Showing papers on "Charge density published in 1981"

Gauge-Invariant Theory of the Dynamical Interaction of Charge Density Waves and Superconductivity

Abstract: The long-wavelength amplitude mode of the charge-density-wave state modulates the average density of states at the Fermi surface leading to a coupling between it and the order parameter for superconductivity. This is shown, in a gauge-invariant theory, to lead to a bound state below twice the BCS gap. Our results are discussed in the context of recent experiments on $2H$-Nb${\mathrm{Se}}_{2}$.

The structure of electrolytes at charged surfaces: The primitive model

Abstract: The effects of ion size on the structure of a primitive model electrical double layer at a charged surface is considered using the integral equation method based on a combined hypernetted chain and mean spherical approximation (HNC/MSA). The HNC/MSA is shown to be a nonlinear weak field approximation. The contact values of the surface ion distribution functions are shown to have the correct quadratic dependence on the surface charge density, An analytical comparison between the HNC/MSA and earlier work based on the modified Poisson–Boltzmann equation is given.

Electric birefringence of restriction enzyme fragments of DNA: Optical factor and electric polarizability as a function of molecular weight

Abstract: The electric birefringence of restriction enzyme fragments of DNA has been investigated as a function of DNA concentration, buffer concentration, and molecular weight, covering a molecular weight range from 80 to 4364 base pairs (bp) (6 × 104–3 × 106 daltons). The specific birefringence of the DNA fragments is independent of DNA concentration below 20 μg DNA/ml, but decreases with increasing buffer concentration, or conductivity, of the solvent. At sufficiently low field strengths, the Kerr law is obeyed for all fragments. The electric field at which the Kerr law ends is inversely proportional to molecular weight. In the Kerr law region the rise of the birefringence is accurately symmetrical with the decay for fragments ≤ 389 bp, indicating an induced dipole orientation mechanism. The optical factor calculated from a 1/E extrapolation of the high field birefringence data is −0.028, independent of molecular weight; if a 1/E2 extrapolation is used, the optical factor is −0.023. The induced polarizability, calculated from the Kerr constant and the optical factor, is proportional to the square of the length of the DNA fragments, and inversely proportional to temperature. Saturation curves for DNA fragments ≤ 161 bp can be described by theoretical saturation curves for induced dipole orientation. The saturation curves of larger fragments are broadened, because of a polarization term which is approximately linear in E, possibly related to the saturation of the induced dipole in high electric fields. This “saturated induced dipole” is found to be 6400 D, independent of molecular weight. The melting temperature of a 216-bp sample is decreased 6°C in an electric field of 8 kV/cm, because the lower charge density of the coil form of DNA makes it more stable in an electric field than the helix form.

Nature of the charge distribution in proteins

Abstract: The significance of the fairly strong electric fields produced by the electric macrodipole of the α-helix and by ionic charges in stabilizing globular proteins has been recognized for some years1–4. These electrostatic interactions are involved at the active sites of functional proteins in binding a substrate or a coenzyme, or in enhancing enzymatic reaction rates2–5, and are unique among the many interactions affecting protein structure and function because of their long range, extending over the whole protein molecule. We have now analysed the distribution of the distances separating the ionic charges (ionized groups and the apparent charges at the termini of the α-helices) for more than 44,000 charge pairs in 14 proteins. Our results show that charges in the proteins are, on average, surrounded by charges of opposite sign. Previous calculation of the electric potential near the helix termini has shown that the electrostatic effect of the α-helix dipole is equivalent to the effect of one-half of a positive unit charge at the N-terminus of the α-helix and one-half of a negative unit charge at the C-terminus3. We report here that the macrodipole of the α-helix has the same order of contribution to stabilizing the native protein conformation as ionized groups.

Atomic structure and properties of polar Ge-GaAs(100) interfaces

Abstract: We present self-consistent calculations of the total energy, charge density, and electronic states of "compensated" (100) interfaces of Ge and GaAs. In this paper we test the methods using the simplest compensated interfaces---an abrupt single mixed layer, either (1/2)(Ge + Ga) or (1/2)(Ge + As), using the "average atom" approximation for the interface plane. The (1/2)(Ge + As) interface is found to be the more stable (however, this may be an artifact of the averaging). We find a large variation in the interface dipole with stoichiometry. This is not observed experimentally, suggesting that the stoichiometry is, in fact, varying only slightly in experimental Ga-rich and As-rich interfaces. No interface states are found in the fundamental gap although there are localized states at $J$ and $K$ in the two-dimensional Brillouin zone. Several points are discussed improving the efficiency of the self-consistent calculations. Finally, variation of the dipole with relaxation is used to determine interface effective charges.

Calculation of partition coefficients by the charge density method

Abstract: A new method is presented for the calculation of partition coefficients of solutes in water lipid systems. Log P values are calculated based on the charge densities of the atoms as determined by quantum mechanical methods. In this article, the results have been obtained from charge densities calculated by two methods: MINDO/3 and Huckel-type calculation based primarily on topology. Results are favorably compared with those obtained by fragment analysis.

Ground-state properties of diamond

Abstract: The ground-state properties of diamond are investigated using an ab initio density-functional pseudopotential scheme. The calculated equilibrium lattice constant, cohesive energy, and bulk modulus are in excellent agreement with experiment. Unlike Si and Ge, a double hump is found in the valence-electron charge density along the tetrahedral bonds.

Charge-Density-Wave Transport in Ta S 3

Abstract: We report the observation of strongly nonlinear conductivity with a sharp threshold field and a giant dielectric constant in the charge-density-wave state of Ta${\mathrm{S}}_{3}$. We argue that these properties reflect the dynamical behavior of the charge-density-wave condensate, and compare the observed behavior with the properties of Nb${\mathrm{Se}}_{3}$.

EPR Determination of Membrane Potentials

Abstract: A large number of membrane processes are electrically active; that is, they involve the movement of charge within or through the membrane. Membrane electrical potentials are primary variables that must be measured in investigating such processes. A possible electrostatic potential profile across a membrane that delineates an interior and exterior space is shown in Figure l. �I/I, the transmembrane potential, is the potential difference between the bulk aqueous phases on either side of the membrane and results from separation of mobile charge across the membrane. The surface poten­ tials, 1/1'0,1/1.;, are the electrostatic potentials at the surfaces of the membrane relative to the potential of the bulk aqueous solution, and arise from fixed charge at the membrane-solution interface. Recently, Andersen et al (3) have described an internal membrane potential that arises from the adsorption of hydrophobic ions like tetraphenylborate in a region within the membrane low dielectric. This potential is termed the boundary potential and in this review is defined as 1/10,1/1;, the potential difference between the site of ion adsorption and the corre­ sponding bulk aqueous phase (see Figure 1). An important characteris­ tic of the boundary potential is that it arises from the charge density within a low dielectric and thus cannot be completely screened by high ionic strength (3). The potentials shown here ignore contributions from dipoles within the membrane structure and include only variable poten­ tials arising from charge separations involving mobile ions.

Electric field structure in an active part of a small, isolated thundercloud

Abstract: A balloon carrying an electric field meter and a standard meteorological radiosonde rose into a relatively small, isolated thunderstorm in central New Mexico on July 16, 1977 The electric field, E, versus altitude between 3200 m (surface) and 7000 m above sea level can be explained reasonably well by the following charge distribution: (1) a 130 m thick layer of positive charge just above the surface with a density of 17 × 10−9 C/m3; (2) a localized 1 C of positive charge moving downward on rain with a velocity of 8 m/s at a horizontal distance of 420 m from the balloon; (3) a 1-km thick ‘layer’ of negative charge between 4800 and 5800 m above sea level with a density of about −5 × 10−9 C/m3 This negative charge spanned a temperature range of about −2 to −5°C Between 7000 and 10,000 m above sea level, the balloon was near the edge of the cloud where E was low and principally horizontal Above 10,000 m the balloon ascended into an anvil cloud that was apparently positively charged

Solution of the kinetic equations governing trap filling. Consequences concerning dose dependence and dose-rate effects

Abstract: The equations governing the traffic of charge carriers during the filling, by ionizing radiation, of traps and luminescence centers in an insulator are numerically solved. The numerical solution is that of a set of four simultaneous differential equations governing the time-dependent functions of concentrations of electrons and holes in the conduction and valence bands and in traps and centers. The results are more general and accurate than those reported previously since no assumptions concerning the proximity to equilibrium have to be made. Moreover, all previous calculations took into account the accumulated concentrations at the end of the irradiation, whereas we have considered an additional period of time after the excitation which allows for the relaxation of carriers in the bands. This simulates the experimental conditions more accurately because during this time any charge carriers which may have accumulated in the conduction and valence bands will relax into the traps and centers and, in doing so, will contribute to the final concentration of trapped charge. In our calculations we have allowed for this by letting the charge in the bands decay for a period of time $T$ following the cessation of the irradiation (which occurs at time $t$). Thus, the level of trapped charge $n$ is calculated at time $t+T$ and this is taken to be a better representation of the trapped charge density. Results were obtained for very high and very low dose rates (intensities) of the radiation. Experimental findings of the dose dependence of thermoluminescence (TL) are susceptible to analysis by the approach developed by us. By adding a competing trapping level and changing the set of equations appropriately, we get a set of five simultaneous differential equations. In this way we can test the previous approximative results yielding a superlinear filling of one of the traps. It is found that, under an appropriate choice of parameters, superlinearity emerges, although the results are not identical to those of the previous approximations. In addition, an important result to emerge from the analysis is the possible dependence of TL output on the dose rate for a constant total dose. Recent experimental results of such a dependence on TL in quartz are shown to be in general accord with the numerical results.

Silicon nitride for the improvement of silicon inversion layer solar cells

Abstract: It is demonstrated that CVD Si-nitride films as transparent dielectric satisfy all requirements for achieving MIS/IL solar cells with high efficiency and long term stability. Deposited on silicon by the SiH4/NH3 reaction at temperatures lower than usual (between 600° and 650°C) fixed positive interface charge densities QN/q up to 7 × 1012 cm−2 with excellent stability have been obtained. Utilizing the Si-nitride charge storage effect, the highest known QN/q values (> 1013cm−2) combined with low values of Nit have been achieved. The charge distribution is discussed and an energy band diagram modified according to new analytical results is presented. MIS/IL solar cells with AM1 efficiencies of 15% (active area) and high UV sensitivity have been obtained.

The electrostatic potential and field in the surface region of lamina and semi‐infinite point charge lattices

Abstract: General and readily adaptable equations are derived for the Coulomb potential in the surface region of infinite lamina and semi‐infinite point charge lattices using the Bertaut method. The expressions which resemble the Ewald formulas used for comparable bulk lattices are cast in terms of charge spreading functions of arbitrary analytical form and are evaluated at real and reciprocal space lattice sites. The significance of the zero reciprocal space vector term is discussed. The convergence behavior of a modified direct summation scheme over the lattice charges compares favorably with that of the Bertaut method.

Variational theorems for the single-particle probability density and density matrix in momentum space

Abstract: The existence of Hohenberg-Kohn---like density-functional theorems in momentum space is demonstrated. Invoking principles employed by Levy to construct universal variational density and density-matrix functionals in position space, it is found that (1) there exists a universal variational functional for the one-electron reduced density matrix in momentum space, and (2) for any given external potential, there exists a proper variational functional for the one-electron momentum-space probability density.

Measurement of charge distribution in polymer electrets by a new pressure-pulse method

Abstract: The charge distribution in the thickness direction of 10 to 100 μm thick polymer electrets can be determined with a new method utilizing a <1 ns laser pulse to launch a pressure pulse in the sample. Propagation of the pressure pulse through the film causes electrode currents which yield the charge distribution. The method has been applied to electron-beam charged PETP and FEP samples.

Neutron charge form factor in a quark model with hyperfine interactions

Abstract: It is known that the charge radius of the neutron can be quantitatively understood as being due to color hyperfine interactions which mix nonsymmetric components into the nucleon spatial wave function. We calculate the electric form factor of the neutron in this model and show that it compares favorably with the data on G/sup n//sub E/(q/sup 2/).

Force calculations in the density functional formalism

Abstract: A force formula for an arbitrary system of electrons and nuclei is derived within the frozen core approximation to the density functional scheme. The formula has the important computational advantage over a direct application of the Hellmann–Feynman theorem in requiring only stationarity of a modified energy functional with respect to the valence charge density. The usefulness of the method is illustrated by its application to light dimers.

Electronic structure of Li 3 N

Abstract: We present the results of a self-consistent calculation of the electronic structure of the fast ionic conductor lithium nitride (${\mathrm{Li}}_{3}$N). The first-principles pseudopotential method is used to obtain the band structure, the charge density, and the density of states at zero temperature. Apart from treating the nitrogen and the lithium $1s$ state in the frozen core approximation, no ad hoc assumptions on the electron density are made. The calculation confirms the ionic picture of the chemical bonding in ${\mathrm{Li}}_{3}$N. A negative charge of approximately 2.8 electrons can be associated with each nitrogen atom. The calculated ground-state wave functions are used to obtain the autocorrelation function of the one-electron density matrix. The observed anisotropies in the autocorrelation function are quantitatively reproduced.

Charge storage and distribution in the nitride layer of the metal-nitride-oxide semiconductor structures

Abstract: Electronic charge trapping in the low‐pressure chemically vapor deposited thin film of Si3N4 has been investigated by internal photoelectric‐effect technique. Charge‐storage characteristics and the spatial distribution of electrons trapped in the Si3N4 layer of a biased Al‐Si3N4‐SiO2‐Si structure have been determined using photocurrent and high‐frequency C‐V measurements. Electrons generated by internal photoemission at the Si surface were captured by traps located within the Si3N4 layer which gave rise to a charge‐storage mechanism. Some of these trapped charges were then photoionized by photons with an energy of 4.14 eV and removed from the insulator. This resulted in a discharging mechanism. The flatband voltage shifts for various charging and discharging processes were determined and characterized in detail. The total trapped electronic charge was on the order of 3.14×1012/cm2, with a retention time of 48 days in the dark. The etch‐back experiments, where the charged Si3N4 layer was etched back in ste...

Ground-state properties of GaAs and AlAs

Abstract: Ground-state properties of GaAs and AlAs are studied and compared using the self-consistent pseudopotential method. The calculated equilibrium lattice constant, the bulk modulus, and the cohesive energy of these compounds are in good agreement with experiment. This agreement allows for the investigation of a subtle difference in the bond strength between GaAs and AlAs. The density of states for the two compounds is found to be similar, but there exists a characteristic difference which can be used as a sensitive probe for distinguishing different species. The valence charge density and the kinetic energy density functions for these compounds are illustrated for comparison.