Showing papers on "Electric field published in 1968"

Electric Fields Produced by the Propagation of Plane Coherent Electromagnetic Radiation in a Stratified Medium

Abstract: Explicit formulas are derived for the mean-square electric fields induced by plane electromagnetic radiation in a two-phase, three-phase, and N-phase stratified medium. The first (incident) and last phases are semi-infinite in extent. Boundaries separating phases are plane and parallel. Phases are isotropic with arbitrary optical constants. Simple relationships follow for special cases such as at the critical angle for a two-phase system. Equations for reflectance, transmittance, and phase changes on reflectance and transmittance are given. Details are given concerning the energy absorption process, especially in the two- and three-layer cases. Equations for the N-layer case are in terms of characteristic matrices which can be readily programmed for a computer.

Mössbauer Spectra in a Fluctuating Environment

Abstract: The M\"ossbauer line shape in the presence of time-dependent electric field gradients and magnetic fields is considered. Two specific soluble stochastic models are treated: (1) a static electric field gradient with a randomly fluctuating magnetic field which takes on values $+h$ and $\ensuremath{-}h$, each directed along the axis of the field gradient, and (2) as in (1), but with the fluctuating magnetic field perpendicular to the axis of the field gradient. Example (2) is more complex than (1), since the fluctuating field is in this case capable of inducing transitions between the nuclear levels, while in (1) this is not possible. Specific calculations for the two cases illustrate the differences between them.

The Poole-Frenkel effect with compensation present.

Abstract: Poole-Frenkel model for internal field assisted thermal emission with compensation for relative densities of donor and acceptor sites

The stochastic computer simulation of ion motion in a gas subjected to a constant electric field

Abstract: The stochastic computer simulation of ion motion in a gas subjected to an electric field involves the problem of statistically choosing free times between the collisions, when the velocity cannot be assumed constant during a free time. It is shown how free times can be chosen, without any approximations or numerical integrations, by the use of a piecewise velocity-independent mean collision frequency chosen to be greater than the actual collision frequency, combined with the assumption that an appropriate part of the collisions results in zero momentum transfer.

Transport Properties of GaAs

Abstract: The transport properties of the electrons in GaAs have been investigated; i.e., the absolute values of the electron drift velocity, the diffusion coefficient, and the trapping time have been measured for the first time as a function of the electric field. To measure the velocity at the electron, the response of a reversed-bias Schottky barrier-$I\ensuremath{-}{n}^{+}$ GaAs device to a short pulse (0.1 nsec) of high-energy electrons was measured. The incident electrons create a sheet of charges in the semiconductor very close to the cathode. The electrons move across the diode under the influence of the applied electric field and induce a current in the contacts until they reach the anode. The width of the induced current pulse is a measure of the transit time of the electrons. With a knowledge of the width of the field region ($I$ layer), the drift velocity corresponding to the particular bias field can be accurately determined. The specimen used in this experiment consists of a slab of semi-insulating boat-grown GaAs cut in the [100] and [111] directions. Thin contacts were evaporated on each face; one, the cathode contact, less than 1000 \AA{} thick, forms the noninjecting Schottky barrier. The other, the anode, is ohmic. The experimental results are in excellent agreement with the Butcher-Fawcett theory, with a low-field mobility of 7500 ${\mathrm{cm}}^{2}$/V sec, a threshold field of 3300 V/cm, and an initial negative mobility of 2600 ${\mathrm{cm}}^{2}$/V sec. Signs, but no strong evidence, of the minimum velocity being reached up to the highest field used, 14 kV/cm, were observed. We have also measured by the same method the velocity-field relation over a range of ambient temperatures from 160 to 340\ifmmode^\circ\else\textdegree\fi{}K. From the measurement of the difference between the rise and the fall time of the induced current pulse, which is a measure of the spread of the electron layer created at the cathode by electron bombardment, it was possible to obtain the diffusion coefficient as a function of the electric field. The diffusion coefficient is sharply peaked (900 ${\mathrm{cm}}^{2}$/sec) at the threshold field and decrease to a value slightly less than 200 ${\mathrm{cm}}^{2}$/sec at high field. The experimental results are in considerable disagreement with the theoretical prediction. This disagreement may possibly be due to the velocity fluctuation arising from the rapid electron intervalley transfers which have not been taken into account in the theory by Butcher and Fawcett. Finally, measurement of the number of electrons trapped during the electron transit time across the specimen yields the variation of the trapping time as a function of the electric field.

Electrocaloric Effects in Some Ferroelectric and Antiferroelectric Pb(Zr, Ti)O3 Compounds

Abstract: Variations of sample temperature arising from the electrocaloric effect, antiferroelectric to ferroelectric transitions, and hysteresis‐loss heating have been recorded together with the hysteresis loops for four different ceramic ferroelectrics near room temperature. Both the electric‐field dependence and the temperature dependence of the electrocaloric effect were accurately predicted by the usual thermodynamic expression for the effect. Values for the heat capacity and the pyroelectric constant were also obtained from the data. The sample temperature of one composition which becomes antiferroelectric under zero applied field near 40°C was monitored at various temperatures as antiferroelectric to ferroelectric transitions were forced by an electric field. Above 55°C, the temperature change at the transition could be fitted with a theory originally developed for the field‐enforced antiferroelectric to ferroelectric transition in PbZrO3.

Display device containing minute droplets of cholesteric liquid crystals in a substantially continuous polymeric matrix

Abstract: This disclosure is directed to articles of manufacture, chiefly display devices, containing minute ''''naked'''' droplets or inclusions of cholesteric liquid crystal material in a substantially continuous polymeric matrix, said liquid crystal material changing color or shade of color not only upon application of an electric potential but also upon removal of the field. The image produced has a comparable outline to that of the path of the electric field. Three chromatic states are evident, the normal color (before the electric potential is applied), the color given off when the electric field is applied, and the color observed when the electric field is removed. All three chromatic states are readily discernible from one another. The polymer matrix protects the cholesteric liquid crystal droplets from aging and enhances electric field behavior because the third chromatic state (electric potential removed) has a greater longevity with the matrix-bound material versus unprotected material of identical composition but no polymeric matrix. Other advantages are also discussed.

Interfacial Relaxation Overstability in a Tangential Electric Field

Abstract: It is well known that electromechanical polarization surface waves propagate along the lines of electric field intensity imposed tangential to the interface between perfectly insulating fluids. These waves have a stiffening effect on electrohydrodynamic equilibria that is analogous to that of the Alfven surface waves on hydromagnetic equilibria. An investigation is presented of the dynamical effects of charge relaxation on these waves. A self‐consistent model represents the relaxation in terms of an Ohmic conduction process in the bulk of the fluids, with surface shears induced by the free interfacial charges placed in dynamic equilibrium by viscous stresses. The dominant effect of the charge relaxation is to produce overstability. Experiments are described where this instability appears as a spontaneous oscillation of the interface with wavenumbers directed along the lines of electric field intensity. Detailed analytical results are given for liquid‐gas and liquid‐liquid interfaces. The field required to produce incipient instability and the propagation direction of the observed instability are satisfactorily predicted. It is found that in this liquid‐vapor case, relatively simple explicit expressions can be given for the incipient instability conditions. A discussion is given of the significance of this work for the dielectrophoretic orientation of liquids in the zero‐gravity environments of space.

Microwave Surface Resistance of Superconducting Niobium

Abstract: The unloaded Q's of pure Nb cavities in the TE011 mode at 11.2 GHz and in the TM010 mode at 8.4 GHz were measured between 1.2° and 4.2°K to determine whether the residual surface losses in Nb could be made sufficiently low to make it a useful material for practical microwave devices. Three forms of reactor grade Nb were evaluated: recrystallized fully wrought, arc melted, and electron‐beam melted. Each of these materials was subjected to additional metallurgical processing including high‐temperature vacuum firing and polishing to determine their effects on surface losses. In the TE011 mode, a residual Q of 3.8×1010 was achieved. Also in the TE011 mode, the unloaded Q was observed to be essentially a constant up to the ac critical magnetic field Hcae, where it abruptly decreased by a factor of 100 or more. An Hcae as large as 436 Oe was observed for unstrained Nb. In the TM010 mode, which has electric fields terminating on the surface of the Nb, a residual Q of 2×109 was achieved, the limitation on the max...

Electric-Field-Induced Phase Change in Cholesteric Liquid Crystals

Abstract: Optically-negative cholesteric liquid-crystal films can be transformed to an optically-positive state by applying high d-c electric fields A phase transformation occurs in which the helicoidal cholesteric structure is converted to either a planar smectic, or a linear nematic, structure The threshhold field for this transformation has been studied as a function of sample thickness, temperature and composition Bulk fields are responsible for the phenomenon It is inferred that the compositional dependences are due to size factors and internal molecular dipole moments

Electro-optic and Piezoelectric Coefficients and Refractive Index of Gallium Phosphide

Abstract: Measurements of the constant‐strain electro‐optic coefficient of GaP between 0.56 and 3.39 μ wavelength were made using both pulsed electric field and optical heterodyne techniques. At 0.6328 μ wavelength its value is −0.97×10−12 m/V. A dispersion of only 10% is present between 0.56 and 3.39 μ. Approximate values of the piezoelectric coefficient and the constant‐stress electro‐optic coefficient are also obtained from the analysis of the experiments. For GaP the magnitude of the constant‐stress coefficient is less than the constant‐strain coefficient. The analysis also points out an often ignored effect: electro‐optic measurements made with electric field frequencies in the range of resonant acoustic frequencies may not exhibit the expected symmetry of the crystal plus field but may also evidence effects from the crystal shape. Accurate refractive index measurements made on many GaP crystals between 0.545 and 0.70 μ wavelength are also reported.

Kinetic properties and the electric field effect of the helix-coil transition of poly(gamma-benzyl L-glutamate) determined from dielectric relaxation measurements.

Abstract: Dielectric relaxation of poly(γ-benzyl L-glutamate) in solution has been studied in the 5 kcps-10 Mcps range for various values of the helix content. The results give first experimental evidence for three effects of major significance. (1) The system exhibits dielectric relaxation due to a chemical rate process (namely helix formation). This confirms recent theoretical predictions. (2) The mean relaxation time τ* of the helix–coil transition could be evaluated as a function of the degree of transition. The results are in excellent agreement with a previously developed theory. At the midpoint of transition it is found τ*max = 5 × 10−7 sec. The elementary process of helical growth turns out to be practically diffusion-controlled (with a rate constant of hydrogen bond formation of 1.3 × 1010 sec−1). (3) There is a considerable electric field effect of the helix–coil transition. This indicates that conformation changes in biological systems could be potentially caused by direct action of an electric field.

The observations of a near monoenergetic flux of auroral electrons

Abstract: Rocket-borne observations were made of a nearly monoenergetic beam of electrons in association with a prebreakup auroral display. The characteristic energy of this beam was about 4 kev and was very stable over a 150-second period of time. The nature of this energy spectra strongly suggests that the electron energization was by the electric fields associated with some static 4-kev potential difference in the magnetosphere. A model in which this potential was established directly along a magnetic field line cannot be uniquely excluded by these observations but is discounted on other grounds. The alternative is a model in which the required electric field is transverse to the magnetic field lines. Such electric fields are known to exist and have been measured directly. Energization of low-energy electrons by an electric field of this geometry, would appear to require that these electrons be stably trapped on closed lines of magnetic force, the energization occurring by virtue of gradient and line curvature drift across equipotential lines. It is this process, which is similar to that proposed by Taylor and Hones (1965), that is believed to have produced the auroral particles observed in this aurora. The electrical potential differences known to be available in the magnetosphere although sufficient to produce these 4-kev electrons are inadequate to produce the 100-kev electrons seen in association with the aurora, thus implying the existence of more than one acceleration mechanism.

Correlation of Reduction in Optically Induced Refractive‐Index Inhomogeneity with OH Content in LiTaO3 and LiNbO3

Abstract: It has recently been demonstrated that crystals of LiTaO3 and LiNbO3 can be made more resistant to optically induced refractive‐index inhomogeneities caused by laser irradiation by annealing the crystals in the present of an electric field. The explanation given for the improvement was that some impurity entered the crystal during the annealing cycle, modifying the conductivity of the material. Additional data presented here suggest that the impurity is hydrogen. The exact mechanism by which the susceptibility to index inhomogeneity is reduced is not understood at this time.

Preliminary results of electric field measurements in the auroral zone

Abstract: In April 1967 five Nike-Apache rockets were launched from Kiruna, Sweden, carrying barium release and electron concentration experiments. The component of the electric field transverse to the magnetic lines of force is derived from the motion of the ion clouds. The electron concentration measurements provide ionospheric conductivities. The magnitudes of the electric fields varied between 2 and 20 v/km. Their directions were northwestward for evening conditions and southwestward for morning conditions. The magnetic perturbations on the ground that would result from ionospheric currents derived on the basis of the measured fields and conductivities closely resemble the actual ground magnetometer data. The electric fields vary appreciably with space and time; they can even reverse their directions over distances of the order of 60 km.

Detection of electric-field turbulence in the earth's bow shock.

Abstract: Detection of electric field turbulence in earth bow shock, noting wave amplitude correlation with magnetic field structure

Path-integral calculation of the quantum-statistical density matrix for attractive coulomb forces.

Abstract: A method for the calculation of the two‐particle statistical density matrix for attractive Coulomb forces is described. The path‐integral expression for the density matrix is reduced to a modified path integral which involves summation over only one‐dimensional paths. This expression is then approximated by an iteration procedure using direct numerical quadratures. The results obtained are related directly to the quantum‐mechanical radial‐distribution function for a plasma at small ion‐electron separations.

Electroconvective Instability with a Stabilizing Temperature Gradient. I. Theory

Abstract: A uniform vertical electric field produces an instability in a poorly conducting liquid subject to a vertical temperature gradient A gradient in conductivity resulting from the temperature gradient causes free charge to accumulate in the fluid when an electric field is applied For the cases considered, the gradient in dielectric constant can be neglected with the significant electric force being that due to the free charge The threshold conditions for the instability are predicted using linear perturbation theory Approximations are made which allow the equations with space‐varying coefficients to be solved The analysis shows that, for liquids with short or moderate electrical relaxation times, the electric field causes the internal gravity wave propagating downward to become unstable

Digital computer calculation of the electric potential and field of a rod gap

Abstract: The electric field and potential distribution in the gap between a cylindrical rod having a hemispherical tip and an infinite plane perpendicular to the cylinder axis was determined using a charge simulation technique. This method assumes a charge at the center of the hemisphere and a finite number of semi-infinite axial charges in the cylindrical portion of the rod electrode. Boundary conditions in the cylindrical and spherical portions enabled the formulation of simultaneous equations whose digital solution yielded the assumed lumped charges. The digital computer program provided values of the potential and both field components anywhere in the gap with an accuracy of about 2 percent.

Method and apparatus for interleaved charged particle acceleration

Abstract: A method and apparatus for accelerating charged particles is described in which charged particles of opposite sign but of similar charge to mass ratio are accelerated through the same structure by means of an alternating electric field. The two sets of particles can be accommodated in the same apparatus by grouping particles of the same charge in bunches which are spaced by a phase difference of approximately pi radians with respect to the accelerating field frequency from the bunches of the opposite charge sign.

Effect of an electric field on boiling heat transfer.

Abstract: The results of previous experiments to determine the effect of an electric field on boiling heat transfer are reviewed. It is pointed out that the major effects occur in the regions of peak heat flux, transition boiling, and minimum transition boiling. These boiling regions are known to be heavily influenced by hydrodynamic stability. Hydrodynamic stability is influenced by the presence of an electric field. The hydrodynamic theory of boiling heat transfer is therefore reconstructed, including the effects of a perpendicular electric field across the interface separating vapor and liquid phases. The result is the ratio of the boiling heattransfer rate in the presence of an electric field to that with zero field, as a function of the electric-field strength and the physical parameters of the boiling medium. The calculations are restricted to nonconducting fluids, that is, good dielectrics. The analytical results are compared with recent, but somewhat restricted experimental results and, within the inherent complexities of the problem, the agreement is good. Apparently the theory predicts the correct functional form of the data.

Interband dielectric properties of solids in an electric field.

Abstract: : A general expression for the imaginary part of the one-electron interband dielectric function of a solid in the presence of an electric field is derived. The result is valid for all regions of k-space and explicitly takes into account the variation of the dipole matrix element and effective mass throughout the Brillouin zone. Under certain approximations the finite electric field dielectric function reduces to the convolution of the zero field dielectric function with an Airy function. This result can be used in conjunction with band structure calculations which have already been done in order to predict the line shape of the electroreflectance spectra as a function of electric field. The convolution expression reduces further at non-degenerate critical points, and the forms for the four types of critical points are presented. In certain instances it is possible to unfold the convolution integral and obtain the zero field dielectric function from either the finite field dielectric constant or the electroreflectance data at isolated critical points. (Author)

Gravitationally Induced Electric Fields in Conductors

Abstract: Gravitationally induced electric field in conductor including effect of lattice compressibility

Electromodulation of the Optical Properties of Gold

Abstract: When a strong electric field is applied to the surface of a thin gold-film electrode in contact with aqueous electrolyte, we observe a change in the attenuated total reflection (ATR) spectrum of as much as 7%. The change was measured as a function of photon energy, angle to incidence, polarization, and electric field modulation frequency. It is shown that this effect is due to changes in the optical properties of the gold and not to changes in the electrolyte. The peak in this derivative ATR spectrum is at 2.5 eV, where the optical properties of gold are changing most rapidly with frequency. The effect is quantitatively explained by a shift in the optical constants of gold due to the change in electron concentration.

An experimental determination of the dipole moments of the X(1Σ) and A(1Π) states of the BH molecule

Abstract: The optical emission spectrum of the BH radical produced in a discharge tube designed to give high electric fields shows Stark effects. From these, the dipole moments of the X(1Σ+) and A(1Π) states...