Showing papers on "Electric field published in 1969"

Power spectrum of light scattered by two-level systems

Abstract: The power spectrum of the light scattered by a two-level atom driven near resonance by a monochromatic classical electric field is evaluated. The atom is assumed to relax to equilibrium with the driving field via radiation damping, which is treated by explicitly coupling the atom to the quantized electromagnetic field modes. The power spectrum of the scattered field is directly obtainable from the two-time atomic dipole moment correlation function, which is evaluated by a method based on a Markoff-type assumption analogous to that used to evaluate the time evolution of single-time atomic expectation values.

Investigation of the Electrical Properties of Sr1−xBaxNb2O6 with Special Reference to Pyroelectric Detection

Abstract: The dielectric constants, electrical conductivity, specific heat, and pyroelectric coefficients of ferro‐electric Sr1−xBaxNb2O6 (SBN) are investigated as a function of temperature in the range 10°–500°K, and as a function of the Sr/Ba composition of the material. A simple technique for measuring absolute pyroelectric coefficients and spontaneous polarizations of ferroelectrics is described. The electric field and frequency dependence of the dielectric properties are also investigated. The theory of pyroelectric detection is discussed from a materials point of view, and the experimental data are considered in terms of the usefulness of SBN as a pyroelectric detector of electromagnetic radiation.

A Novel Electrophotographic System

Abstract: A novel electrophotographic system is described which consists of a thin film of a transition metal oxide and a thin film photoconductive layer sandwiched between a pair of electrodes. When an electric field is applied across this composite structure and an optical image is projected on it, the resulting modulation of the conductivity pattern in the photoconductive layer causes a similar coloration pattern in the oxide layer, thereby forming a visible image. The formation of color centers and the associated optical and electrical properties are discussed.

Limiting Laws and Counterion Condensation in Polyelectrolyte Solutions II. Self‐Diffusion of the Small Ions

Abstract: The concepts developed in Part I of this series are applied to the self‐diffusion of counterions and co‐ions. The condensed counterions are assumed to have no mobility, while the uncondensed counterions and all co‐ions execute Brownian motion subject to the locally inhomogeneous electric field created by the polyelectrolyte molecules. The polyelectrolyte field is assumed to have small amplitude, in analogy to the Debye–Huckel treatment of uncondensed mobile ions in Part I. The self‐diffusion coefficients of the counterion and the co‐ion are both less than the coresponding values in the absence of polyelectrolyte, but the coefficient for the counterion is much less than that for the co‐ion. The results for the self‐diffusion coefficient of the counterion in a salt‐free polyelectrolyte solution are in good agreement with experimental data.

Shapes of Inhomogeneously Broadened Resonance Lines in Solids

Abstract: Inhomogeneous broadening has been observed in resonance lines in solids over the wide range of energies spanned by nuclear magnetic resonance, electron spin resonance, optical, and M\"ossbauer methods. The broadening arises from random strains, electric fields, and other perturbations from the defects in the lattice containing the centre whose transitions are studied. This paper reviews the calculation of the shapes of such resonance lines. The most important method used is the so-called statistical method. This method determines the line shape as a function of the distribution of the defects with respect to the centres studied, the density of the defects, and the perturbation fields of the individual defects. Emphasis is laid on the physical assumptions and approximations in this method and on its relation to other approaches. The theory is applied to a variety of broadening mechanisms, both in the widely used continuum approximation for the lattice containing the defects and in the more realistic discrete-lattice model. Two classes of experimental work are reviewed. The first deals with the ways in which resonance lines are recognised as being inhomogeneously broadened. These methods show that a wide range of phenomena can be used to check the theory of the line shapes. The second discussion of the experimental work compares theory and experiment for each of the various broadening mechanisms. These mechanisms include broadening by the strains from dislocations and point defects, by the electric fields and field gradients from charged defects, and by unresolved hyperfine structure. In each case theory and experiment are compared in detail for the system for which the most complete data are available. The conclusion is that the statistical method provides a satisfactory approach in all cases for which there are adequate data.

Erratum: Conduction‐Induced Alignment of Nematic Liquid Crystals: Basic Model and Stability Considerations

Abstract: It is shown by way of model orientation patterns how conduction in nematic liquid crystals may cause a torque per unit volume that more than offsets the dielectric torque felt in the insulating state. This conduction‐induced torque arises from an anisotropy of the conductivity or of the dielectric constant. It is partly dielectric but its more important part is shear‐induced, the shear flow being a consequence of space charge which is generated by and interacts with the applied electric field. The stability of a uniform orientation pattern in electric fields parallel or perpendicular to the preferred axis is investigated, with respect to a complete set of sinusoidal perturbations and including distortional and (stabilizing) magnetic torques. The analysis shows that the shear‐induced torque cannot be treated like its dielectric counterpart. It depends in a peculiar way on the direction of the perturbational wave vector and may contain a nonconservative contribution. Formulas for the instability threshold, ...

Atomic susceptibilities and shielding factors.

Abstract: A calculation of electric and magnetic susceptibilities and shielding factors for closed-shell atomic systems based on relativistic Hartree-Fock-Slater (RHFS) electron theory is presented. Numerical results are given for the electric dipole and quadrupole, and for the magnetic dipole and octupole cases for closed-shell atoms and ions from $Z=2$ to $Z=92$. Comparison is made with previous nonrelativistic uncoupled Hartree-Fock calculations and with experiment.

Electrochromism and Solvatochromism

Abstract: The position and the intensity of electronic bands are influenced by an electric field Pronounced changes in the position of absorption bands are mainly due to the dipole moment of the molecule in the ground state and the change in the dipole moment during the excitation process, and pronounced changes in intensity are due to the field dependence of the transition moment, which can be described by the transition polarizability The effect of an external electric field on the optical absorption (electrochromism) of suitable molecules can be used to determine the dipole moment in the ground state, the change in dipole moment during the excitation process, the direction of the transition moment of the electronic band, and certain components of the transition polarizability tensor These data largely determine the strong solvatochromism (solvent-dependence of the position and intensity of electronic bands), which is observed in particular with molecules having large dipole moments Smaller contributions to solvatochromism result from dispersion interactions, which predominate in the case of nonpolar molecules The models developed have been experimentally checked and verified by a combination of electro-optical absorption measurements (influence of an external electric field on absorption) and investigation of the solvent-dependence of the electronic bands

Acceleration of particles reflected at a shock front

Abstract: Reflected particle acceleration at magnetospheric bow shock front attributed to interplanetary electric field

Magnetospheric electric field measurements with balloons

Abstract: Two balloons have been flown from Fort Churchill, Canada, in August 1968 to measure horizontal and vertical electric fields. It is shown theoretically and experimentally that horizontal upper atmospheric electric fields are generally equal to one-second, several hundred kilometer averages of the horizontal ionospheric electric field. Analyses of thirty-five hours of data obtained during quiet and active periods yield the following conclusions. The average equatorial electric field strength was 0.6 mv/m during a quiet period and 1.0 mv/m during an active period. During magnetic bays, ground-based magnetometers principally measured Hall currents whose variations depended on conductivity changes as much or more than on electric field variations. For a period of about 30 minutes before the onset of a negative bay near local midnight, the ionospheric electric field pointed westward, indicating radially inward flow in the equatorial plane. This result is interpreted in terms of a model of the magnetospheric substorm in which bays are triggered deep within the magnetosphere by an instability associated with a radial plasma density gradient established by convective flow. The average electric field in the dayside ionosphere was about an order of magnitude smaller than that at night, suggesting that the dayside magnetosphere tended to corotate with the earth. Equipotential contours of the electric field in the equatorial plane are constructed based on this and other hypotheses.

Theory of Electronic States and Transport in Graded Mixed Semiconductors

Abstract: Semiconductors which are slowly graded in composition can be shown to have position-dependent band gaps and position-dependent effective masses, describable in terms of an effective Hamiltonian in an effective-mass equation. The effective Hamiltonian previously obtained is, in the present work, rendered Hermitian. Electronic minority-carrier transport for graded systems is described in terms of an effective field which includes the electrostatic field plus a term in the gradient of the band edge and another in the gradient of the effective mass. The local radiative-recombination lifetime and local density of states for inhomogeneous semiconductors are discussed. The equation for the excess minority-carrier concentration in an inhomogeneous semiconductor is deduced and is found to differ from that in an homogeneous system, by the effective field replacing the electric field, by the position dependences of lifetime and mobility, and by terms in the mobility gradient. Some phenomena specific to graded mixed semiconductors are considered on the basis of the theoretical analysis.

A source for field‐aligned currents at auroral latitudes

Abstract: Energetic plasma drifting toward the earth from the plasma sheet in the earth's magnetospheric tail is terminated by an inner boundary (the Alfven layer). Sheets of field-aligned current (Birkeland currents) flow down to the ionosphere from this plasma boundary. The mechanism for generating the current is essentially that originally proposed by Alfven in 1939. A certain amount of charge separation develops in the Alfven layer because the geocentric drift paths of ions and electrons are not coincident within the layer. The charge separation produces an electric field that is perpendicular to B and is transmitted along lines of force to the ionosphere. Currents flow across magnetic field lines in the ionosphere and along the magnetic field lines to the Alfven layer. The electric field is reduced by these currents, which flow to neutralize the charge separation. The position of the Alfven layer is such as to produce a current sheet that enters the ionosphere at auroral latitudes. The Birkeland current necessary to neutralize the Alfven layer is calculated to be about 106 amps, which is sufficient to produce the transverse magnetic disturbances of about 100 γ that have been observed by satellites in auroral latitudes; the thickness of the Alfven layer corresponds to the width of the zone of these transverse magnetic disturbances. Finally, the Birkeland current is related to a larger current system that includes the DS current system.

Electrohydrodynamic Charge Relaxation and Interfacial Perpendicular‐Field Instability

Abstract: The small‐amplitude motions of a plane interface between two fluids stressed by an initially perpendicular electric field are investigated. The fluids are modeled as Ohmic conductors and the convection of the surface charge caused by the dynamic interplay of interfacial electric shear stresses and the viscous stresses is highlighted. The influence of viscosity on instability growth rates in the zero‐shear stress limits of perfectly conducting and perfectly insulating interfaces is described and compared to cases involving electrical shear stresses. Detailed attention is given to the instability of an interface between fluids having electrical relaxation times long compared to times of interest. It is shown that, for many common liquids, even a slight amount of surface charge makes the interface unstable at a considerably lower voltage than would be expected from theories based on the dielectrophoretic limit of no interfacial free charge. Experiments, performed using high‐frequency ac stresses, gradually increased dc fields, and abruptly applied dc fields, support the theoretical model. In the general case, the electric Hartmann number is identified as an index to the dominance of the electric shear stresses over the viscous shear stresses in determining the interfacial convection of free charge.

Influence of Electric Fields on the Molecular Alignment in the Liquid Crystal p-(Anisalamino)-phenyl Acetate

Abstract: The effects of electric fields on the molecular alignment in the liquid crystal p-(anisalamino)-phenyl acetate are discussed The behavior of this material is similar to that of p-azoxyanisole in that the ordering which is normally observed shows an alignment with the molecular axis preferring a direction parallel to the field at low audio frequencies, and perpendicular to the field for frequencies of a few hundred kHz Results comparing the effectiveness of electric and magnetic fields for producing molecular alignment are presented which support a theory presented earlier to explain the ordering in p-azoxyanisole at audio frequencies This theory involves the anisotropy associated with the electrical conductivity Results are also presented which show that the effectiveness of dc electric fields is comparable to that for low audio frequencies A few comments are made concerning recent work employing electric fields to produce ordering in liquid crystals for NMR studies

Electric fields in the vicinity of auroral forms from motions of barium vapor releases.

Abstract: Electric field variations in vicinity of auroral forms from motions of Ba vapor clouds released from Nike-Tomahawk rockets

Resonance cones in the field pattern of a short antenna in an anisotropic plasma

Abstract: We report experimental observation of resonance cones in the angular distribution of the radio-frequency electric field of a short antenna in a plasma in a static magnetic field. The cone angle is observed to vary with incident frequency, cyclotron frequency, and plasma frequency in agreement with simple plasma dielectric theory. We discuss the relationship of these cones to the limiting phase- and group-velocity cones which appear in the theory of plane wave propagation.

Theory of Electron Diffusion Parallel to Electric Fields. I. Theory

Abstract: Recent experimental results in many gases demonstrate that the apparent rate of electron diffusion parallel to an electric field can differ significantly from that in the perpendicular direction. We have explained this anomalous behavior by taking account of the effect of electron density gradients on the solution of the Boltzmann equation representing a pulse of electrons under the influence of a uniform electric field. When the electron collision frequency increases with energy, the theory predicts that the leading edge of the pulse has a reduced mobility because of a higher-average electron speed and collision rate. Similarly the mobility of the trailing edge is enhanced, and consequently the half width of the pulse in the field direction is characterized by a new diffusion coefficient. The ratio of the longitudinal diffusion coefficient to the transverse diffusion coefficient is given as integrals involving the momentum transfer cross section and the unperturbed energy distribution. This ratio at high electric fields is found to be approximately one-half and one-fifth, for momentum transfer cross sections that are independent of and linearly proportional to the electron energy.

Dynamo currents, winds, and electric fields

Abstract: After a brief presentation of Sq and L electric current systems deduced from geomagnetic data, electric conductivities and wind models in the dynamo region are discussed. It is then shown that the solar negative-mode thermal diurnal tide and the lunar semidiurnal gravitational tide having a phase shift with altitudes produce the best fit Sq and L current systems, respectively. Distributions of electrostatic fields are computed, and electromagnetic drift speeds in the ionospheric F region are also examined. These calculated values agree well with observed results.

Optical second-harmonic generation by electrically polarized isotropic media

Abstract: In relation to recent experiments, a quantitative discussion of the microscopic mechanisms accounting for second-harmonic generation (SHG) and harmonic mixing in isotropic media immersed in a dc electric field is given, comprising the temperature-independent effect of nonlinear electronic polarizability and the temperature-dependent effect of reorientation of permanent electric dipoles. It is shown that in the case of a very strong dc electric field, when all the microsystems undergo complete alignment in the direction of the field vector, these two mechanisms attain saturation, raising considerably the intensity of SHG in the case of prolate microsystems and lowering it in the case of oblate ones. Electric saturation effects can take place in strongly dipolar molecular substances but are especially intense in solutions of macromolecules and colloid particles. The discussion covers, moreover, the symmetry relations between the nonzero elements of the nonlinear susceptibility tensor of the medium in weak as well as in strong electric fields, and the application of these relations in determining the values of elements of the tensors of second- and third-order polarizabilities of the individual microsystems. A preliminary interpretation of the available experimental data is proposed, and new procedures of SHG measurements are suggested.

One-Dimensional Hydrogen Atom

Abstract: The time-independent Schrodinger equation is solved for the bound state solutions of the one-dimensional Coulomb potential, −e2/|x|. The wave functions obtained are normalizable and continuous. The energy spectrum consists of a set of discrete levels with energies equal to the Bohr energies of hydrogen, and a set of continuum levels with energies lying strictly between the discrete levels. The odd wave functions associated with the discrete levels are differentiable everywhere, but the even wave functions associated with the continuum levels have a cusp with infinite slope at the origin. The energy levels are not degenerate. Following the method of Loudon, the bound state solutions of a truncated Coulomb potential −e2/(|x|+a), a>0, are also obtained. For small a, the discrete spectrum of this potential contains: (1) energy levels which approach arbitrarily close to the discrete levels of the true Coulomb potential as a→0, with odd wave functions identical to the odd wave functions of the true Coulomb pote...

Theory of electron diffusion parallel to electric fields. ii. application to real gases.

Abstract: The theoretical analysis of the preceding paper is extended to include inelastic collisions. The resulting theory is then used to obtain values for the diffusion coefficient ${D}_{L}$, which is appropriate for electrons diffusing parallel to an electric field. Theoretical values of $\frac{{D}_{L}}{\ensuremath{\mu}}$ are compared with values of $\frac{{D}_{T}}{\ensuremath{\mu}}$ as a function of $\frac{E}{N}$ for the gases helium, argon, hydrogen, deuterium, nitrogen, oxygen, carbon dioxide, water vapor, carbon monoxide, krypton, and xenon; $\ensuremath{\mu}$ is the electron mobility, ${D}_{T}$ is the diffusion coefficient for electron diffusion perpendicular to the electric field, $E$ is the electric field strength, and $N$ the number density of the gas. A comparison is also made between theoretical values of $\frac{{D}_{L}}{\ensuremath{\mu}}$ and the available experimental values of this quantity. Experimental differences that have been observed between ${D}_{L}$ and ${D}_{T}$ of the order of a factor of seven in argon and a factor of two in helium, hydrogen, and nitrogen are satisfactorily explained.

Excitation and Detection of Surface Elastic Waves in Piezoelectric Crystals

Abstract: The amplitude of the surface elastic wave produced by the application of an alternating voltage to an interdigital arrangement of electrodes on the surface of a piezoelectric medium is determined. The electric field produced by the surface electrodes is calculated subject to the assumption that the piezoelectric coupling of the material can be neglected. This electric field acts as the forcing term for the inhomogeneous elastic equation, which is then solved to obtain the amplitude of the surface wave generated by the transducer. A reciprocal relationship between the excitation and detection problems is used to obtain the power extracted from the surface wave by an interdigital arrangement of surface electrodes. Measurements made on single crystals of quartz and cadmium sulfide are found to be in good agreement with theoretical predictions. The maximum value of the product (efficiency×fractional bandwidth) for a surface‐wave transducer is calculated. For an interdigital surface‐wave transducer on the basal plane of CdS, the maximum value of this product is found to be 7.8×10−2. It is shown that one can connect linear pairs of surface electrodes in an appropriate binary code so as to obtain high efficiency and large bandwidth transducers.

Electrical control of holographic storage in strontium‐barium niobate

Abstract: Single‐crystal Sr0.75Ba0.25Nb2O6 when used in conjunction with an externally applied electric field is shown to be a sensitive volume holographic medium capable of high‐resolution information storage. Optically induced refractive index changes of 5 × 10−4 have been observed with laser exposure of 14 J/cm2. A simple relation between the applied electric field and optical exposure is determined which allows electrical control of optically induced refractive index change. In addition to control of the optically induced effect it is found that the applied electric field can also be used to control the diffraction efficiency of the holographic reconstruction process.

Analysis of yagi-uda-type antennas

Abstract: A method of analyzing Yagi-Uda-type antennas is presented. Since the Yagi-Uda array is a fairly well-known antenna, it is used as an example to demonstrate the application and accuracy of the method. However, the method of solution is not limited to a planar array, such as the Yagi, but can be applied to arrays of nonplanar linear elements. The approach taken in analyzing Yagi-Uda antennas is based on rigorous equations for the electric field radiated by the elements in the array. All interactions are taken into account. Calculated results are presented for the Yagi-Uda array that show excellent agreement with experimental results reported in the literature. In addition, the dependence of the far-field patterns on the phase velocity is shown. It is also demonstrated that the phase velocity is generally nonuniform along the array.

Scattering by a Cylindrical Post of Complex Permittivity in a Waveguide

Abstract: An exact solution of the discontinuity problem of a circular cylindrical post of arbitrary complex permittivity centered in a rectangular waveguide with the axis parallel to the electric field vector of the dominant mode has been set up and numerical results based directly on this solution have been found rising an electronic computer. The method used divides the waveguide up into three different regions by introducing two imaginary plane walls perpendicular to the waveguide walls. In the center region, which contains the cylindrical rod, the electromagnetic field is expanded in cylindrical waves and in the outer regions the field is expanded in waveguide modes. By setting up the boundary conditions at all discontinuity surfaces and performing numerical matching of the fields at the two imaginary walls, a system of linear equatious determining the coefficients of reflection, transmission, and absorption of the field due to the cylindrical rod is found. The structure which is of most interest in the case of a plasma column is a coaxial structure consisting of an inner dielectric cylinder with complex permittivity (the plasma) surrounded by a dielectric sleeve with real, positive permittivity (the glass tube). The theory is therefore developed to apply generally for such structures. From the numerical results, curves have been obtained showing the relationship between the coefficients of reflection and transmission and the (complex) permittivity of the rod material. Such curves maybe used for deducing the microwave properties of a cylindrical rod from measurements of the reflection and transmission coefficient of the rod.

Gamma Response of Semi‐insulating Material in the Presence of Trapping and Detrapping

Abstract: The photoelectric and Compton response to γ rays for a detector made from semi‐insulating material (i.e., a monocrystalline semiconductor containing trapping centers) was calculated for the case of charge transport by one type of carrier (electron or hole), and also for the case of two‐carrier transport (electron and hole) with each carrier having the same mean free path. Approximate expressions were then derived for the observable γ‐ray efficiency of the detector as a function of material parameters for CdTe crystals used in the fabrication of surface barrier detectors; this efficiency is dependent upon temperature and electric field intensity for specified values of the carrier mobility‐trapping time product and the threshold used in the recording of spectra. Spectra and values for efficiency were determined at room temperature for γ rays at energy values from 0.393 to 1.33 MeV, and also as a function of temperature over the range 24° to 68°C for γ rays from a 60Co source. The observations show behavior...

Dielectric Breakdown of Polymer Films

Abstract: Strong electric fields have been applied to dielectric polymer films to study field‐induced mechanical stresses. For voltages near breakdown the mechanical forces are sufficient to cause localized deformation in some polymer films. Considerable field enhancement can occur at the deformation sites, leading to reduced nominal dielectric strength for the samples. Experimental evidence for electromechanical deformation is presented as a possible explanation for the poor agreement between the observed dielectric strength and that predicted theoretically.