Showing papers on "Charge density published in 1986"

Focusing of electric fields in the active site of Cu‐Zn superoxide dismutase: Effects of ionic strength and amino‐acid modification

Abstract: In this paper we report the implementation of a finite-difference algorithm which solves the linearized Poisson-Boltzmann equation for molecules of arbitrary shape and charge distribution and which includes the screening effects of electrolytes. The microcoding of the algorithm on an ST-100 array processor allows us to obtain electrostatic potential maps in and around a protein, including the effects of ionic strength, in about 30 minutes. We have applied the algorithm to a dimer of the protein Cu-Zn superoxide dismutase (SOD) and compared our results to those obtained from uniform dielectric models based on coulombic potentials. We find that both the shape of the protein-solvent boundary and the ionic strength of the solvent have a profound effect on the potentials in the solvent. For the case of SOD, the cluster of positive charge at the bottom of the active site channel produces a strongly enhanced positive potential due to the focusing of field lines in the channel—a result that cannot be obtained with any uniform dielectric model. The remainder of the protein is surrounded by a weak negative potential. The electrostatic potential of the enzyme seems designed to provide a large cross-sectional area for productive collisions. Based on the ionic strength dependence of the size of the positive potential region emanating from the active site and the repulsive negative potential barrier surrounding the protein, we are able to suggest an explanation for the ionic strength dependence of the activity of the native and chemically modified forms of the enzyme.

Illustration of the linear-muffin-tin-orbital tight-binding representation: Compact orbitals and charge density in Si

Abstract: Plots of the tight-binding (TB) orbitals recently derived by exact transformation of the conventional set of linear muffin-tin orbitals (LMTO's) are presented for crystalline silicon. The TB-LMTO's are found to be extremely compact. As a simple application we show how non-spherically-averaged charge densities may be obtained from standard LMTO calculations. For silicon this charge density is found to be in excellent agreement with the one obtained from a linear augmented plane-wave full-potential calculation. This is true even when the LMTO calculation employs the atomic-sphere approximation for the one-electron potential. A self-contained account of the TB-MTO formalism is presented and a simple way of including the quadratic energy dependence of the MTO's is derived.

Ion—Aerosol Attachment Coefficients and the Steady-State Charge Distribution on Aerosols in a Bipolar Ion Environment

Abstract: Calculations of the ion—aerosol attachment coefficients are carried out for Fuchs' theory (as corrected in this paper) and for a theory which includes three body trapping. The resulting charge distributions agree quite well for particles with radii greater than about 0.007 μm. For smaller particles three-body trapping becomes increasingly important. Comparison of theoretically predicted charge distributions with recently measured charge distributions at radii smaller than 0.02 μm show good agreement. Asymmetric charging due to differences in the physical properties of positive and negative ions can result in large differences in the number of positively and negatively charged particles, particularly at larger radii. The asymmetric charge distribution is also shown to depend on the ionization rate. For the case when aerosol concentrations are comparable to the ion concentrations the effect of polydispersity on the charge distribution is difficult to predict. It is shown that a dominant size particle can es...

Some considerations on the electric field induced in insulators by electron bombardment

Abstract: Starting from a simple model of the distributions of charge created in an insulator by bombardment with electrons, the components of the electric field are evaluated by using Maxwell’s equations and image effects. The results are applied to the most common experimental situations: a semi‐infinite sample (i) bounded by a vacuum or (ii) covered by a conducting film, and a sample in the form of a film (iii) unsupported or (iv) covering a conducting substrate. The results are compared to some experimental data concerning, for instance, electromigration and electron‐stimulated desorption. In surface analysis the decay of the Auger signal from ions of opposite charges and the opposite behavior of ions of the same charge are explained. Similar effects observed in electron‐probe microanalysis of glasses are also elucidated. The results concern scanning electron microscopy, transmission electron microscopy, and electron‐beam lithography applied to biological objects, polymers, ceramics, minerals, glasses, and electronic devices. With slight modifications, the same model can be applied to cases of irradiation with ions or x rays. The evolution of the trapped charges with time is suggested, and the need to indicate the electric parameters (e and γ) of the investigated samples is outlined.

Electronic and structural properties of BN and BP.

Abstract: We present a pseudopotential study within the local-density formalism of the structural and electronic properties of zinc-blende BN and BP. The ground-state properties of these systems such as bulk moduli, lattice constants, cohesive energies, and frequencies of the TO phonon mode are in good agreement with experimental results. The valence charge density of BP shows two local maxima along the bond, which is similar to the case of diamond. In contrast, the charge density of BN is similar to that of a typical III-V compound semiconductor. The resulting band structures have some important features which are in disagreement with previously published work. Like most III-V compound semiconductors, the fundamental gap in BP decreases with decreasing volume. The corresponding gap in BN, however, increases with decreasing volume as was also found in diamond.

Self‐consistent calculation of electron and hole inversion charges at silicon–silicon dioxide interfaces

Abstract: The charge distribution at a semiconductor‐insulator interface has been calculated, both for holes and electrons, by solving Schrodinger’s and Poisson’s equations self‐consistently for particles obeying Fermi–Dirac statistics. The results have been applied to carriers in the channel of a crystalline MOSFET (metal‐oxide‐semiconductor field‐effect transistor) with the (100) axis perpendicular to the gate oxide. For weak inversion, the self‐consistent results do not deviate significantly from those obtained assuming a triangular potential well, but for strong inversion the carriers tend to move closer to the oxide. The energy and occupation levels of the subbands are only affected by a few percent on passing from weak to strong inversion, keeping the transversal interface electric field fixed. Finally the gate capacitance has been calculated and found to agree with the experimental data published elsewhere.

Pseudopotential total-energy study of the transition from rhombohedral graphite to diamond.

Abstract: The path maintaining rhombohedral symmetry in the transition from graphite to diamond which minimizes the energy at each value of the bond length between layers is determined. The energy barrier for this path is found to be 0.33 eV. The total energy of the solid is calculated using local-density-functional theory with ab initio pseudopotentials. Results are presented for the charge density and density of states along the transition path. In contrast to recent extended-H\"uckel-theory results, throughout most of the transition the structure is found to remain semimetallic or semiconducting. A final rapid opening of the gap to the insulating diamond phase develops as the interlayer carbon-carbon bonds form. The behavior of rhombohedral graphite under conditions of isotropic pressure is also examined. We predict that rhombohedral graphite will transform to diamond, without thermal or catalytic activation, at an isotropic pressure of 80 GPa if it maintains its rhombohedral symmetry. Our analysis moreover suggests that, in general, cross linking of hexagonal-ring carbon compounds leading to local tetrahedral coordination should be favored when the interlayer distance between hexagonal rings is between 2.1 and 2.3 A\r{}.

Linear augmented-plane-wave calculation of the structural properties of bulk Cr, Mo, and W

Abstract: A scalar-relativistic procedure for calculating the valence-electron contribution to the total energy of bulk and thin-film solids has been developed and applied to the fcc and bcc phases of the group-VIB transition elements Cr, Mo, and W. This approach, which is based on the linear augmented-plane-wave method and local-density-functional theory, contains no shape approximations for either the charge density or potential. The formulation adopts a rigid-core approximation and incorporates an exact treatment of the core-charge tails that extend beyond the muffin-tin spheres. The application of this procedure to bcc Cr, Mo, and W yields calculated lattice parameters and bulk moduli that are in good (Cr) to excellent (Mo and W) agreement with experiment. The present calculated properties also agree quite well with the results of previous calculations involving a variety of band-structure methods. The calculated fcc-bcc energy difference for Cr, Mo, and W increases in a nearly linear manner from 0.4 to 0.5 eV/atom. The magnitude of the Cr energy difference raises questions concerning the proposed formation of fcc Cr in thick epitaxial layers.

Dielectric properties of water and water/ethylene glycol mixtures for use in pulsed power system design

Abstract: One class of modern pulse power generators use deionized water as an energy storage, switching and transmission dielectric. Water is chosen for its high dielectric constant and relatively high resistivity, which allows reasonably sized and efficient low-impedance high-voltage pulse lines where pulse durations are less than 100 µs. Water/ethylene glycol mixtures are being researched, so that rotating machinery, rather than the usual Marx generator, can be used as the primary energy store. The high resistivity and high dielectric constant of these mixtures at low temperature permit low-loss operation on millisecond time scales. Simple design criteria linking load parameters and charging circuit characteristics to the liquid dielectric are developed which show that the dielectric constant, breakdown strength, and relaxation time are the primary properties of interest to the pulse power engineer. On time scales greater than 100 µs, injection of space charge, with density q and mobility µ, affects the charging and discharging circuit characteristics, introduces the time constant of the time of flight for injected charge to migrate between electrodes, and increases the effective ohmic conductivity σ to σ + qµ. A drift-dominated conduction model is used to describe measured space-charge effects. Kerr electrooptic field mapping measurements show strong space-charge effects with significant distortions in the electric field distribution a few hundred microseconds after high voltage is applied. The injected charge magnitude and sign depends on the electrode material. Thus by appropriate choice of electrode material combinations and voltage polarity, it is possible to have uncharged liquid, unipolar-charged negative or positive, or bipolar-charged liquid. An important case is that of bipolar injection, which has allowed up to a 40 percent higher applied voltage without breakdown than with no charge injection, and thus a doubling of stored energy due to the space-charge shielding which lowers the electric field strengths at both electrodes. Although injected space charge increases the stored electric energy over the capacitive space-charge-free energy, (1/2)CV2, more energy is required from a source during charging and the energy delivered to a resistive load is reduced because of internal dissipation in the capacitor as the charge is conducted to the electrodes. However, it appears that this extra dissipation due to injected charge can be made negligibly small and well worth the price if the space charge allows higher voltage operation for long charging time or repetitively operated machines.

Charge density in cuprite, Cu2O

Abstract: Etude de l'influence des procedures de reduction de donnees, des modeles de facteur de temperature et des modeles de deformation sur les resultats

Electrostatic interactions in phospholipid membranes I: Influence of monovalent ions

Abstract: Electrostatic interactions in monolayers and vesicles of acidic phospholipids are studied by thermodynamical and optical techniques in conjunction with numerical calculations. A nonmonotonic ionic strength dependence with an extremum at 0.1 M (NaCl) is observed for the phase transition temperature of vesicles as well as for the surface pressure of monolayers at low molecular density. This finding is in accordance with the calculations predicting the dominance of charge screening by monovalent counterions only for concentrations above 0.1 M. For lower salt content, however, its increase causes an elevation of the degree of dissociation and thus also electrostatic repulsion. This leads to a higher surface pressure, a lower transition temperature and a smaller size of solid domains observed in the liquid/solid coexistence range of monolayers. This supports the previously published idea, that finite size and repulsion of the domains arise from a different surface charge density in fluid and solid lipid phases.

Nuclear ground-state properties in a relativistic Meson-Field theory

Abstract: We investigate the ability of a relativistic Mean-Field theory to reproduce nuclear ground state properties by an exhaustive fit to experimental data. We find that the bulk properties of nuclei from16O to208Pb can be adjusted very well. There remain problems with level density and fluctuations in the charge density similar as in fits using the conventional Skyrme Hartree-Fock model.

The distribution of charged groups in proteins.

Abstract: The distributions of charged groups in 32 proteins of known three-dimensional structure have been analyzed to determine how regularly the groups are spread over the molecule's surfaces, and to identify and to study those proteins where charge asymmetry would seem important for their function. The distributions have been analyzed in terms of charge “polarity,” surface “charge density,” and electric dipole moments. More detailed studies of the distributions for individual proteins are made using map projections specifically developed for this purpose. In the light of the results obtained we discuss the role of charged groups in relation to protein function.

An examination of the formation and characteristics of charge-density waves in inorganic materials with special reference to the two- and one-dimensional transition-metal chalcogenides

Abstract: The authors have attempted to provide a relatively brief outline of the current situation regarding the formation and properties of charge density wave states in low-dimensional transition metal compounds. They have chosen to illustrate the types of problem involved by specifically focusing on 1T- and 2H-TaS2 and TaSe2, VSe2, NbSe3 and NbTe4, where their own research has centred.

Study of the effect of deposited platinum particles on the surface charge of titania aqueous suspensions by potentiometry, electrophoresis, and labeled-ion adsorption

Abstract: Potentiometry, electrophoresis, and radiotracer technique have been used to study the charge density in different parts of the double layer at the TiO/sub 2/-electrolyte interface for powder samples supporting 0.5-10 wt % platinum as particles of homogeneous size (ca. 2 nm diameter). Less positive charge densities were measured by potentiometry for increasing Pt content; the value of point of zero charge decreases accordingly. More negative electrophoretic mobilities were found for the metal-loaded samples from pH 3 to pH approx.8; as a result the value of the point of zero zeta potential decreases with increasing Pt content. From a comparison between the surface charge density, the charge density of the diffuse part of the double layer, and the charge density of the counterions (determined from adsorption isotherms of labeled Na/sup +/ and Cl/sup -/ ions for TiO/sub 2/ and 10 wt % Pt/TiO/sub 2/), it is deduced that a part of the Cl/sup -/ ions form neutral adsorbed species at the Pt surface. Therefore, the changes observed in potentiometric titrations and electrophoretic mobilities are attributed to an increase in TiO/sub 2/ acidity induced by the Pt particles, which reflects the decreased TiO/sub 2/ electron density.

Noninvasive sheet charge density probe for integrated silicon devices

Abstract: We report a sensitive new technique for probing dynamic sheet charge density variations in integrated silicon devices. Using a specially designed noninvasive Nomarski phase contrast interferometer a sheet charge density sensitivity of 2.6×108 e/cm2/(Hz)1/2 is extracted from experimental data for 1 mA of detected photocurrent. This charge density sensitivity makes possible μV signal level detection in an active device, and with digital signals the corresponding signal/noise level is sufficiently high that multimegabaud data can be captured in real time.

Total (elastic+absorption) cross sections for e-CH4 collisions in a spherical model at 0.10-500 eV.

Abstract: Electron-${\mathrm{CH}}_{4}$ scattering is investigated in a wide energy range (0.1--500 eV) by using a previously proposed spherical model [A. Jain, J. Chem. Phys. 81, 724 (1984)]. The study is divided into three energy regions: the 0.1--1.0-eV region, where a well-known minimum occurs in the total cross section [Ramsauer-Townsend (RT) effect], between 2 and 20 eV, with a d-wave broad structure around 7--8 eV, and from 20 to 500 eV, where inelastic channels (mainly ionization and dissociation) dominate over the elastic process. It is shown that a simple model, in which the total optical complex potential is spherical, is capable of reproducing qualitative features in the total, differential, and momentum-transfer cross sections in the present energy region.The present results are almost equivalent to more rigorous close-coupling calculations at low energies (El20 eV), where the rotationally elastic channel dominates. Below 20 eV, the total optical potential is real and consists of three spherical terms, namely, a static term calculated accurately from near-Hartree-Fock one-center methane wave functions, a parameter-free polarization potential of Jain and Thompson (however, below 1 eV we employ a cutoff-type phenomenological polarization potential), and a local-exchange interaction in the asymptotically adjusted version of the Hara free-electron-gas exchange (AAHFEGE) potential (however, below 1 eV, we consider just the HFEGE form). At and above 20 eV, we employ a complex optical potential with the same real part as in the 2--20-eV region, while the imaginary part is an energy-dependent absorption potential calculated from target electron density and short-range static-exchange potential in the quasifree model with Pauli blocking [Staszewska et al., J. Phys. B 16, L281 (1983)].Two versions of this absorption potential are used; one with undistorted charge density and the other with a polarized density. The final complex total optical potential is treated exactly in a partial-wave analysis to yield various cross sections. The results are compared with the available experimental and other theoretical data. The absorption cross sections are not sensitive to various forms of real part of the optical potential, and in general, reduce the elastic cross sections significantly.

Method and means for optical detection of charge density modulation in a semiconductor

Abstract: Disclosed is a technique for probing dynamic sheet charge density variations in integrated semiconductor devices. Using a specially designed non-invasive Nomarski phase contrast interferometer a sheet charge density sensitivity of 2.6×108 e/cm2 / √Hz is extracted from experimental data for 1 mA of detected photocurrent. The charge density sensitivity makes possible μV signal level detection in an active device, and with digital signals the corresponding signal/noise level is sufficiently high that multi-mega-baud data can be captured in real time.

Electronic structure of AlN

Abstract: The self-consistent electronic structure of AlN has been calculated using a first-principles linear-combination-of-atomic-orbitals method. Theoretical values for the lattice constant, internal parameter, bulk modulus, and cohesive energy were determined as well as the pressure-dependent frequency of the A/sub 1/ TO phonon. The charge density has also been calculated and is in good agreement with results from x-ray diffraction experiments which indicate the lack of a distinct bond charge.

Theory of structural and electronic properties of BAs

Abstract: The pseudopotential method within the local density approximation is used to investigate the structural and electronic properties of the most covalent of the III-V semiconductors, BAs. Results are given for the bulk modulus, lattice constant, cohesive energy and frequency of the TO phonon mode at Gamma . The electronic band structure and charge density are also discussed and shown to have features that differ from those of the other III-V compounds. In particular there is evidence for a reversal of the usual charge transfer between cation and anion.

Dimensionality and size effects in simple metals.

Abstract: A detailed discussion of quantum-size effects and the dimensionality in simple (s-p bonded) metals is presented by using aluminum as a prototype. The density of states, work function, surface energy, surface relaxation, and subband energies are calculated for films of varying thicknesses. Oscillatory variations of various physical properties correlate well with the surface charge density which itself varies with the film thickness. All calculations are performed using the self-consistent pseudopotential method and the planarly averaged one-dimensional potential generated from it. Total-energy calculations and forces on various atomic planes lead to important conclusions about surface relaxation.

Sliding charge-density waves: A numerical study.

Abstract: Numerical results are presented for the behavior of a one-dimensional, classical, deformable charge-density wave pinned by random impurities in applied electric fields below and close to the threshold field for sliding. We discuss the properties of metastable states and show how they can give rise to hysteresis and memory. Both the linear and nonlinear response functions are enhanced at low frequencies. The enhancement of the dielectric response is consistent with the existence of a low-frequency cusp in infinite systems. At fields just exceeding threshold, we find very long relaxation times for the system to reach dynamic equilibrium, with strong broadband noise produced as a transient effect. The effect of thermal hopping at low temperatures is briefly discussed.