charge profiles in polymethylmethacrylate (PMMA) during and after electron-beam irradiation >

Prediction of the structures of hydrogen-bonded complexes using the laplacian of the charge density

Abstract: The laplacian of the charge density is used to predict the structures and geometries of hydrogen-bonded gas-phase complexes of the type BASE-HF. The bases used are N2, OC, SC, OCO, SCO, HCN, N2O, HCP, H2O, H2S, H3N, H3P, O3, OSO, H2CO, HF, HCl, N2S and H2CS. Many of the weaker complexes have not been characterized experimentally, and so, results of full 6–31 G** geometry optimizations are presented. The laplacian of the charge density, ∇2ρ, determines where charge is locally concentrated and depleted. The point where ∇2ρ attains its maximum magnitude in a region of charge concentration in the base defines the site of electrophilic attack by the acidic H of HF. The angle of electrophilic attack predicted in this manner is compared with the ab initio equilibrium angle that the hydrogen of HF makes with the base. In general, the angles predicted using the laplacian are in good agreement with the ab initio and experimental results. The present results are also compared with those obtained from electrostatic m...

First-principles calculation of highly asymmetric structure in scanning-tunneling-microscopy images of graphite

Abstract: Using a first-principles calculation, we have computed the charge density for states near ${E}_{F}$ that is related to the current density observable in scanning-tunneling-microscopy experiments for surfaces of hexagonal, rhombohedral, and a model stage-1 intercalated graphite. In hexagonal and rhombohedral graphite, the tunneling current is predicted to be considerably smaller at surface atomic sites which have nearest neighbors directly below them than at sites with no such neighbors. This asymmetry is explained by the particular symmetry of the wave functions at the Fermi surface of graphite near K\ifmmode\bar\else\textasciimacron\fi{} in the surface Brillouin zone. The calculated asymmetry is nearly independent of the polarity and decreases with increasing magnitude of the bias voltage. Our results show that no asymmetry is expected on surfaces of stage-1 AA stacked intercalated graphite. The dependence of the asymmetry on the tip-to-surface separation has also been evaluated using a tight-binding model for different bias voltages. For the surface of hexagonal graphite, our predictions have been confirmed by recent experiments.

Density-functional theory of many-electron systems subjected to time-dependent electric and magnetic fields.

Abstract: A time-dependent density-functional formalism is developed for many-electron systems subjected to external electric and magnetic fields with arbitrary time dependence. The single-particle current density is shown to determine uniquely the time-dependent scalar and vector potentials characterizing the system, and hence also the many-particle wave function. A Levy-type universal functional is defined, and practical schemes for the calculation of electron density and the current density through hydrodynamical as well as a set of single-particle Kohn-Sham-like equations are proposed. The gauge invariance of the present self-consistent formalism is also proved.

A DIM model for homogeneous noble gas ionic clusters

Abstract: The method of diatomics-in-molecules (DIM) is applied to the calculation of the energy of the homogeneous noble-gas ionic clusters Ar + and Xe + forn=3, 4, ..., 22. The trimers are stable symmetric linear molecules exhibiting chemical binding, a result in agreement both with ab initio calculations and with previous DIM work. The clusters up ton=13 are best described as a trimer ion surrounded by neutrals, whereby the charge distribution changes slightly with increasingn. Both noble gases exhibit a special stability associated with the completion of the first shell of neutral atoms atn=13. Asn increases from 13 to 22, there is a greater delocalization of the positive charge, the central ion tending to become a linear tetramer, symmetric for Xe and unsymmetric for Ar. Energies of the excited electronic states are reported and the possibility of developing simpler DIM models for the clusters and for mixed noble gases is discussed.

The shell structure of atoms and the Laplacian of the charge density

Abstract: It is shown that the form of the Laplacian of the charge density provides a more complete resolution of the shell structure of atoms than the radial density function. The complete shell structure is resolved for s‐block and most p‐block atoms, but only the inner shells are resolved in the d‐block elements. The shell structures revealed by the Laplacian of the charge density and the radial density function parallel one another where direct comparison is possible.

Analysis of electronic structure and charge density of the high-temperature superconductor YBa2Cu3O7

Abstract: Self-consistent linearized augmented plane wave (LAPW) method calculations of the band structure, density of states, Fermi surface, Coulomb potential, charge density, core-level shifts, and electron-phonon interaction are presented for Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/. The calculated Sommerfield parameter ..gamma.. is 4.35 mJ(mole Cu)/sup /minus/1/ K/sup /minus/2/, roughly about a factor of 2 smaller than experimentally deduced values of the enhanced value ..gamma.. = (1 + lambda)..gamma../sub 0/, suggesting that the Fermi surface mass enhancement is of the order of unity. The crystal charge density is best represented by overlapping spherical ionic densities when the Cu and O ions are assigned charges of +1.62 and -1.69, respectively, corresponding to about 0.3 holes per oxygen atom. Core-level energies for the inequivalent atoms differ by as much as 0.45 eV for Cu and 0.7 eV for O, amounts which may be detectable by core-level spectroscopies. These results provide important information on the character and magnitude of ionic contributions to bonding in these materials. Within the rigid muffin-tin approximation, calculated McMillan-Hopfield parameters yield estimates for the electron-phonon strength lambda that appear to be too small to account for the observed T/sub c/. We point out an unusual band of oxygen-derived chain states below, butmore » within 0.1 eV of, the Fermi level.« less

The Laplacian of the charge density and its relationship to the shell structure of atoms and ions

Abstract: The Laplacian of the spherically averaged charge density ∇2ρ(r) has been computed from nonrelativistic SCF wave functions for the neutral atoms from hydrogen to uranium, and the singly positive ions, from helium to barium and lutetium to radium, in order to examine the shell structure. ∇2ρ(r) exhibits a number of extremal points and zeros with the absolute value of the function becoming smaller at each successive extremal point. The zeros, in particular the odd numbered zeros, are shown to exhibit good correlation with the Bohr theory of an atom while the extremal points correlate to a lesser extent. At most five shells are seen in the studied atomic cases based on the fact that the odd numbered zeros are the topological feature of ∇2ρ(r) most indicative of a shell.

Structural and electronic properties of WC.

Abstract: We present the results of a pseudopotential local-orbital calculation on hexagonal WC. The calculated lattice constants and cohesive energy are in good agreement with experiment, and the calculated bulk modulus lies within the wide range of measured values. The band structure and Fermi surface obtained are also generally consistent with experimental data, and the Fermi level is found to lie in a deep minimum of the density of states. The calculated valence electron charge density indicates that the bonding in WC consists of a metallic component similar to that found in solid W, and strong W-C bonds.

Experimental studies of the behaviour of single adsorbed atoms on solid surfaces

Abstract: The behaviour of single adsorbed atoms (adatoms) on solid surfaces can now be studied with the field ion microscope (FIM) and the atom-probe FIM With the FIM, adsorbed atoms can be seen With the atom probe, adatoms can be analysed The substrate surface for experimentation can be developed to atomic perfection by low-temperature field evaporation The number of adsorbed atoms participating in an experiment can be specified and controlled Quantitative information on diffusion parameters, interatomic forces between adsorbed atoms, adatom-impurity-atom interactions and adatom-plane-edge interactions, etc, can be measured Association of single atoms into clusters and their dissociation, and the information of adsorption layer superstructure, can also be studied in atomic detail The charge distribution of a surface atom or the localised electronic density of states of an adatom, can be studied by measuring both its dipole moment and polarisability Basic principles and results of these FIM studies with single atoms are presented and discussed Possible future studies are also considered, and their principles described

Vacuum polarization in a strong external field

Abstract: Vacuum polarization in the field of a high-Z finite-size nucleus is examined, and the polarization charge density in coordinate space of order alpha(Z alpha)exp n not less than 3 is calculated. Energy-level shifts of K- and L-shell electrons in hydrogenlike systems are given.

Donor cell-finite element descriptions of wire-duct precipitator fields, charges and efficiencies

Abstract: Computations of electric field and charge density structures and resultant efficiencies in wire-duct electrostatic precipitators are described. The computational method is based upon the finite-element method as a means for computing the potential and electric field for a known charge distribution and a donor cell method that imposes conservation of charge in integral form as a means for computing charge densities for a known field structure, with iterative convergence to self-consistent solutions. The solution region is discretized by the Delaunay algorithm. This division simultaneously provides the triangles needed for the finite-element method and the Voronoi polygons over which charge conservation is imposed. Thus, a natural geometric interface is established between the finite-element method and the donor cell description. Results are shown in models that include the time-averaged effect of turbulence through a diffusivity coefficient, bipolar ionic species modeling back ionization, and the effects of particulate and ionic space charge. >

Current transport mechanisms in atomically abrupt metal-semiconductor interfaces

Abstract: A comprehensive model for electron transport mechanisms across a fully formed Schottky-barrier junction is proposed in which the metal-semiconductor interface is approximated as an abrupt quantum mechanical transition. Improved formulations of the barrier-lowering mechanisms and carrier tunneling effects are derived where the dipole barrier lowering is modeled as a single exponential decay of the total surface charge density. Quantum calculations follow a two-band model in which the imaginary component of the electron wave vector in the semiconductor energy gap is obtained by including the effect of both conduction and valence states. The energy band profile effects are included in the calculation of tunneling current, and it is shown that the finite negative charge residing at the metal-semiconductor interface considerably modulates the tunneling transmission probability of carriers. Experimental results obtained from atomically clean Al-n/sup +/GaAs-nGaAs interfaces fabricated by in situ molecular-beam epitaxy (MBE) are shown to be in excellent agreement with the transport calculations. >

Effect of electron correlation on the topological properties of molecular charge distributions

Abstract: This paper reports on the effect of electron correlation on the topological properties of the charge density and its associated gradient vector and Laplacian fields. The properties of these fields define the atoms, their reactivity, and the structure of a molecular system within the theory of atoms in molecules. The singlet and triplet states of CH2, CF2, and SiH2 are investigated using a configuration interaction method which includes all single and double excitations with respect to suitable zero‐order reference wave functions, together with a number of hydrocarbon molecules including unsaturated and geometrically strained systems for which the correlation is introduced via the generalized valence bond approach. It is found that the correlated charge distributions possess the same number and kind of critical points in both the ρ(r) and ∇2ρ(r) fields as are found for SCF charge distributions. Thus the topology of a charge distribution and the structure it defines are unaffected by the addition of Coulomb correlation. The quantitative changes in the properties of the charge density at the critical points in both ρ(r) and ∇2ρ(r) induced by correlation are found to be small in magnitude and to be more pronounced for shared or covalent atomic interactions than for systems with pronounced charge transfer between the atoms. The properties of the atoms in these molecules also exhibit correspondingly small changes in value.

Field-effect analysis for the determination of gap-state density and Fermi-level temperature dependence in polycrystalline silicon

Abstract: The field-effect conductance has been used in two distinct ways to determine the gap-state density in polycrystalline silicon. The relationship between the surface potential and the gate voltage, which determines the gap-state density, has been deduced according to the incremental method, already proposed by Suzuki et al., and a new method. The new method is based on the temperature dependence of the derivative of the field-effect conductance with respect to the gate voltage. The results from the two methods are in good agreement and show a rapidly increasing gap-state density in the upper half of the gap. The temperature analysis of the field-effect conductance indicates that the position of the Fermi level is temperature dependent. The contribution to this dependence from the statistical shift has been determined.

Analytical method for evaluating surface charge distribution on a dielectric from capacitive probe measurement-application to a cone-type spacer in +or-500 kV DC-GIS

Abstract: The measurement of surface charging on a spacer in a +or-500-kV HVDC gas-insulated switchgear (GIS) is performed by a capacitive probe set inside the gas tank. The various factors that cause measurement errors are examined using a simple model. An analytical method to correct the measurement errors and transform the probe output into the charge distribution on the surface of the spacer is developed. The method is an extension of a numerical method for 3-D surface charge calculation. It is shown that the charge distributions from the probe measurement agree fairly well with dust figures. >

In-situ insulator surface charge measurements in dielectric bridged vacuum gaps using an electrostatic probe

Abstract: Surface charge measurements on alumina and polymer insulators were carried out after stressing them with DC voltages in a high vacuum. The order of magnitude of surface charge density was found to be the same for materials with supposedly widely varying secondary-emission yields. Surface coatings on alumina insulators reduced charge accumulation because of increased surface conductivity and/or reduced secondary-emission yield, which led to significant improvement in voltage hold-off for alumina ceramics. Removing the cathode triple junction from the main body of the cylindrical insulator, reducing the X-ray activity in the gap, or relieving the stress at the critical junction did not significantly alter the surface charge characteristics of cylindrical insulators. Wet hydrogen firing of plain alumina reduced the voltage hold-off by 25% without altering the surface charge density. It is postulated that the charging of insulators in bridged vacuum gaps with DC stresses is due to internal secondary emission produced by ionization of the lattice in the surface layer of the insulating material, by primary electrons injected at the cathode triple junction. This mechanism of charge production differs from the current models where charging is believed to occur due to electrons hopping along the surface/vacuum interface. >

Theoretical study of the total (elastic+inelastic) cross sections for electron -H2O (NH3) scattering at 10-3000 eV

Abstract: By employing semi-empirical spherical complex optical potentials (SCOP) for the electron-H2O and NH3 systems, the author reports first theoretical results on the total (elastic+inelastic) cross sections over the energy range of 10-3000 eV. The real part of the local SCOP term for each molecule is a sum of three isotropic interactions, namely the static, the polarisation and the exchange; all three potentials are defined in a functional form of the target charge density evaluated from near-Hartree-Fock one-centre molecular wavefunctions. The contribution from the non-spherical interaction terms (such as the permanent dipole, quadrupole, etc.) is included incoherently via the first Born approximation under the rotating molecule model in which the integral cross section is finite.

Ionic contributions to lattice instabilities and phonon-dispersion in la2cuo4

Abstract: We have applied the potential-induced breathing model, a nonempirical ionic model, to La/sub 2/CuO/sub 4/. The ionic model predicts the observed tetragonal-to-orthorhombic distortion, predicts a lower-symmetry ground state, and predicts a stable oxygen breathing mode. We find unstable phonon branches that we relate to phase transitions. Harmonically unstable double-well modes will couple to the charge density in the copper-oxide planes and possibly contribute to high-T/sub c/ superconductivity. This ionic coupling is not included in recent calculations.

dc surface flashover mechanism along solids in vacuum based on a collision‐ionization model

Abstract: Selected experimental results are presented from an extensive experimental investigation of dc prebreakdown and breakdown processes in vacuum along solid insulators made of a variety of alumina ceramics and polymers. The investigations included measurements of predischarge current, x‐ray intensity, insulator surface charge density, breakdown behavior as influenced by insulator surface characteristics and insulator profiles, and the analysis of the spectral characteristics associated with insulator surface luminescence. The prebreakdown and breakdown phenomena were found to be strongly influenced by surface microstructure and the chemical state of the insulator surface. The experimentally observed phenomena in our studies strongly point to a new breakdown model based on collision‐ionization by electrons of defect sites and/or traps within the dielectric subsurface at the vacuum interface.

Counterion condensation in micellar and colloidal solutions

Abstract: Asymptotic solutions are obtained to the Poisson–Boltzmann equation for large, highly charged spheres in an ionic solution. It is proved that as the size of the sphere is increased, keeping the surface charge density fixed, there is a critical value for the radius beyond which counterion condensation sets in. This critical radius is much larger than the Bjerrum length but small compared to the Debye length and depends on the ionic strength. An expression is derived for the effective charge. When the radius becomes much larger than the Debye length, it is shown that sufficiently many counterions condense in a shell of thickness small compared to the polyion radius to essentially neutralize the polyion charge.

The radial charge distribution and the shell structure of atoms and ions

Abstract: The radial charge distribution, D(r), for the neutral atoms, hydrogen through uranium, and the singly positive ions, helium through barium and lutetium through radium, are computed from the non-rel...