Showing papers on "Charge density published in 1983"

A simple model for the ionization potential, electron affinity, and aqueous redox potentials of small semiconductor crystallites

Abstract: Large semiconductor crystals have intrinsic electronic properties dependent upon the bulk band structure. As the crystal becomes small, a new regime is entered in which the electronic properties (excited states, ionization potential, electron affinity) should be strongly dependent upon the electron and hole in a confined space. We address the possibility of a shift in the photochemical redox potential of one carrier, as a function of crystallite size. As a semiquantitative guide, one might expect a shift on the order of h2/8em*R2 due to the kinetic energy of localization in the small crystallite. We model the elementary quantum mechanics of a charged crystallite using (a) the effective mass approximation, (b) an electrostatic potential for dielectric polarization, and (c) penetration of the carrier outside the crystallite in a cases of small effective mass. Shifts of several tenths of an eV appear possible in crystallites of diameter 50 A. The carrier charge density reside near the crystallite surface if ...

Surface magnetism of Fe(001)

Abstract: Results of all-electron self-consistent semirelativistic full-potential linearized augmented-plane-wave local-density and local-spin-density studies are reported for a seven-layer Fe(001) thin film. The calculated work function for the ferromagnetic state is found to be in excellent agreement with experiment, whereas that calculated for the paramagnetic state is significantly worse (namely, 0.5 eV too large), indicating the importance of spin polarization on this electrostatic property. For both states, partial densities of states (projected by layer and by orbital angular momentum), surface states, and charge (and spin) densities are presented and their differences employed to discuss the origin of surface magnetism. No Friedel oscillation is found in the layer-by-layer charge density. The surface-layer magnetic moment is found to have been increased by 0.73${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ from the center layer to 2.98${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$/atom; a very small Friedel oscillation is obtained for the spin density, which indicates possible size effects in this seven-layer film. Layer-by-layer Fermi contact hyperfine fields are presented: While the core-polarization contributions are proportional to the magnetic moment, the conduction-electron contribution shows a pronounced Friedel oscillation in the central layer and, significantly, a change of sign and increase in the magnitude for the surface-layer contribution. The hyperfine field at the nucleus of the center-layer atoms is found to be in excellent agreement with experiment. The net result for the surface-layer atoms is a predicted decrease in magnitude of the total Fermi contact hyperfine field despite the large increase of their magnetic moments. The relevance of this prediction to experiment is discussed.

Hartree–Fock exchange energy of an inhomogeneous electron gas

Abstract: We examine the short-range behavior of the spherically averaged Hartree–Fock exchange charge density by performing a simple Taylor expansion. On the basis of this expansion, a theoretical model is constructed that generates gradient correction terms to the local density approximation for the exchange energy of an inhomogeneous electron gas. In particular, we derive the Xαβ exchange energy functional and a theoretical value for the parameter β. Our value for β agrees well with previous empirical estimates, and with empirical calculations in the present work.

Fitting the Coulomb potential variationally in Xα molecular calculations

Abstract: The method of Mintmire and Dunlap which variationally fits the Coulomb potential rather than the charge density is extended to linear combination of Gaussian‐type orbitals Xα calculations on molecules. This approach is more efficient than fitting the charge density since the same integrals are used to treat the Coulomb and exchange potentials. Furthermore, the required Fock matrix elements are simple overlap rather than Coulomb integrals. However, the method is somewhat less accurate than fitting the charge density in a test on H2.

Size of sodium dodecyl sulfate micelles in the presence of additives i. alcohols and other polar compounds

Abstract: The steady-state fluorescence quenching method has been used to examine how 12 polar additives affects the size of SDS micelles. The picture that emerges is that the controlling factor for micelle size is the surface area available per charged headgroup, or its inverse the surface charge density. In first approximation, the surface charge density is the same (linear in a wide range) function of the mole fraction additive in the micelle for all the alcohols from C4 and up. Depending on the volume of the hydrophobic part of the additive the micelle either grows directly or first passes a minimum size on increasing amounts of additives. The fluorescence quenching methods primarily give the number of micelles in the solution, from which number average aggregation number and micelle size are calculated.

Threshold Behavior of Charge-Density Waves Pinned by Impurities

Abstract: A time-dependent mean-field theory of the nonlinear electric field response of a charge-density wave pinned by impurities is implemented. It is found that above a threshold electric field ${E}_{T}$, the charge-density wave moves with velocity $v\ensuremath{\propto}{(E\ensuremath{-}{E}_{T})}^{\frac{3}{2}}$. Some general discussion of the behavior as a function of dimension is included.

All-electron local-density theory of alkali-metal bonding on transition-metal surfaces: Cs on W(001)

Abstract: A theoretical study of the nature and the mechanism of the bonding of an alkali metal (Cs) on a transition-metal surface [W(001)] in the high-coverage limit is presented in order to understand and explain the lowering of the work function and to elucidate the role of W surface states and surface resonance states in the adsorption process. The analysis is based on all-electron local-density-functional results obtained with our self-consistent full-potential linearized augmented-plane-wave method for thin films for (1) a five-layer slab of W, (2) an unsupported Cs monolayer, and (3) Cs in a $c(2\ifmmode\times\else\texttimes\fi{}2)$ structure on both sides of the five-layer W slab for three different Cs-W separations. We find that Cs forms a polarized-metallic rather than ionic overlayer: The Cs valence electrons originating from the atomic $6s$ states are polarized toward the W surface leading to an increase of electronic charge in the Cs/W interface region and a depletion of electronic charge on the vacuum side of the overlayer. In addition, the semicore Cs $5p$ electrons are markedly counterpolarized. The net result of these multiple surface dipoles is a lowering of the work function upon cesiation from 4.77 eV (clean five-layer W slab) to 2.77, 2.55, and 2.28 eV, corresponding to heights of the Cs atoms above the W surface of 2.60, 2.75, and 2.90 \AA{}, respectively. The Cs-induced changes in the charge density are essentially localized outside the surface W atoms. The W $d$ surface states and surface resonance states which are so characteristic of the W(001) surface are found to persist on the cesiated W(001) surface. The main effect of the Cs overlayer on these states is their energetic stabilization; this effect is most pronounced for the contamination-sensitive ${\overline{\ensuremath{\Gamma}}}_{1}$ surface state just below the Fermi energy, which is lowered in energy by 1 eV due to hybridization with Cs $s$ states.

Charge-density excitations at the surface of a semiconductor superlattice: a new type of surface polariton

Abstract: A cause de la quantification des niveaux d'energie electronique par le potentiel de superreseau, ce nouveau mode de polariton a la propriete remarquable de ne pas avoir d'amortissement de Landau

Counter-ion condensation and system dimensionality.

Abstract: Condensation of the counter-ions around a highly charged infinitely long cylindrical molecule, such as DNA, can be described in detail in terms of the solutions of the Poisson-Boltzmann (Gouy-Chapman) equation. By using the Alfrey-Berg-Morawetz (1951) solution of this equation one can show that a certain fraction of the counter-ions remain within finite distances of the poly-ion even when the volume of the system is expanded indefinitely; these ions can be appropriately called "condensed". The fraction of the macromolecule's charge represented by these ions is just 1-1/xi, where xi is the linear charge-density parameter of the macromolecule; this is also the value given by Manning's theory. The question arises: Is this property unique to the infinite cylinder? Using the same PB equation, we can consider the infinite charged plane and a large finite charged sphere for comparison. In the case of the plane all of the counter-ions are condensed in the above sense, not just a fraction, for any surface charge density of the plane. These ions form the classical Gouy double layer. On the other hand, none of the counter-ions of the charged sphere are condensed in the above sense, no matter how high the surface charge density. Thus the cylinder is a unique intermediate case in which a fraction of the counter-ions are condensed if the linear charge density is higher than the critical value of unity.

Ab initio study of structural and electronic properties of beryllium

Abstract: An ab initio calculation of the structural and electronic properties of beryllium is presented. The calculational method used is the self-consistent pseudopotential approach within the local-density-functional scheme. The calculated lattice constants, cohesive energy, bulk modulus, Poisson's ratio, electronic band structure, density of states, and charge density are all in good agreement with the experimental measurements.

Lithium-intercalated graphite: Self-consistent electronic structure for stages one, two, and three

Abstract: First-principles electronic structure calculations were carried out for Li${\mathrm{C}}_{6}$, Li${\mathrm{C}}_{12}$, and Li${\mathrm{C}}_{18}$ representing first-, second-, and third-stage model graphite intercalation compounds. By comparing the charge density of these compounds to that of reference graphite compounds, we could define a "total difference density" in order to quantify charge transfer and polarization in these materials. The total difference density is found to be highly concentrated near the intercalant ions. However, the conduction electrons (those in partially occupied bands) are found to have the distribution of virtually undistorted $\ensuremath{\pi}$ wave functions and have a much more delocalized distribution than that of the total difference density. These two types of charge distributions account for many of the unusual electronic properties of graphite intercalation compounds.

High voltage electric field and space‐charge distributions in highly purified water

Abstract: High voltage Kerr electro‐optic field mapping measurements in highly purified water show significant field distortions due to injected positive space charge over the temperature range of 8–25 °C using parallel plane stainless steel electrodes stressed to average field strengths up to 150 kV/cm. The net space charge was always positive with density of ≂2 C/m3 which is about 40% of the background value of charge density of thermally generated hydronium and hydroxyl ions. Because measurements only showed a net positive space charge, a unipolar drift dominated conduction model was developed where positive charge would migrate in an ohmic medium. This model predicts the observed propagating charge front, electric field enhancement at the noncharge injecting electrode, and field decrease at the charge injecting electrode. The time of flight of positive charge between electrodes gave an estimate of the positive ion mobility of μ≂4×10−7 m2/(V s) which is inbetween the values of the hydronium ion mobility measured at low voltages and the electrohydrodynamic mobility which accounts for fluid convection effects due to the Coulombic force putting the fluid into motion. The dielectric relaxation time at T≂8 °C (τ≂320 μsec) measured at low voltage was much less than the measured charge transport time between electrodes, τmig ≂2 msec, so that the calculated effects of space charge were much less than measured because the charge relaxes quickly as it migrates. It was found that a dielectric relaxation time of τ≂1.6 msec would provide a good fit between analysis and experiments.

Theory and experiment on the surface‐photovoltage diffusion‐length measurement as applied to amorphous silicon

Abstract: The collection‐efficiency theory for low‐mobility solar cells proposed by Reichman has been adapted to the analysis of the surface‐photovoltage experiment in amorphous silicon. From measurement of the apparent diffusion length at low‐ and high‐bias light intensity the ambipolar diffusion length and space‐charge width can be obtained. In order to fit experimental observations to the theory it was necessary to assume that the positive space‐charge density in the surface barrier increases with the light level. This was independently confirmed by capacitance measurements.

Blocking capability of planar devices with field limiting rings

Abstract: The results of breakdown voltage investigations of planar devices with field limiting rings are described. The two-dimensional Poisson equation is solved using the finite difference method. The question of the optimal ring spacings for devices with more than one field ring and the influence of surface charges on the blocking capability are extensively studied. The influence of the following device parameters is discussed: ring spacing, ring width, doping gradient and surface charge density. An analytical model for simplified calculations showing the principal characteristics of field ring devices is presented. Furthermore some experimental results of EBIC measurements are shown.

Three-dimensional theory of the free-electron laser in the collective regime

Abstract: A fully self-consistent theory of the free-electron laser is derived in the collective regime which includes all transverse variations in the wiggler field as well as the effects of a finite waveguide geometry. A general orbit theory is derived by perturbation about the steady-state trajectories in a configuration which consists of an axial guide field in addition to the helical wiggler field, and used to obtain the source current and charge density for the Maxwell-Poisson equations. By this means, a set of coupled differential equations is found which describes an arbitrary radial beam profile. A dispersion equation is obtained under the assumption of a thin monoenergetic beam, and solved numerically for the growth rates of the TE/sub 11/ and TM/sub 11/ modes in a cylindrical waveguide. A selection rule is found by which the TE/sub l/m or TM/sub l/m modes are resonant at the lth free-electron-laser Doppler upshift.

Charge distribution and atomic thermal vibration in lead chalcogenide crystals

Abstract: Charge distribution in crystals of lead chalcogenides, PbX (X = S, Se and Te), has been studied by X-ray diffraction. The structure of PbX is of the rock salt type: cubic, Fm3m, a = 5.934 (1), 6.128 (1) and 6.462 (1)A for PbS, PbSe and PbTe, respectively. Final agreement factors for PbS, PbSe and PbTe were R = 0.015, 0.011 and 0.015 for 144, 147 and 130 independent reflections, respectively. The direct integration of charge density, the observed atomic scattering factors, and the population analysis of the valence electrons unanimously indicated that the Pb atom is negatively charged. This observation suggests predominant covalency for the chemical bond in lead chalcogenide crystals. No overlap density was observed in the deformation density map. The bonding electrons in these crystals may be delocalized like free electrons in metals. The thermal parameters of the Pb atoms were found to be larger than those of the chalcogen atoms and the ratio Bpb/B x increases in the order: PbS < PbSe < PbTe. Anharmonic thermal vibration of the Pb atom was detected in the difference electron density maps. A positive peak of 2 (0.3) e A -3 was observed in the (111) direction at 0.4 A from the Pb nucleus for all three compounds. Its height was reduced to 1 (0-3) e A -3 by introducing anharmonic potential parameters for the Pb atom.

Charge-density waves in the quasi-one-dimensional compounds NbTe4 and TaTe4

Abstract: Superlattice spots observed by X-ray and electron diffraction in the tetragonal, metallic, quasi-one-dimensional compounds NbTe4 and TaTe4 are interpreted in terms of charge-density waves. It is shown that three slightly different types of columns occur in the crystals. In NbTe4 at room temperature two types of chains have incommensurate distortions with wavevectors q1=(0, 0, 0.311c*) and q2=(0.5a*, 0.5b*, 0.344c*): the third type has a very weak distortion with q3=(0.5a*, 0, 1/3c*). The wavevectors for TaTe4 are as above except that the component along the metal chain is exactly 1/3c* in each case. The vectors q1 and q2 are independent of temperature but can be varied by substitution of Ta for Nb. These are structurally the simplest inorganic crystal structures in which CDW have been observed and are the only reported crystals in which three CDW coexist.

Calculation of potential cutoff for one-phonon atom-surface scattering

Abstract: A cluster calculation for Cu(111), Ag(111), and Au(111) shows that the lateral Fourier transform of the charge deformation due to a displacement of a single surface atom is a Gaussian exp(-Q/sup 2//2Q/sub c//sup 2/). This is proportional to the scattering amplitude for one-phonon exchange atom-surface scattering and produces a cutoff in the cross section. The values of Q/sub c/ are evaluated for the above-mentioned surfaces, explaining very well the experimental observations. We also find a simple formula that should be useful to experimentalists in calculating the cutoff.

The effect of surface states and fixed charge on the field effect conductance of amorphous silicon

Abstract: We have developed a computer program to calculate the field effect conductance for an amorphous semiconductor including the effects of surface states and fixed charge at both surfaces of the thin semiconductor film. For undoped films with a bulk density of states of less than 1017 cm−3 eV−1, the space‐charge region extends to a depth of 0.5 μm. A complete description of the potential distribution in the semiconductor must include the contribution of surface charge from the surface opposite the gate electrode. This is of practical importance in thin film transistors, for example, where different transistor structures and processing of devices can affect the charge density of this surface.

Reactively rf magnetron sputtered AlN films as gate dielectric

Abstract: AlN films were deposited on silicon (100, 111), fused quartz and GaAs (100) by sputtering in an Ar:N2 gas mixture. We report on the physical and chemical analysis of the films with the aid of scanning electron microscopy, x‐ray diffraction, and Auger electron spectroscopy. The films were found to be transparent with a c‐axis orientation. Their bulk resistivity was 1015 Ω‐cm. Quasistatic C‐V analysis of Al/AlN/Si gave a flat‐band voltage of 0.85 V, density of fixed charge of 6×1010 cm−2 and a density of fast interface charge of 1.5×1011 cm−2 eV−1 at midgap. These films were formed at 250 °C and offer an attractive fabrication alternative for gate dielectric.

Statistical mechanics of electrolytes and polyelectrolytes. III. The cylindrical Poisson-Boltzmann equation

Abstract: The Poisson–Boltzmann equation for the potential due to an infinitely long, cylindrical polyion in a dilute ionic solution is studied and certain properties of its solution are proved. It is shown that when the charge density on the polyion exceeds a critical value, the surrounding ionic solution separates into two essentially independent phases. The inner phase which is close to the polyion consists predominantly of counterions. An explicit solution to the distribution function in this phase is obtained to the leading order and its validity is proved. This function depends only on the polyion charge density, and not on the concentration of the ionic solution. The distribution function in the outer phase depends upon the ionic strength but, to the leading order, not on the polyion charge density as long as it exceeds the critical value. It is shown that as the charge density on the polyion is increased, a proportionate number of counterions appears in the inner phase so as to neutralize the effect of this increase on the outer phase. It is proved that the potential, to its leading order, in the outer phase for any polyion charge density higher than the critical value, is the same as that due a polyion carrying a charge density equal to the critical value. The nonlinear dependence of the surface value of the potential on the polyion charge density is proved.