Average Charge Density Research Articles

Real-space calculations of nonspherically averaged charge densities for substitutionally disordered alloys.

Based on screening transformations of muffin-tin orbitals introduced by Andersen and Jepsen [Phys. Rev. Lett. 53, 2571 (1984)], we have developed a formalism for calculating the nonspherically averaged charge densities of substitutionally disordered alloys using the Korringa-Kohn-Rostoker coherent-potential-approximation (KKR CPA) method in the atomic-sphere approximation (ASA). We have validated our method by calculating charge densities for ordered structures, where we find that our approach yields charge densities that are essentially indistinguishable from the results of full-potential methods. Calculations and comparisons are reported for Si, Al, and Li. For substitutionally disordered alloys, where full-potential methods have not been implemented so far, our approach can be used to calculate reliable nonspherically averaged charge densities from spherically symmetric one-electron potentials obtained from the KKR-ASA CPA. We report on our study of differences in charge density between ordered AlLi in the L${1}_{0}$ phase and substitutionally disordered ${\mathrm{Al}}_{0.5}$${\mathrm{Li}}_{0.5}$ on a face-centered-cubic lattice.

Coherent-potential approximation in the tight-binding linear muffin-tin orbital method.

We describe a consistent approach for applying the coherent-potential approximation (CPA) to the various representations of the linear muffin-tin orbital method. Unlike the previous works of Kudrnovsk\'y et al. [Phys. Rev. B 35, 2487 (1987); 41, 7515 (1990)], our results for the ensemble-averaged Green functions in the tight-binding representation yield E- and r-dependent quantities that are consistent with the traditional applications of the single-site CPA. To illustrate the reliability and the usefulness of our approach we compare the nonspherically averaged charge densities, calculated in real space, of ordered NiPt in L${1}_{0}$ structure and the substitutionally disordered ${\mathrm{Ni}}_{0.5}$${\mathrm{Pt}}_{0.5}$ on a face-centered-cubic lattice.

Lattice fermions at non-zero temperature and chemical potential

We study the free fermion gas at finite temperature and chemical potential in the lattice regularized version proposed by Hasenfratz and Karsch and by Kogut et al. Special emphasis is placed on the identification of the particle and antiparticle contributions to the partition function. In the case of naive fermions we show that the partition function no longer separates into particle-antiparticle contributions in the way familiar from the continuum formulation. The use of Wilson fermions, on the other hand, eliminates this unpleasant feature, and leads, after subtracting the vacuum contributions, to the familiar expressions for the average energy and charge densities.

Modification of ion flow field by insulator surface with rod-plane geometry in SF/sub 6/

The effect of an insulator inserted along the axis of a rod-to-plane gap on an ion flow field in SF/sub 6/ gas is investigated experimentally in the pressure region where a preceding corona discharge exists. Without the insulator, the calculated electric field on the plane electrode agrees fairly well with the measured one. The high density space charge near the rod electrode produces the corona stabilization effect. With the insulator, the accumulated charges on the insulator affect the current and electric field distribution on the plane electrode which are variable with pressure, while the normalized charge density near the plane electrode is less dependent on pressure. These characteristics may be attributed to the change in the electric field distribution by the accumulated charge, the increase in the ratio of the dielectric strength in gas gap to that along the insulator surface with the gas pressure, and the thermal diffusion of ions near the insulator. On the basis of measured results, the ion flow field in the presence of the insulator is discussed. >

Surface area and CEC as related to qualitative and quantitative changes of forest soil organic matter after liming

AbstractWater vapor adsorption isotherms were used for estimation of (apparent) surface areas of samples of limed and unlimed plots of an acidic sandy forest soil. Samples were taken at two microrelief (ridge and furrow) positions from five subsequent 10 cm layers. Values of surface area and CEC correlated linearly with organic matter content but only for four bottom layers. Surface areas and CEC values calculated per the unit mass of organic carbon were higher in upper layers than in lower layers for control samples. For limed samples an opposite trend was observed. The estimated average (apparent) charge densities of organic matter showed a better correlation with humic to fulvic acids ratio. Values of surface charge densities for every investigated profile increased with depth and they were lower in limed than in unlimed profiles.

A comparative analysis of the effects of in-vivo and in-vitro abscisic-acid treatment on the surface electrical properties of barley chloroplast membranes

The effects of in-vivo and in-vitro abscisic acid (ABA) treatments on the surface charge density (σ) of barley (Hordeum vulgare L.) thylakoids were compared using 9-aminoacridine fluorescence. The estimated surface charge density of isolated thylakoid membranes from control (non-treated) barley leaves was-0.065 C · m(-2). The net negative surface charge density decreased after application of various concentrations of ABA (10(-6), 10(-5) M) for 7 d in-vivo, the more pronounced effect being observed at 10(-5) M ABA. When ABA was added to the suspension of isolated thylakoids the opposite effect was observed. The average charge density increased in in-vitro-treated thylakoids at 10(-5) M ABA to -0.081 C · m(-2). The results are discussed in terms of a specific ABA-induced influence of the composition and-or stoicheometry of charged protein complexes within the thylakoid membranes.

Many-atom screening effects on diffusion in dense helium.

The structure and dynamics of helium at 5 eV over the density range 1.5{le}{rho}{le}8 g/cm{sup 3} were studied using self-consistent-field molecular dynamics. Representative results for the pair-correlation function, velocity autocorrelation function, and the average charge density surrounding an atom are presented. Many-atom screening is found to have an important effect on all of these quantities above 0.3 g/cm{sup 3}. Above 2 g/cm{sup 3}, screening effects appear to saturate, and the coefficient of diffusion tends to the limit of a one-component plasma.

Charge density of total organic carbon in Finnish lakes

Ion balance calculations were used to estimate the charge density of total organic carbon (TOC) and for calibrating the model of Oliver et al. (1983) for Finnish lakes. The database consisted of samples collected in the autumn of 1987 from 955 randomly selected lakes covering the whole country. Organic anion concentrations were estimated as the difference between Σ cations and Σ inorganic anions. The charge density of TOC was estimated by dividing the organic anion concentration by TOC. The median charge density was 7·5 μeq (mg TOC) −1. In lakes with pH values lower than 4·5 the average charge density was 5·2 μeq (mg TOC) −1 and it increased to 10·3 μeq (mg TOC) −1 in lakes with pH higher than or equal to 7·0. The calibrated model explained 79% of the observed variability in organic anion concentration. The value for the total amount of acidic functional groups per mol organic carbon in the model of Oliver et al. (1983) was slightly higher compared to the Finnish calibration. At pH values higher than 4·4 the Finnish calibration gives lower organic anion concentrations compared to the model of Oliver et al. (1983).

Evidence on the breakdown of the Born-Oppenheimer approximation in the charge density of crystalline 7LiH/D

Accurate X-ray diffraction data on LiH and LiD measured at three different temperatures are analysed in terms of multipolar radial densities searching for phenomenological indications on the nature of bonding. The average spherical charge density around the atomic positions shows typical features of an ionic crystal. The central peaks are slightly contracted compared with superimposed free ions. The Li + peak contains, however, a small but significant excess of electrons, and the H-/D- peak is low and diffuse. Li + is spherical and its Debye-Waller parameters agree with the neutron diffraction values obtained by Vidal& Vidal-Valat [Acta Cryst. (1986), B42, 131137]. This indicates that, within the experimental accuracy, Li + is rigid. The non-spherical multipoles are significantly stronger in the hydrogen than in the deuterium derivative. They accumulate charge along the (100) directions giving a phenomenological indication of 'long-distance covalency' of H-H and D-D bonding with Li ÷ ions embedded in the middle but not contributing to the covalency. The significant deviation of the charge distribution of the anion in LiH from that in LiD indicates breakdown of the Born-Oppenheimer approximation due to coupling of the vibrations and the electronic states, which is much stronger in LiH. This is the first case - and probably the only possible - where such a breakdown can be seen by X-ray diffraction.

A rapid quantitative determination of pectin and carboxymethyl cellulose in solution using poly(hexamethylenebiguanidinium chloride)

There is a need for an effective wet method for the determination of pectin or carboxymethyl cellulose in solution. The poly(hexamethylenebiguanidinium chloride) complexation method has proved to be of value in this respect. Fears that heterogeneous distributions of the carboxyl groups might prevent a meaningful assay result from being obtained have proved groundless. In fact investigations of the complexation of such polysaccharides by poly(hexamethylenebiguanidinium chloride) have established that the assay procedure can be considered as a means of characterizing pectins or other carboxylated polysaccharides in terms of average charge density, i.e. equivalent weight, or, if the latter is known, determining polycarboxylate concentrations in dilute aqueous media.

Multilevel measurement of the electric field underneath a thundercloud: 2. Dynamical evolution of a ground space charge layer

In August 1989 the electric field was measured below a thunderstorm at Kennedy Space Center for more than 1 hour at several levels up to 800 m. No substantial precipitation reached the ground at the experimental site until after this data collection period, which included the thunderstorm approach and the triggering of four lightning flashes. The sensors at altitude were suspended from a tethered balloon and that at the surface was a standard field mill. The maximum value detected was 65 kV/m at 603 m while at the same time the steady surface field did not exceed 5 kV/m. Applying Gauss's law to the electric field data, we have calculated average charge densities within the layers formed by the levels where the sensors were located. Corona ions produced at the surface were detected up to the upper layer between 436 m and 603 m, and the densities calculated reached values close to 1 nC m−3 within each layer. The current densities were also evaluated at the sensor levels; the one calculated at 436 m ranges about 1.4 nA m−2 which means that a large proportion of corona ions were carried up to several hundreds of meters above ground.

Photosynthetic acclimation of Tradescantia albiflora to growth irradiance: morphological, ultrastructural and growth responses

Tradescantia albiflora (Kunth), a trailing ground species naturally occurring in deep shade in rainforests, has an unusual photosynthetic acclimation profile for growth irradiance. Although capable of increasing its capacity for electron transport, photophosphorylation and carbon fixation when grown in full sunlight, Tradescantia has constant chlorophyll alb ratios, photosystem reaction centre stoichiometry and pigment‐protein composition at all growth irradiances (Chow et al. 1991. Physiol. Plant. 81: 175–182). To gain an insight into the compensatory strategies which allow Tradescantia to grow in both high and low lights, plants were grown under shade cloth (100 to 1.4% relative growth irradiance) and leaf and chloroplast attributes were compared. While shade Tradescantia chloroplasts had three times more chlorophyll per chloroplast and twice the length of thylakoid membranes compared to plants grown in full sunlight, the ratios of appressed to nonappressed thylakoid membranes were constant. The average net surface charge density of destacked thylakoids was the same for plants grown at moderate and low‐irradiance, consistent with their similar stacking profiles. Tradescantia plants grown in direct sunlight had 10‐times more fresh and dry weight per plant compared to plants grown in shade, despite a lower photosynthetic capacity on a leaf area basis with partial photoinhibition. We conclude that having a light‐harvesting apparatus permanently locked into the “shade‐plant mode” does not necessarily prevent a plant from thriving in high light. Analyses of leaf growth at different irradiances provide a partial explanation of the manner in which Tradescantia compensates for very low photosynthetic capacity per unit leaf in sunlight.

A novel procedure for determining the average charge density of pectin chains

A novel procedure for determining the average charge density of pectin chains

On the short-channel theory for MOS transistor

An attempt is made to derive rigorous analysis for the short-channel MOS transistor on the basis of the 2-D Poisson's equation. The analysis is able to predict a correct dependence of the threshold voltage on channel length and drain voltage, avoids the need for the definition of an average depletion charge density, and gives more physical insight into the short-channel effects. >

How far can a carbanion delocalize? Carbon-13 NMR studies on soliton model compounds

For well-defined {pi} systems, {sup 13}C chemical shifts are correlated with {pi}-electron density. The authors have used this correlation to develop models for charge carriers in n-doped polyacetylene ('solitons') that allow them to determine if the charge density distribution predicted by the soliton theory is correct. In this investigation, a linear chemical shift/charge density relationship is obtained for anions of the general structure Ph(CH){sub n}Ph{sup {minus}}, n = 1, 3, 5, 7, 9,{hor ellipsis} (DPN), if the average chemical shift ({delta}{sub c}) of all carbons is plotted against the average charge density ({rho}{sub av}). Such a plot has allowed them to develop the empirical relationship ({rho}{sub c} = {delta}{sub c} - 132.7)/187.3 and thus to calculate the charge density at each carbon atom. Moreover, comparison with neutral compounds of formula Ph(CH{double bond}CH){sub n}Ph allows them to define the soliton length, that is, the point at which carbanion delocalization in a linear chain ceases.

Surface Charge Density Changes in Isolated Photosystem II Membranes Induced by Depletion of the Extrinsic Polypeptides of the Oxygen Evolving System

Abstract Treatment of PS II particles with either 1 M NaCl or alkaline Tris (1 M , pH 8.4) caused a considerable decrease in the average net negative surface charge density, concomitant with depletion of the extrinsic 17, 24 and 33 kDa proteins of the oxygen evolving complex from the membranes. The partial recovery of the values for surface charge in both NaCl- and Tris-treated membranes was registered after reconstitution experiments with the three proteins. These results are compared with the data for the charge densities of the thylakoid membranes, to examine the role of the three extrinsic proteins in the formation of heterogeneous arrangement of surface charge across the appressed (granal) thylakoids.

A theoretical analysis of donnan dialysis across charged porous membranes

A previously developed model [1] for the transport of ions in charged capillary tubes is used to determine the effect of membrane properties (average pore radius and surface charge density) and solution concentration on steady-state ion and solution fluxes in two types of Donnan dialysis separations. Specifically, the concentration of a dilute metal ion solution into an acid stripping solution by ion exchange across a cation exchange membrane and the removal of an acid contaminant from a metal ion solution using an anion exchange membrane are studied. The model is able to predict the changes in metal ion flux caused by changes in acid and metal ion concentrations in the strip and feed solutions in the Donnan dialysis experiments performed by Lake and Melsheimer [2]. These effects could not be explained by the mass-transfer resistance model used by Lake and Melsheimer [2] and by others [3,4] to describe ion transport across a charged membrane in Donnan dialysis separations. The time-dependent ion concentrations in well-stirred solutions undergoing Donnan dialysis are also modeled. The results are shown to match the experimental data of Ng and Snyder [4] when the boundary layer resistance are negligible and reasonable values for the membrane pore parameters are assumed.

Surface Charge Density of Hetero-Fused Plant Protoplasts: an Electrophoretic Study

Electrophoretic mobilities of hetero-fused plant protoplasts, which were obtained by electrofusion of barley mesophyll cell protoplasts and Rauwolfia serpentina cultured cell protoplasts, and those of the unfused parent protoplasts were measured in various media of different pH values. At pH 5.2, the zeta potential of the fused protoplasts was intermediate between those of the barley and R. serpentina protoplasts and the average surface charge density of the fused protoplasts was closer to that of R. serpentina than to that of barley. The distribution of the surface charge density of fused protoplast obtained at pH 5.2 is discussed in terms of the surface charge densities and the sizes of parent protoplasts. These results revealed that the surface charge density of fused protoplasts was determined by the surface charge densities and the ratio of the surface areas of the respective parent protoplasts.

Segmented roller: A device for measuring charge density on a moving conducting web

A device is described which can be used to directly measure average charge density on moving webs which have conducting layers (layer resistivity less than 10 11 ohms/square). This is not possible with other currently available devices because of charge movement on the web caused by the presence of the measuring instruments. The principles by which the segmented roller works are discussed, and experimental results are presented.

Hartree-Fock theory of the inhomogeneous electron gas at metallic surfaces

In this paper we derive fundamental properties of the interacting inhomogeneous electron gas at jellium metal surfaces within the Hartree-Fock approximation. We also discuss the question of what constitutes quantum-mechanically the image charge at a metal surface, and provide a physical interpretation for the quantum-mechanical origin of the image potential. The self-consistent solution of the Hartree-Fock equations is in general formidable due to the integral exchange operator. We show that from a more fundamental physical viewpoint, the difficulty for the metal surface physics problem manifests itself in the complex structure of the orbital-dependent exchange charge densities that give rise to the orbital-dependent potentials. However, it is possible to solve the problem of the Pauli-correlated electron gas within the exchange-only formalism of density-functional theory. By employing the variational principle for the energy with the exchange energy component treated in its non-local form, and the variationally accurate “displaced-profile-change-in-self-consistent-field” expression for the work function, we derive rigorous upper bounds to the surface energy and accurate work functions. In order to understand what constitutes quantum-mechanically the image charge at a metal surface we study the structure of the average exchange charge density or Fermi hole as an electron is removed from within a metal to infinity outside. The study shows that the Fermi hole is localized to the surface region and is part of the image charge only for electron positions close to the surface. As the electron is removed further into the vacuum region, the width of the hole increases. In the asymptotic limit when the electron is removed to infinity, the hole is completely delocalized and spread throughout the crystal, its center of mass being singular. As a consequence it appears that it is the Coulomb hole charge distribution localized at the surface that is the image charge, but this has yet to be shown. Finally, we provide insights into the quantum-mechanical origin of the image potential. These ideas are based on our interpretation of the local exchange-correlation potential of density-functional theory as being the work done to remove an electron against the electric field of its Fermi-Coulomb hole charge density. Since the Coulomb hole charge is zero, the image potential in the asymptotic region far from the surface is the work done against the electric field of its Fermi hole. The results of preliminary calculations confirm this conclusion. Whether the Coulomb hole contributes to making the total effective potential the image potential for electron positions closer to the surface is yet unanswered. The above physical interpretation is based on the fact that the Fermi-Coulomb hole charge distribution is dynamic as a function of electron position. This explains why the exchange potential as calculated by the Slater method is incorrect. As such we also present the complete structure of the Slater potential at a metal surface and show that it leads to an erroneous value in the interior of the metal and that its asymptotic structure though image-potential-like has a coefficient approximately twice as large as that of the image potential.