Showing papers on "Charge density published in 1993"

Wigner crystal in one dimension.

Abstract: A one-dimensional gas of electrons interacting with long-range Coulomb forces [V(r)\ensuremath{\approxeq}1/r] is investigated. The excitation spectrum consists of separate collective charge and spin modes. For arbitrarily weak Coulomb repulsion density correlations at wave vector 4${\mathit{k}}_{\mathit{F}}$ decay extremely slowly and are best described as those of a one-dimensional Wigner crystal. Pinning of the Wigner crystal then leads to the nonlinear transport properties characteristic of charge density waves. The results allow a consistent interpretation of the plasmon and spin excitations observed in one-dimensional semiconductor structures, and suggest an interpretation of some of the observed features in terms of ``spinons.''

High-Density Nanosecond Charge Trapping in Thin Films of the Photoconductor ZnODEP.

Abstract: An electrooptical memory effect is observed with solid thin films of the photoconductor zinc-octakis(beta-decoxyethyl) porphyrin (ZnODEP) sandwiched between two optically transparent electrodes. Upon irradiation with the simultaneous application of an electric field, electron-hole pairs are generated and separated within the photoconductive layer. These electron-hole pairs become "frozen" within the films when the irradiation is interrupted. These trapped charges can be released by irradiation of the cell, resulting in a transient short-circuit photocurrent. No cross talk between adjacent memory elements separated by approximately 0.2 micrometer (a density of 3 gigabits per square centimeter) was detected. The charge storage system is robust and nonvolatile. The response time for the write-read beam is in the subnanosecond range, and no refreshing is required for long-term retention of trapped charges.

The influence of the strain‐induced electric field on the charge distribution in GaN‐AlN‐GaN structure

Abstract: We show that strongly pronounced piezoelectric properties play a key role in GaN‐AlN‐GaN semiconductor‐insulator‐semiconductor (SIS) and related structures. In sufficiently thin AlN layers, the lattice constant mismatch is accommodated by internal strains rather than by the formation of misfit dislocations. These lattice‐mismatch‐induced strains generate polarization fields. We demonstrate that, in a GaN‐AlN‐GaN SIS structure with the growth axis along a (0001) crystallographic direction, the strain‐induced electric fields can shift the flat band voltage and produce an accumulation region on one side and a depletion region on the other side of the AlN insulator. On which side of the insulator the accumulation region is produced depends on the type of atomic plane at the heterointerface (Ga or N). The surface charge density caused by the piezoeffect is on the order of 1012 cm−2. As a consequence of the asymmetry in the space charge distribution, the capacitance‐voltage (C‐V) characteristics of the SIS stru...

Bistable saturation in coupled quantum‐dot cells

Abstract: Model quantum dot cells are investigated as potential building blocks for quantum cellular automata architectures. Each cell holds a few electrons and interacts Coulombically with nearby cells. In acceptable cell designs, the charge density tends to align along one of two cell axes. Thus, a cell ‘‘polarization,’’ which can be used to encode binary information, is defined. The polarization of a cell is affected in a very nonlinear manner by the polarization of its neighbors. This interaction is quantified by calculating a cell–cell response function. Effects of nonzero temperature on the response of a model cell are investigated. The effects of multiple neighbors on a cell are examined and programmable logic gate structures based on these ideas are discussed.

Proton binding to humic substances. 1. Electrostatic effects

Abstract: Ion binding to humic substances is influenced by the chemical heterogeneity and by the variable charge behavior of the humics. In this paper, we concentrate on the variable charge effects. To study these effects, acid/base titration data of 11 humic substances measured at a series of salt levels are analyzed. In a first-order approach, the organic matter molecules are treated as an ensemble of small rigid and impermeable cylindrical or spherical particles of a certain size and a variable surface charge density depending on pH and salt concentration. With double-layer models for such particles, the electrostatic effects on the proton binding can be described reasonably well. When the density of the humic material is used as a constraint, the radius of the spheres or the cylinders is the only adjustable parameter in the model

Theory of structural phase transitions in a trapped Coulomb crystal.

Abstract: Crystalline confined systems of charges exhibit structural phase transitions as a function of the anisotropy of the confining potential. A theory which describes these structural transitions is presented. The theory is based on the transitions which occur in infinite homogeneous 1D and 2D systems. These transitions have been previously explored for 1D systems but not for 2D systems. The charge density in the inhomogeneous crystal is found using a fluid theory as well as a theory which keeps correlations in the local density approximation.

Space Charge Segregation at Grain Boundaries in Titanium Dioxide: II, Model Experiments

Abstract: A quantitative study of space charge solute segregation at grain boundaries in TiO[sub 2] is conducted, using a new STEM method for the measurement of aliovalent solute accumulation. It is shown that the electrostatic potential at grain boundaries can be varied in sign and magnitude with doping, oxygen pressure, and temperature, and that the isoelectric point lies in slightly donor-doped compositions for samples annealed in air. The experimental results closely fit the space charge model in Part 1. Space charge solute segregation is found even in defect regimes of high electron concentration. Approximately one in ten grain boundaries are special in exhibiting no detectable segregation; in one such instance a twin boundary is identified. Among boundaries with significant amounts of segregation, clear differences in potential also exist. From the potential determined in acceptor- and donor-doped composition, the Frenkel energy (assumed to be lower than the Schottky energy in TiO[sub 2]) can be separated into its individual terms. An average value for the titanium vacancy formation energy of g[sub v[sub Ti]] = 2.4 eV and an upper limit to the titanium interstitial formation energy of g[sub Ti[sub i]] = 2.6 eV are obtained.

AM1 study of the electronic structure of coumarins

Abstract: We report our calculations on a series of coumarin molecules. Using semiempirical methods with an AM 1 parametrization, we have calculated the ground-state, first excited triplet state, and first excited singlet state energies and dipole moments as well as their dependence on the geometries of different labile side groups. We find that for all of the coumarins there are excited triplet states in close energetic proximity to the excited singlet states, and the relative ordering of these states depends on the substituents attached to the coumarin chromophore

First-principles study of graphite monofluoride (CF)n

Abstract: The structure and the electronic properties of graphite monofluoride (CF${)}_{\mathit{n}}$ have been studied within the framework of the density-functional technique, using nonlocal ionic pseudopotentials and a large number of plane waves. The chair conformation is shown to be energetically favored with respect to the boat conformation by 0.145 eV per CF unit. The transition between these two states has been investigated: the boat conformation is metastable, with an estimation of the transition barrier on the order of 2.72 eV. This indicates that the material actually synthesized could depend on the kinetics of the intercalation or could as well be a mixing of both conformations. The equilibrium geometries are compared with experimental data. We also present the valence charge density and the band structure of the chair conformation.

Pulsed electro-acoustic method for measurement of space charge distribution in power cables under both DC and AC electric fields

Abstract: A modification of an earlier technique, to measure the distribution of space charge in the insulation of a coaxial cable, under both DC and AC electric fields, is described. The mathematical analysis is developed on the basis of acoustic waves propagating in media with cylindrical geometry. In order to measure the charge distribution in AC electric fields at any phase angle, a phase shift technique has been employed. The method of calibration of charge density is given, and the charge distribution in the insulation (2 mm in thickness) of XLPE cable has been measured under DC voltages (+or-40 kV, at room temperature) and AC voltage (17 kV peak, at 24 degrees C and 105 degrees C).

Derivation of force theorems in density-functional theory: Application to the full-potential LMTO method.

Abstract: A systematic technique for deriving force theorems within density-functional theory is presented. It is based on an arbitrary guess at the response of the electrons to a shift of a nucleus, yielding a valid force theorem for each choice. Application to the full-potential linear-muffin-tin-orbital method gives a workable force expression which is unusually insensitive to deviations from self-consistency.

Electronic and structural properties of GaN by the full-potential linear muffin-tin orbitals method: The role of the d electrons.

Abstract: The structural and electronic properties of cubic GaN are studied within the local-density approximation by the full-potential linear muffin-tin orbitals method. The Ga 3d electrons are treated as band states, and no shape approximation is made to the potential and charge density. The influence of d electrons on the band structure, charge density, and bonding properties is analyzed. Due to the energy resonance of Ga 3d states with nitrogen 2s states, the cation d bands are not inert, and features unusual for a III-V compound are found in the lower part of the valence band and in the valence charge density and density of states. To clarify the influence of the d states on the cohesive properties, additional full- and frozen-overlapped-core calculations were performed for GaN, cubic ZnS, GaAs, and Si. The results show, in addition to the known importance of core-valence exchange-correlation nonlinearity, that an explicit description of closed-shell interaction has a noticeable effect on the cohesive properties of GaN. Since its band structure and cohesive properties are sensitive to a proper treatment of the cation d bands, GaN appears to be somewhat exceptional among the III-V compounds and reminiscent of II-VI materials.

Synchrotron X-ray study of the electron density in α-Al2O3

Abstract: Structure factors for synthetic haematite, α-Fe 2 O 3 , have been measured for two small crystals using focused λ=0.7 A synchrotron radiation. The structure factors from the two data sets are consistent. Approximate symmetry in the concordant densities, related more closely to the Fe-Fe geometry than to the nearest-neighbour Fe-O interactions, is similar to that in the corundum α-Al 2 O 3 structure. Deformation density maxima are located at the midpoint of the Fe-Fe vector along the c axis, on a common face for O-octahedra, perpendicular to c. Maxima also occur at the midpoint of the Fe-Fe vector bisecting the edges of the O-octahedra. These results are in accordance with theoretical predictions for metal-metal bonding

Inclusion of charge correlations in calculations of the energetics and electronic structure for random substitutional alloys.

Abstract: Currently, the coherent-potential approximation (CPA) implemented via the multiple-scattering theory of Korringa, Kohn, and Rostoker (KKR) gives the best first-principles description of the electronic structure for random substitutional alloys. However, the total energy has an important component of electrostatic energy missing, namely, that arising from the correlation of charges with varying atomic environments. We develop a ``charge-correlated'' CPA method (cc-CPA) which includes (some) local environmental charge correlations within the KKR-CPA method. We investigate the cc-CPA energetics for several alloys and show that the formation energies are in better agreement with experimental results. These calculations show that the excess charge on a species is almost completely screened by the first-neighbor shell. We then derive a simplified scheme to include the vast majority of the omitted electrostatic energy from charge correlations which requires only a species-dependent shift of the potentials within the original KKR-CPA method. We also discuss the ramifications on the electronic structure.

Convective cells in nonuniform dusty plasmas

Abstract: It is shown that purely damped convective cell modes acquire a real frequency in the presence of static charged dust grains in a nonuniform magnetized dusty plasma. The frequency of the two‐dimensional mode is induced by the plasma density gradient and corresponds to charge density waves arising out of the plasma non‐neutrality. The dynamics of weakly interacting finite‐frequency convective cell modes is governed by the generalized Navier–Stokes equation. The presence of the new term arising from the dust inhomogeneity in the latter provides the possibility of a two‐dimensional dipolar vortex solution in dusty plasmas.

Charge movement by the Na/K pump in Xenopus oocytes.

Abstract: Pre-steady-state transient currents (1986. Nakao, M., and D. C. Gadsby. Nature [Lond.]. 323:628-630) mediated by the Na/K pump were measured under conditions for Na/Na exchange (K-free solution) in voltage-clamped Xenopus oocytes. Signal-averaged (eight times) current records obtained in response to voltage clamp steps over the range -160 to +60 mV after the addition of 100 microM dihydroouabain (DHO) or removal of external Na (control) were subtracted from test records obtained before the solution change. A slow component of DHO- or Na-sensitive difference current was consistently observed and its properties were analyzed. The quantity of charge moved was well described as a Boltzmann function of membrane potential with an apparent valence of 1.0. The relaxation rate of the current was fit by the sum of an exponentially voltage-dependent reverse rate coefficient plus a voltage-independent forward rate constant. The quantity of charge moved at the on and off of each voltage pulse was approximately equal except at extreme negative values of membrane potential where the on charge tended to be less than the off. The midpoint voltage of the charge distribution function (Vq) was shifted by -24.8 +/- 1.7 mV by changing the external [Na] in the test condition from 90 to 45 mM and by +14.7 +/- 1.7 mV by changing the test [Na] from 90 to 120 mM. A pseudo three-state model of charge translocation is discussed in which Na+ is bound and occluded at the internal face of the enzyme and is released into an external-facing high field access channel (ion well). The model predicts a shift of the charge distribution function to more hyperpolarized potentials as extracellular [Na] is lowered; however, several features of the data are not predicted by the model.

Charge generation and collection in p-n junctions excited with pulsed infrared lasers

Abstract: The author examines optical absorption processes for applications of infrared lasers to the simulation of single-particle effects in silicon and GaAs, which require that the laser is focused to a small area on the device surface. The resulting charge generation is compared with charge generation from heavy ions. Charge funneling is reduced in silicon structures because of the lower charge density unless the LET (linear energy transfer) is above a threshold value. In both materials, the effective LET of a laser is inherently nonlinear because of nonlinear absorption at high intensities. These factors limit quantitative comparisons between lasers and heavy ions, and are increasingly important as devices are scaled to smaller dimensions. >

Electronic charge distribution in crystalline diamond, silicon, and germanium.

Abstract: Recent refinement studies of a consolidated set of Si structure factors have produced information on the Si charge density with an unprecedented level of accuracy, unmatched by any other crystallographic study to date. In this work we examine the extent to which an accurate implementation of the local-density formalism can describe the charge distribution in silicon, as well as that of the experimentally less-refined data on diamond and germanium. Results of a refinement study of recent germanium and diamond measurements are presented and compared with the ab initio calculations. Our ab initio calculated structure factors for Si show a twofold to fivefold improvement in the R factor over previous local-density calculations

On the structural properties of NaCl: an ab initio study of the B1-B2 phase transition

Abstract: The structural properties and the phase transition between the B1 (rock salt) and B2 (CsCl) phases of NaCl have been investigated at an ab initio level. Total energy curves, obtained with periodic Hartree-Fock (HF) calculations, have been corrected a posteriori by integrating the HF charge density according to the correlation-only density-functional-like formulae proposed by Colle and Salvetti in 1975 and by Perdew in 1986 and 1991. The three functionals are 'non-local', containing terms that depend on the gradient of the electronic density. The correlation correction brings the HF data for the lattice energy (LE), lattice parameter (ao) and bulk modulus (B) into good agreement with experiment, the best performance being obtained with the most recent functional, (the percentage error is +0.3%, -0.5% and +5.6% respectively for LE, ao and B).

Interaction of tetradecyltrimethylammonium bromide with poly(acrylic acid) and poly(methacrylic acid). Effect of charge density

Abstract: The effect of polymer chalge density on the interaction of tetradecyltrimethylammonium brolnide

Magnetic field distribution in models of trabecular bone.

Abstract: A magnetostatic model consisting of a tetragonal lattice of struts of diamagnetic material, mimicking vertebral trabecular bone, was developed. The model allows estimation of the magnetic field histogram within the lattice's unit cell as a function of geometric parameters. The field was computed analytically from the induced magnetic surface charge density on the faces of the struts. The contribution from the induced magnetic field to the effective transverse relaxation rate, R2', was obtained as the mean decay rate of the Fourier transformed histograms, for both fixed and randomly oriented lattices. The model predicts the field distribution to increase with both strut thickness and density, paralleling material density. Finally, significant changes in R2' are predicted at constant material density, in that the field distribution widens with simultaneously increasing strut number density and decreasing strut thickness.

Interacting electrons in a one-dimensional quantum dot.

Abstract: The spectral properties of up to four interacting electrons confined within a quasi-one-dimensional system of finite length are determined by numerical diagonalization including the spin degree of freedom. The ground-state energy is investigated as a function of the electron number and of the system length. The limitations of a description in terms of a capacitance are demonstrated. The energetically lowest-lying excitations are physically explained as vibrational and tunneling modes. The limits of a dilute, Wigner-type arrangement of the electrons, and a dense, more homogeneous charge distribution are discussed.

Simulations of solvent effects on confined electrolytes

Abstract: Monte Carlo simulations are reported for the solvent primitive model (SPM) of the electrical double layer at electrically charged and neutral surfaces. Both solvent and ions are modeled as hard spheres with interactions governed by hard sphere and Coulomb interactions. Density profiles of solvent and ions and electrostatic potential profiles are presented for 1:1 electrolyte concentrations of 1 and 2 M at 300 K. Comparison of results at charged and neutral walls indicates that the density distribution of a dense solvent near a wall induces significant layering of ions and decrease of potentials in the double layer. This layering effect cannot be captured in simulations of the conventional primitive model (PM) of the double layer. The solvent induced steric ordering of electrolyte ions at a neutral wall can create a space charge layer.

Effective liquid drop description for the exotic decay of nuclei.

Abstract: The present model describes the exotic decay of nuclei by considering the molecular phase of the fragments in an effective liquid drop description of the process. Shell corrections are included via experimental values of energy released in the disintegration (Q value), which is used to define the effective surface tension of the drop. The Coulomb potential energy is the exact solution of the Poisson equation for a uniform charge distribution in the nuclear volume. The inertial coefficient of the barrier penetrability problem is determined by using the Werner-Wheeler approximation for the velocity field of the nuclear flow. The model is successful in calculating the half-life for both the exotic decay and the \ensuremath{\alpha} disintegration processes.

Adsorption of vapor at a solid interface: a molecular model of clay wetting

Abstract: A molecular description of the clay-water interface is used to simulate the adsorption of water on a solid surface. Grand canonical ensemble and isobaric-isothermal ensemble Monte Carlo simulations are used to describe the organization of the film of liquid water molecules adsorbed on a clay surface. This treatment allows predictions of the wettability of the mineral surface as a function of its charge density and the chemical nature of its different atomic components

Laplacian of charge density for binuclear complexes : terminal vs bridging carbonyls

Abstract: SCF wave functions have been computed for the ground state of Fe 2 (CO) 9 , Fe 2 (CO) 6 (C 2 H 2 ) and Mn 2 (CO) 10 complexes. These wave functions were the starting point for theoretical studies of the topology of the charge density of several carbonyl types. An analysis of the Laplacian of ρ shows that a bridging carbonyl shows a distribution of the electron density similar to that of a keto group, whereas in the terminal coordination the CO fragment retains the axial symmetry. The integration of the electron density suggests a large transfer of charge from the metal atom to each bridging CO. According to the back-bonding model, the electron density in a bridging carbon is more concentrated in the coordination plane. The atomic properties of equatorial and axial carbonyls in Mn 2 (CO) 10 present some differences

Electric charge asymmetry of the Universe and magnetic field generation.

Abstract: If at an early stage of the evolution of the Universe the gauge symmetry of electromagnetism was spontaneously broken, an electric charge asymmetry would develop. After restoration of gauge invariance, the asymmetry should disappear so that the net electric charge density must vanish, the compensating charge being produced from the Higgs vacuum in the form of heavy charged particles. Energetic products of their decay would create an electric current and a local charge asymmetry. Alternatively, such an asymmetry could be created even if the electric current was always conserved but an asymmetry in another nonconserved charge existed. The primary currents which created the asymmetry as well as those damping it via plasma discharge could generate chaotic magnetic fields on astronomically interesting scales. These fields might be large enough to seed the observed magnetic fields in galaxies via a protogalactic dynamo.

Comparison of electron‐density measurements made using a Langmuir probe and microwave interferometer in the Gaseous Electronics Conference reference reactor

Abstract: A Langmuir probe and a microwave interferometer have been combined to measure the electron density of argon glow discharges in the Gaseous Electronics Conference reference reactor [Bull. Am. Phys. Soc. 36, 207 (1991)]. The two techniques indicate the same charge density at 100 mTorr to within 30%. This 30% difference is easily explained by the experimental peculiarities. While the predicted charge densities obtained from the two techniques track one another as the applied rf voltage is varied at constant pressure, they do not track one another as the pressure is varied at constant rf voltage. In fact, the charge densities predicted from the Langmuir probe using Laframboise’s theory (Tech. Rep. 100, Univ. Toronto Inst. Aerospace Study, 1966) are factors of 2 and 4 times lower than those from the interferometer at 250 and 500 mTorr, respectively. It appears that the probe alters the charge density in its vicinity when the probe radius becomes greater than the ion mean free path. The interferometer has also ...