Showing papers on "Electric field published in 1985"

Theory of gel electrophoresis of DNA.

Abstract: A theory of the electrophoresis of DNA through gels with large interfiber spacing, such as dilute agarose, is presented. We assume that the DNA molecule moves along its axis through a “tube” in a neutral gel under the influence of the electric field. The tube is random except for possible bias due to the effects of the field. When the field is small, we easily recover the inverse-length dependence of the mobility found previously by de Gennes and by Doi and Edwards. At higher fields, a new effect appears; the tube becomes oriented because the field biases the direction of the leading end of the chain as it moves to form an extension of the tube. This leads to an increase of the mobility with increasing field by adding a field-dependent but length-independent term to the mobility expression. In agreement with experiment, we find that the field effect can be important at fields as low as 1 V/cm and that the effect can seriously decrease the sensitivity of the mobility to chain length. We also examine the fluctuation of the migration distance, the degree of orientation induced by the field, and the transient effects occurring when the feld direction is rotated by a right angle.

Theory of negative corona in oxygen.

Abstract: Theoretical predictions are given of the development of the current and the distributions of charge and electric field in negative corona, or Trichel current pulses [G W Trichel, Phys Rev 54, 1078 (1938)], in oxygen at a pressure of 667 kPa (50 Torr) For a 10-mm-diam negative sphere located 20 mm from a positive plane, the calculated current pulse has a rise time of 11 ns, a pulse width of 50 ns, and a peak amplitude of 13 mA These results agree satisfactorily with experimental values The predicted velocity of the cathode-directed light pulse also agrees well with observations The theory is based on the accurate numerical solution of Poisson's equation in conjunction with the continuity equations for electrons, positive ions, and negative ions The effects of ionization, attachment, recombination, electron diffusion, and photoemission and ion secondary-electron emission from the cathode are all included The initial steep rise of the current pulse is largely due to rapid ionization and electron motion in the high Laplacian field near the cathode As the discharge develops, a dense plasma forms near the cathode, leading to strong space-charge distortion of the field A prominent cathode fall region is formed immediately adjacent to the cathode, an almost zero field is formed within the plasma and the field is enhanced over the region to the anode The current pulse is quenched because the low electric field in the plasma immobilizes the majority of the electrons which then undergo three-body attachment; furthermore, the cathode fall region becomes reduced to such a short distance that insignificant current is produced from this region Because of the low mobility of the negative ions, the current remains low and the structure of the space-charge fields changes only slowly with time between pulses

Interplanetary magnetic field control of high-latitude electric fields and currents determined from Greenland Magnetometer Data

Abstract: To determine the effects of the interplanetary magnetic field (IMF) on the electric potential as well as on ionospheric and field-aligned currents, a recently available numerical algorithm is applied to an empirical model of high-latitude magnetic perturbations, parameterized in terms of the By and Bz components of the IMF. The empirical model is derived from 20-min average magnetometer data observed during summer at the chain on the west coast of Greenland and the corresponding IMF information from the HEOS 2 satellite. The calculated results reproduce fairly well overall features of the influence of the IMF on high-latitude electric fields which have been reported on the basis of more direct measurements. This confirms the validity of the numerical method and the conductivity distribution models. In addition, our results indicate that the system of ionospheric and Birkeland currents near the polar cusp, which has been shown to depend strongly on By, exists independently of the system of region 1 and region 2 field-aligned currents, which, on the other hand, depends strongly on Bz. The direction of the field-aligned currents in the dayside polar cap is uniquely controlled by the sign of the By component of the IMF, namely upward currents for By > 0 in the northern polar cap and oppositely directed for By 0 and By small the ionospheric and field-aligned currents are localized near the dayside polar cusp, and the electric field has a dusk-dawn component in a narrow region near magnetic local noon in agreement with reported satellite measurements. The associated distribution of field-aligned currents consists of the region 1 current system and an additional pair of oppositely directed currents located poleward of the region 1 currents.

Boundary effects on electrophoretic motion of colloidal spheres

Abstract: An analysis is presented for electrophoretic motion of a charged non-conducting sphere in the proximity of rigid boundaries. An important assumption is that κa → ∞, where a is the particle radius and κ is the Debye screening parameter. Three boundary configurations are considered: single flat wall, two parallel walls (slit), and a long circular tube. The boundary is assumed a perfect electrical insulator except when the applied field is directed perpendicular to a single wall, in which case the wall is assumed to have a uniform potential (perfect conductor). There are three basic effects causing the particle velocity to deviate from the value given by Smoluchowski's classic equation: first, a charge on the boundary causes electro-osmotic flow of the suspending fluid; secondly, the boundary alters the interaction between the particle and applied electric field; and, thirdly, the boundary enhances viscous retardation of the particle as it tries to move in response to the applied field. Using a method of reflections, we determine the particle velocity for a constant applied field in increasing powers of λ up to O(λ6), where λ is the ratio of particle radius to distance from the boundary. Ignoring the O(λ0) electro-osmotic effect, the first effect attributable to proximity of the boundary is O(λ3) for all boundary configurations, and in cases when the applied field is parallel to the boundaries the electrophoretic velocity is proportional to ζp − ζw, the difference in zeta potential between the particle and boundary.

First-Principles Calculation of the Electric Field Gradient of Li 3 N

Abstract: The electric field gradient can be obtained from self-consistent energy-band calculations by the linearized-augmented-plane-wave method provided that a general potential is used. This first-principles method, which does not rely on any Sternheimer antishielding factor, is tested for ${\mathrm{Li}}_{3}$N and yields electric field gradients for Li(1), Li(2), and N in excellent agreement with NMR experiments. The electric field gradient is mainly determined by local distortions of the electronic charge density, especially in the case of the polarizable ${\mathrm{N}}^{3\ensuremath{-}}$ ion.

Electromagnetic radiations from rocks

Abstract: To test the possibility of the emission of electromagnetic waves from rocks, experiments have been made to measure the electric field by using mainly granite samples that were struck together or struck by a hammer or a weight and were fractured by a bending moment. The wide-band (10 Hz to 100 kHz) waveforms of electric signals were digitally recorded. Roughly four kinds of signals have been observed: 30 kHz, 5 kHz, 10 Hz and, in addition, intermittent pulses. Using these measurements of the electric fields, the average electric dipole moment generated was estimated to be 10−14 C m. The possibilities of producing such a dipole moment are discussed in terms of contact electrification (or separating electrification) and piezoelectrification, and both possibilities are shown.

Acceleration Factors for Thin Gate Oxide Stressing

Abstract: Time dependent dielectric breakdown (TDDB) data for 100A of thermally grown SiO2 has been analyzed using an Eyring model based on thermodynamic free energy considerations. The model describes well the following features of the data: (1) an apparent activation energy which is a function of the stressing electric field and (2) a field acceleration parameter that is a function of temperature. Quantitatively, the model suggests the proper field dependence for the activation energy and the observed temperature dependence of the field acceleration in the 100A oxide material. The apparent activation energy is found to decrease from > leV at low field stressing (Eb(50%) - Es > 5 MV/cm) to <0.3eV at higher fields Eb(50%)- Es < 3 MV/cm). Also, the field acceleration was found to be approximately 6 decades/MV/cm at room temperature but reduces to 2 decades/MV/cm at 150C.

Auroral zone electric fields from DE 1 and 2 at magnetic conjunctions

Abstract: Nearly simultaneous measurements of auroral zone electric fields are obtained by the Dynamics Explorer spacecraft at altitudes below 900 km and above 4,500 km during magnetic conjunctions. The measured electric fields are usually perpendicular to the magnetic field lines. The north-south meridional electric fields are projected to a common altitude by a mapping function which accounts for the convergence of the magnetic field lines. When plotted as a function of invariant latitude, graphs of the projected electric fields measured by both DE-1 and DE-2 show that the large-scale electric field is the same at both altitudes, as expected. Superimposed on the large-scale fields, however, are small-scale features with wavelengths less than 100 km which are larger in magnitude at the higher altitude. Fourier transforms of the electric fields show that the magnitudes depend on wavelength. Outside of the auroral zone the electric field spectrums are nearly identical. But within the auroral zone the high and low altitude electric fields have a ratio which increases with the reciprocal of the wavelength. The small-scale electric field variations are associated with field-aligned currents. These currents are measured with both a plasma instrument and magnetometer on DE-1.

Total-energy differences and eigenvalue sums

Abstract: We derive a variational expression for the total-energy difference between two crystal structures in terms of the difference in the sum of the one-electron eigenvalues and additional exchange-correlation and Coulomb contributions. These terms can be made to vanish under certain approximations, but for compound formation and for changes in volume, these terms can be of significance. Our derivation clearly shows that it is the same density, not the same potential as is commonly asserted, that is used in calculating the eigenvalue sums of the two structures; this formal distinction is of importance in order to maintain the variational nature of the results. Our results provide a framework in which many commonly used expressions can be justified and generalized.

The electronic structure of the calcium monohalides. A ligand field approach

Abstract: The electronic structure of the calcium monohalides is addressed using a ligand field model which approximates the halide as a polarizable negative charge perturbing the one electron valence structure of the Ca+ ion. A simple, zero‐free‐parameter model is shown to predict accurately electronic energies, transition moments, permanent dipole moments, and several other molecular constants that have been experimentally determined. The molecular properties and electronic wave functions are interpreted in terms of the polarization (s/p/d/f mixing) and radial expansion (nl/n+1l mixing) of the low lying, free ion, basis functions caused by the electric field of the ligand.

Aerosol Wall Losses in Electrically Charged Chambers

Abstract: Wall deposition of aerosol particles in spherical, electrically charged chambers is investigated theoretically and experimentally. The theory accounts for transport by convection, Brownian diffusion, gravitational sedimentation, and electrostatic drift. Experiments involved measurements of loss rates for particles of known size and charge in 250-liter FEP Teflon film bags. Agreement between theory and experiment is satisfactory provided that charging and neutralization of aerosol particles by ions is taken into account. It is shown that under these experimental conditions, charge can have a dominant effect on deposition rates, depending on particle size and on electric field strength. For example, wall loss rates of singly charged 0.1 μm diameter particles were a factor of 100 greater than wall loss rates of neutral particles of the same size in these experiments. The theory is valid for any distribution of electric fields near the interior surface of the chamber. A general result of the theory is that po...

Distribution of ion energies incident on electrodes in capacitively coupled rf discharges

Abstract: The energy and angular distribution of ions striking the electrodes in rf discharges are of interest with respect to the application of such discharges to the processing of semiconductor materials. The ability to fabricate small (< 1 μm) semiconductor features using the plasma etching process results, in part, from the energetic and anisotropic flux of ions which strike the semiconductor surface. In this paper the energy and angular distribution of ions striking the electrodes in low‐pressure capacitively coupled rf discharges are studied using a Monte Carlo model for ion trajectories and a parametric model for the time‐dependent electric field within the sheath. Energy and angular distributions are discussed as a function of rf frequency, ion mass, and the mean‐free path between charge exchange collisions within the sheath. The ion energy distribution is found to be characterized by a scaling parameter proportional to (rf frequency × sheath thickness)2 × ion mass/(sheath voltage); small values of this parameter yield bimodal distributions, intermediate values yield distributions peaked at the maximum sheath potential, and high values yield distributions peaked at the average sheath potential. The ion energy distribution is also examined for different values of the dc and rf components of the sheath potential and for different models for the electric field within the sheaths. When the dc component of the sheath potential is small compared to the rf amplitude, a large thermal component to the ion energy distribution results. The implication of this result and that for the angular distribution of ions incident on the electrodes is discussed with respect to the isotropy of the etch obtained during plasma etching of semiconductor materials.

Aero-optical foundations and applications

Abstract: Nomenclature A = coefficient defined in Eq. (A40) A Q = area B = magnetic field c = speed of light C = correlation function Cn = index of refraction structure function constant Cp = specific heat D = diameter of aperture D = electric displacement E = spectrum E = electric field H = magnetic induction / = intensity // = Bessel function of order / k = wave number / = turbulence scale size L = path length

Electron heating in silicon dioxide and off‐stoichiometric silicon dioxide films

Abstract: Electron heating in silicon dioxide (SiO2) at electric fields ≲5 MV/cm is demonstrated using three different experimental techniques: carrier separation, electroluminescence, and vacuum emission. Gradual heating of the electronic carrier distribution is demonstrated for fields from 5 to 12 MV/cm with the average excess energy of the distribution reaching ≳4 eV with respect to the bottom of the SiO2 conduction band edge. Off‐stoichiometric SiO2 (OS‐SiO2) layers are shown to behave similarly to very thin SiO2(≲70 A in thickness) with a transition occurring from ‘‘cool’’ to ‘‘hot’’ electrons as the conduction mechanism changes from direct tunneling between silicon (Si) islands in the SiO2 matrix of the OS‐SiO2 material to Fowler‐Nordheim emission into the conduction band of the SiO2 regions. The relationship of electron heating to electron trapping, positive charge generation, interface state creation, and dielectric breakdown is treated. The importance of various scattering mechanisms for stabilizing the el...

Generation of positive charge in silicon dioxide during avalanche and tunnel electron injection

Abstract: Avalanche and Fowler–Nordheim tunneling electron injections have been performed at constant current on a broad variety of differently processed Al‐gate metal‐oxide‐semiconductor capacitors. It is found that the same type of positive charge (the ‘‘slow states’’) is generated during low‐field and high‐field electron injection. The maximum amount of positive charge which can be generated at a given electric field depends on processing and increases linearly with the average field in the oxide. However, the rate at which the positive charge is generated is controlled uniquely by the anode field, for a given polarity of the gate voltage. It follows that the role of the electron traps in the bulk SiO2—independent of their nature—is that of increasing both the rate and the total number of created defects by enhancing, respectively, the anode field, as a result of the distortion of the potential in SiO2, and the average field which must be increased to maintain a constant injected current. Processes described ear...

Green's-function approach to nonlinear electronic transport for an electron-impurity-phonon system in a strong electric field.

Abstract: A Green's-function approach to nonlinear electronic transport in a static electric field is developed microscopically for the system composed of interacting electrons with impurities and phonons. The essential idea is to separate the center-of-mass motion from the relative motion of electrons. An electron temperature is introduced as a measurement of the internal energy of the relative electrons without reference to any distribution function. By allowing different temperatures for decoupled electrons and phonons in the initial state, we obtain the density matrix for the electron-lattice system to the first order of interaction but under arbitrarily strong electric field. The frictional force experienced by the center of mass of electrons and the energy transfer rate from electron system to phonon system are derived by means of the Green's-function technique, and the force- and energy-balance equations for steady state are obtained. These equations are applied to the calculations of the ratio of electron temperature to the lattice temperature and the electron resistivity as functions of drift velocity for impurity, acoustic-phonon, and optical-phonon scatterings. The dynamic nature of Coulomb screening by charge carriers is studied numerically. One of the interesting predictions is the possible cooling of electrons at low temperatures in samples with low impurity concentration.

Electric-field-induced dissociation of excitons in semiconductor quantum wells.

Abstract: We have calculated the effect of a constant electric field on the energy position of the exciton ground state in semiconductor single-quantum wells. We discuss only the case of quantum wells where electron and hole wave functions are mostly localized within the same layer at zero electric field (e.g., GaAs-${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As structures). In such quantum wells an electric field applied parallel to the growth axis polarizes free-electron and -hole wave functions in opposite directions and therefore weakens the excitonic binding. Our theoretical results are in qualitative agreement with recent absorption data.

Auroral electrodynamics with current and voltage generators

Abstract: The electrodynamic structure of auroral currents is studied in the steady state by coupling Maxwell's equations with effective Ohm's laws for ionospheric Pedersen currents, field-aligned currents and currents in the generator region. An effective generator conductivity is introduced which allows consideration of a variety of states from pure current (ΣG = 0) to pure voltage (ΣG → ∞) generators. On short time scales when Alfven waves are important, this conductivity mapped to the ionosphere is lower than the ionospheric conductivity, implying a currentlike generator. This is in contrast with previous electrodynamic models of the aurora, which have assumed voltage-fixed generators. Current generators differ from the voltage generators in that the natural scale size is smaller for current generators, indicating that the narrow discrete arcs may be current driven, while the overall large-scale auroral currents are voltage driven. In addition, reversals of the electric field occur naturally in the current-driven case, leading to V-shaped potential structures. Examination of the effects of ionospheric conductivity gradients and comparison with a time-dependent MHD model of auroral currents indicate that either regions of nonlinear resistivity or generator motions can decouple the ionospheric currents from the generator. In this case the effective conductivity becomes the Alfven wave conductivity, which, being small compared to the ionospheric conductivity, leads to a structure typical of a current-driven system. This implies that the current structure is not affected by ionospheric conductivity gradients, while the ionospheric electric field is enhanced on the low-conductivity side.

The cleft ion fountain: A two‐dimensional kinetic model

Abstract: The transport of ionospheric ions from a source in the polar cleft ionosphere through the polar magnetosphere is investigated using a two-dimensional, kinetic, trajectory-based code. The transport model includes the effects of gravitation, longitudinal magnetic gradient force, convection electric fields, and parallel electric fields. Individual ion trajectories as well as distribution functions and resulting bulk parameters of density, parallel average energy, and parallel flux for a presumed cleft ionosphere source distribution are presented for various conditions to illustrate parametrically the dependences on source energies, convection electric field strengths, ion masses, and parallel electric field strengths. The essential features of the model are consistent with the concept of a cleft-based ion fountain supplying ionospheric ions to the polar magnetosphere, and the resulting plasma distributions and parameters are in general agreement with recent low-energy ion measurements from the DE 1 satellite.

Basic mechanisms of the optical nonlinearities of semiconductors near the band edge

Abstract: Semiconductors are known to show in the band-gap region relative large changes of their optical properties with increasing light intensity. These changes are due to the creation of electron–hole pairs, which modify by band filling the intraband and interband contributions to the complex optical dielectric function. Most important, with increasing concentration of electron–hole pairs, the band gap shrinks, and the Coulomb forces are strongly reduced so that excitonic effects disappear. The theoretical description of these phenomena in three- and quasi-two-dimensional semiconductors is reviewed.

The ambipolar electric field in stellarators

Abstract: In a three-dimensional device like a stellarator, the ambipolar electric field must be determined self-consistently from the ambipolarity constraint and can have a significant effect on transport through the diffusion coefficients. A differential formulation and an algebraic formulation for the electric field are solved, together with the density and temperature equations. The results are compared, and in both cases multiple electric field solutions can exist, with bifurcations occurring between different solutions. It is shown that heating of the electrons encourages bifurcation to the more favourable positive electric field root.

Trapping of ion conics by downward parallel electric fields

Abstract: Several examples of upflowing field-aligned electrons at altitudes between 1000 and 15000 km at high latitudes have been reported recently. The velocity-space distributions of these electrons suggest that they were accelerated out of the ionosphere by downward parallel electric fields, and parallel potential drops of a few tens to a few hundred volts over the altitude range 1000–6000 km are implied. The electron beams are associated with ion conics. Data from electrostatic analyzers onboard the S3-3 satellite show that ion conics can be trapped at low altitudes by the downward electric field. Evidence for the parallel electric field is obtained not only from observations of upflowing electron beams but also from observations of retardation of downflowing electrons and local acceleration of downflowing ions. The inferred parallel electric fields have magnitudes large enough to locally balance the magnetic mirror force on perpendicular ion conics in the few hundred electron volt energy range. This electrostatic trapping has important consequences for wave heating of ion conics since it extends the residence time of ions in the heating region. It is demonstrated that this multipass heating mechanism leads to perpendicular ion heating far in excess of that predicted by present theories of ion conic heating.

Electrical measurements in the atmosphere and the ionosphere over an active thunderstorm: 2. Direct current electric fields and conductivity

Abstract: On August 9, 1981, a series of three rockets was launched over an air mass thunderstorm off the eastern seaboard of Virginia while simultaneous stratospheric and ground-based electric field measurements were made. The conductivity was substantially lower at most altitudes than the conductivity profiles used by theoretical models. Direct current electric fields over 80 mV/m were measured as far away as 96 km from the storm in the stratosphere at 23 km altitude. No dc electric fields above 75 km altitude could be identified with the thunderstorm, in agreement with theory. However, vertical current densities over 120 pA/sq m were seen well above the classical 'electrosphere' (at 50 or 60 km). Frequent dc shifts in the electric field following lightning transients were seen by both balloon and rocket payloads. These dc shifts are clearly identifiable with either cloud-to-ground (increases) or intercloud (decreases) lightning flashes.

High‐field and current‐induced positive charge in thermal SiO2 layers

Abstract: The generation of a bulk positive charge in SiO2 layers of silicon gate metal‐oxide‐silicon (MOS) devices, under the conditions of high‐field and charge injection is studied. The time dependence of the positive charge and its spatial distribution as a function of the oxide thickness and electric field are all consistent with an impact ionization‐recombination model which takes into account both the spatial and the field dependence of the ionization probability. The nature of the ionization, either band‐to‐band, or traps ionization, is still unknown. Bulk positive charge of the same nature is also formed in Al gate oxides. Nevertheless, it was not always observed in previous works since a much larger Si‐SiO2 interfacial positive charge is also generated in these samples.

Electron-hole pair production at metal surfaces

Abstract: We discuss the response of a metal surface to an external electric field which varies slowly in space and time. An earlier calculation of the power absorption due to the creation of electron-hole pairs is improved with the inclusion of an interference term between ``bulk'' and ``surface'' excitation processes. With these improvements, we obtain excellent agreement with the measured inelastic-electron-scattering cross section for Cu(100).