Showing papers on "Charge density published in 1975"

Negative surface charge near sodium channels of nerve: divalent ions, monovalent ions, and pH.

Abstract: Evidence is given for a high density of negative surface charge near the sodium channel of myelinated nerve fibres The voltage dependence of peak sodium permeability is measured in a voltage clamp The object is to measure voltage shifts in sodium activation as the following external variables are varied: divalent cation concentration and type, monovalent concentration, and pH With equimolar substitution of divalent ions the order of effectiveness for giving a positive shift is: Ba equals Sr less than Mg less than Ca less than Co approximately equal to Mn less than Ni less than Zn A tenfold increase of concentration of any of these ions gives a shift of +20 to +25 mV At low pH, the shift with a tenfold increase in Ca-2+ is much less than at normal pH, and conversely for high pH Soulutions with no added divalent ions give a shift of minus 18 mV relative to 2 mM Ca-2+ Removal of 7/8 of the cations from the calcium-free solution gives a further shift of minue 35 mV All shifts are explained quantitatively by assuming that changes in an external surface potential set up by fixed charges near the sodium channel produce the shifts The model involves a diffuse double layer of counterions at the nerve surface and some binding of H+ions and divalent ions to the fixed charges Three types of surface groups are postulated: (1) an acid pKa equals 288 charge density minus 09 nm- minus 2; (i) an acid pKa equals 458, charge density minus 058 nm- minus 2; (3) a base pKa equals 628, charge density +033 nm- minus 2 The two acid groups also bind Ca-2+ ions with a dissociation constant K equals 28 M Reasonable agreement can also be obtained with a lower net surface charge density and stronger binding of divalent ions and H+ ions

Landau theory of charge-density waves in transition-metal dichalcogenides

Abstract: A Landau theory is proposed for charge-density waves (CDW) in transition-metal dichalcogenides with the charge density as an order parameter. The theory predicts the sequence of phases, normal-state---incommensurate-CDW---commensurate-CDW with decreasing temperature, separated by first-order phase transitions. The peaks in charge density lie at "lattice sites" of a hexagonal crystal and for the incommensurate case the theory predicts phononlike distortions of the CDW "lattice" as well as dislocations. Impurities pin the charge density wave, broaden the phase transitions, and stabilize the incommensurate state relative to the commensurate state.

The role of charge density waves in structural transformations of 1T TaS2

Abstract: Electron and X-ray diffraction studies have identified three metastable phases in 1T TaS2. Extra reflexions are interpreted as arising from charge density waves coupled to periodic distortions modulating a trigonal lattice. These waves are incommensurate with the matrix in 1T1 and 1T2, but form a commensurate super-lattice in 1T3. Within 1T2, the temperature dependence of the wave vector is correlated with a change in the cross-section of the Fermi surface, while the diffuse scattering which accompanies the discrete reflexions in 1T1 is discussed as an image of this surface in the phonon spectrum. Changes in the stacking of distortions and in their amplitude are also reported.

Role of ab initio calculations in elucidating properties of hydrated and ammoniated electrons

Abstract: The properties of solvated electrons are analyzed in terms of a self- consistent modified continuum model based on the techniques of ab initio molecular quantum mechanics. The model is semiclassical in spirit, employing the quantum mechanical density for the excess charge and the first solvation shell in conjunction with classical electrostatics, and is developed in a general form which can be straightforwardly applied to special cases of interest, such as the solvated mono- and dielectron complexes. The advantages and disadvantages of the technique are discussed in relation to other more empirical approaches. Computational results are presented for excess electrons (mono- and dielectrons) in water and ammonia, and the role of long-range polarization of the medium in localizing the excess charge is analyzed. The variationally determined ground states are characterized in terms of equilibrium solvation shell geometry (appreciable cavities are implied for both water and ammonia), solvation energy, photoionization energy, and charge distribution. The finding of negative spin densities at the first solvent shell protons underscores the importance of a many electron theoretical treatment. Preliminary results for excited states are also reported. The calculated results are compared with experimental and other theoretical data, and the sensitivity of the results to variousmore » features of the model is discussed. Particular attention is paid to the number of solvent molecules required to trap the excess electron. (auth)« less

A misconception concerning the electronic density distribution of an atom

Abstract: It is shown that the electronic charge density of a ground-state atom decreases monotonically as a function of radial distance from the nucleus, contrary to the widespread belief that the shell structure is reflected by relative maxima in the density. Any proposed relationship between chemical bonding and the maxima in the radial density functions of atoms should therefore be regarded with caution. It is proven that the electrostatic potential of an atom must be monotonically decreasing. The changes in charge distribution upon molecule formation are also discussed.

Surface‐potential decay in insulators with field‐dependent mobility and injection efficiency

Abstract: Surface‐potential decay characteristics of corona‐charged insulators are analyzed theoretically. The effects of field‐dependent drift mobility and injection efficiency, as well as time‐dependent injection, are included. The theoretical results are compared with existing surface‐potential decay data on polyethylene. At the highest initial surface potentials all the surface charge is injected into the insulator on a time scale which is small compared to that of the measurement, and the injected charge drifts under its own self‐field to the collecting electrode by means of a field‐dependent transport process without significant range limitation. At the lowest initial surface potentials little or no charge is injected, and the surface potential is constant in time. For intermediate initial surface potentials the injection process is time and field dependent, with the time dependence being stronger than an injection process associated with a surface charge density that decreases exponentially in time, but weaker than that for instantaneous complete injection.

Electronic structure of trigonal and amorphous Se and Te

Abstract: The electronic structure of trigonal and amorphous Se and Te is investigated using the empirical pseudopotential method (EPM), charge-density calculations, and simple tight-binding models. Band structures and electronic densities of states are obtained which are in excellent agreement with recent photoemission measurements. The tight-binding models are used to obtain analytic expressions for the energy bands and to interpret the EPM band structures in terms of real-space orbital-orbital interactions. Charge-density calculations obtained as a function of energy and evaluated within specific energy intervals are used to interpret various structure in the density of states. Specifically certain easily resolvable peaks in the experimental photoemission spectra are associated with intrachain and interchain localized states, respectively. By taking only short-wavelength components of the charge density, a bonding charge can be defined which gives an estimate of the intrachain vs interchain bonding strengths. The trigonal results along with model calculations to investigate the effects of bond-angle variations on chains and the presence of eight- and six-fold rings of bonds are used to interpret the changes observed in the experimental spectra of amorphous Se and Te. A new model of amorphous Se is proposed.

Distribution of charge in electrets

Abstract: Measurements are reported of the total charge and its mean position in liquid‐charged FEP electrets. A new nondestructive technique has been used. The large difference in decay rates of positive and negative liquid‐charged electrets is accompanied by quite different behavior of the deposited charge as it migrates through the electret. The total volume charge density present in untreated FEP has been measured: it typically amounts to 0.1–0.6 C m−3 for 25‐μ‐thick material.

Magnitude and location of surface charges on Myxicola giant axons.

Abstract: The effects of changes in the concentration of calcium in solutions bathing Myxicola giant axons on the voltage dependence of sodium and potassium conductance and on the instantaneous sodium and potassium current-voltage relations have been measured. The sodium conductance-voltage relation is shifted along the voltage axis by 13 mV in the hyperpolarizing direction for a fourfold decrease in calcium concentration. The potassium conductance-voltage relation is shifted only half as much as that for sodium. There is no effect on the shape of the sodium and potassium instantaneous current-voltage curves: the normal constant-field rectification of potassium currents is maintained and the normal linear relationship of sodium currents is maintained. Considering that shifts in conductances would reflect the presence of surface charges near the gating machinery and that shape changes of instantaneous current-voltage curves would reflect the presence of surface charges near the ionic pores, these results indicate a negative surface charge density of about 1 electronic charge per 120 A2 near the sodium gating machinery, about 1 e/300 A2 for the potassium gating machinery, and much less surface charge near the sodium or potassium pores. There may be some specific binding of calcium to these surface charges with an upper limit on the binding constant of about 0.2 M-1. The differences in surface charge density suggest a spatial separation for these four membrane components.

Density Functional Theory of Chemisorption on Metal Surfaces.

Abstract: : The density functional theory of chemisorption is developed by means of a self-consistent linear response formalism. Any chemisorbed species which can be represented as an external charge distribution perturbing the metallic surface can be studied with this approach. The formalism is applied to hydrogen chemisorbed on a tungsten surface. Theoretical results for the ionic desorption energy, adatom vibrational frequency, relative size of the dipole moment, resonance levels associated with the adsorbate, and the question of dissociation agree well with experimental measurements. The calculated level width is too large. Differential scattering cross sections for chemisorbed hydrogen are compared with those of the isolated atom. The rather satisfactory overall agreement of our theory with experimental results suggests that the linear response formalism may have a wider usefulness.

Charge Density-NMR Chemical Shift Correlations in Organic Ions

Abstract: Publisher Summary This chapter discusses theoretical and empirical studies that emphasize the need to consider several factors in addition to linear charge polarization to interpret either proton or carbon-13 chemical shifts in terms of charge distribution. Prime among these are (1) the effect of neighboring group anisotropy on proton shifts, (2) the E2 term in proton shifts at high positive or negative charge densities, (3) the paramagnetic term in proton shifts in systems with lone pair electrons, (4) a possible reversal of slope for the carbon-1 3 shift-charge density correlation at high positive charge density, (5) the importance of bond order terms in determining carbon-13 shifts, and (6) the importance of average excitation energies (ΔE) in determining carbon-13 shifts, particularly in systems with lone pairs. The related phenomenon of ion association is reported in the cases of carbanions.

Carrier-density fluctuations and the IGFET mobility near threshold

Abstract: The behavior of the inversion‐layer mobility observed in the IGFET at biases close to threshold is explained in terms of small fluctuations in the density of minority carriers. These fluctuations are in response to localized charges at the oxide‐semiconductor interface. The theory predicts the abrupt drop of mobility with decreasing carrier density near threshold which is observed in the dc conductance mobility, field‐effect mobility, and Hall mobility. The analysis also indicates that the dc mobility drop occurs at higher carrier densities at 4.2 °K than at 300 °K due to the larger fluctuations of the minority carriers at low temperatures (when they are all in a single quantum subband) than at high temperatures (when they are distributed over a large number of subbands). The anomalous threshold shift of the IGFET with temperature for temperatures below 77 °K is attributed to carrier population changes with temperature, i.e., with the freezeout with decreasing temperature of carriers from higher subbands, where fluctuations are small, into the lowest fluctuation‐dominated quantum subband. The Chen‐Muller mobility step with increasing carrier density is well described. A large number of other experimental facts are qualitatively accounted for by the analysis, which can be made more quantitative with more accurate evaluation of various integrals, etc. In particular, orientation dependence, doping level dependence, and field dependence are all contained in the theory. A striking feature of the calculation is the marked dependence of the dc mobility dropoff upon interface quality as measured, e.g., by the interface charge density. This suggests that the low‐field mobility provides a good monitor for process control in device fabrication and that greater control may lead to sharper turn on, lower leakage, or closer threshold tolerances especially for low‐voltage complementary metal‐oxide‐semiconductor (CMOS) applications.

The interactions of calcium with mpyxicola giant axons and a description in terms of a simple surface charge model.

Abstract: 1. Myxicola giant axons were examined under voltage clamp in solutions with [Ca2+] ranging from 2 to 200 mM. 2. The magnitude of the shifts in the Na and K conductance-voltage curves, as well as membrane rate constants, caused by changes IN [Ca22+] were comparable and consistent with a model in which there exists a negative surface charge density of -0.013 charges/A-2, but there is no significant binding of divalent cations to the external membrane surface. 3. There was no effect of equimolar substitution of magnesium for Ca on the positions of the conductance-voltage curves along the voltage axis, consistent with the above conclusion. 4. Ca2+ has no effect on the shape of the steady-state Na inactivation relation but simply causes a translation along the voltage axis. 5. It is concluded that a simple surface charge model in which divalent cations do not bind is adequate for Myxicola axons, and that there is no evidence to support a specific involvement of Ca with gating processes.

Heterogeneity of charge density distribution in montmorillonite as inferred from cobalt adsorption

Abstract: Abstraet--A comparison is made of the ion exchange behavior, towards the cobalt ion, of five sodium montmorillonite clays. The selectivity for the bivalent ion at low cobalt loading is correlated with the average dimensions of the particles in the various clays, as characterized by several methods. The data are interpreted in terms of a higher selectivity of the bivalent ions for the broken bonds located at the edges of the clay crystals. Using a model comprising two areas of different charge densities, the experimental differences in behavior can be predicted reasonably well.

Energy dissipation in a thin polymer film by electron beam scattering: Experiment

Abstract: A series of experiments of scanning electron beam exposure of PMMA films on silicon, copper, and gold substrates is described, and compared with the calculations of Monte Carlo and analytic models of energy dissipation per unit volume. Film thickness was varied from 1000 to 10 000 A, accelerating voltage from 10 to 20 kV, and linear charge density from 1×10−9 to 1×10−5 C/cm. Good agreement was obtained in the comparison with the predictions of Monte Carlo calculations, whereas some discrepancies were observed in the comparison with the analytic models. The assumption inherent in the theoretical approaches that PMMA acts like a linear recording medium, in the sense that exposure is additive, is experimentally evaluated by means of an Abel inversion. The assumption that developed profiles represent surfaces of equal energy dissipation was examined by means of a computer simulation of the development process.

Carbon‐13 NMR spectra of coumarin and methoxycoumarins—a reinvestigation of charge density/Chemical Shift Relations

Abstract: The carbon-13 NMR spectra of coumarin, 6-, 7-, 8-methoxycoumarin, and 5,7-, 7,8-, 5,8- and 6,7-dimethoxycoumarin have been measured and assigned. It is shown that substituent induced chemical shifts S(δ) in the mono- and disubstituted systems correlate well with the HMO atom-atom polarisibilities πij of the parent compound: Si(δi) = 80.13 πij with a standard deviation of 1.42 ppm and a correlation factor of 0.994. Correlations between δ(13C) values and charge densities calculated by various semi-empirical methods are less successful.

Shell structure of the $sup 58$Ni charge density

Abstract: A very-high momentum-transfer (q=780 MeV/c), elastic electron-scattering experiment on $sup 58$Ni has been performed using the 600-MeV linac of Saclay. The charge density, extracted from (e,e) and muonic-x-ray data, exhibits considerably less structure than predicted by Hartree-Fock calculations.

Diffuse double layer interaction between two parallel plates with constant surface charge density in an electrolyte solution

Abstract: The diffuse double layer interaction between two parallel dissimilar plates with constant surface charge density in a symmetrical electrolyte solution is considered. The influence of the electric field which may be induced within the plates by the overlapping of the two double layers is discussed in theDebye-Huckel approximation. The solution of the non-linearPoisson-Boltzmann equation is also obtained for the case of thick plates. It is shown that if the surface charges of the two plates are of like sign, or if either of the plates is uncharged, the interaction force is always repulsive, whereas if the charges are of opposite sign, the force may be either attractive or repulsive.

Anisotropic scattering of conduction electrons on point defects in Al

Abstract: New Hall effect experiments on Al containing Zn, Mg or Ge impurities are discussed and compared with previous experiments on Al containing Frenkel defects; interstitials and vacancies. The low field Hall coefficient is found to change always in the positive direction, with respect to its strong negative isotropic value, this effect being largest for Ge doped Al and Frenkel defect Al, where even a positive sign for Hall coefficient is obtained. Kohler rule investigations show that anisotropic scattering of electrons on point defects is the origin of this effect. The relaxation time anisotropies are explained as originating from corresponding strong differences of the wavefunctions involved. This is proved by OPW charge density calculation for k states on the realistic Fermi surface of Al. The symmetrized results, as contour plots, show strong, expected differences. A simple qualitative measure of anisotropy is derived for both impurity and Frenkel scattering.

An investigation of the silicon—Sapphire interface using the MIS capacitance method

Abstract: Measurements of the MIS capacitance as a function of voltage were carried out on aluminum-sapphire-silicon structures. The results were used to determine the doping of the silicon and the interface-state density and flat-band charge density at the silicon-sapphire interface. The density of states function D s (cm-2eV-1) is found to be qualitatively similar to that reported for the Si-SiO 2 interface but is larger in magnitude by a factor of 5-10.