Showing papers on "Electric field published in 1991"

Plasma sheet instability related to the westward traveling surge

Abstract: The detailed analysis of an isolated dispersionless substorm is performed on the basis of field and particle data collected in situ by the geostationary satellite GEOS 2 and of data from ground-based instruments installed close to the GEOS 2 magnetic footprint. These data give evidence for (1) quasi-periodic variations of the magnetic field configuration, which is alternatively taillike and dipolelike, (2) in-phase oscillations of the flux of energetic electrons, which is high when the configuration is dipolelike and vice versa, (3) a gradient in the flux of energetic ions, which is, on the average, earthward but undergoes large fluctuations around this average direction, and (4) large transient fluctuations of the quasi-dc electric field, which reverses its direction from eastward to westward. It is shown that these results are consistent with the development of an instability which leads to a westward propagating “wave”. The source of the instability is the differential drift of energetic electrons and ions in a highly stressed magnetic field configuration (in a high β plasma). Evidence is given for a system of localized field-aligned currents flowing alternately earthward and equatorward at the leading and trailing edges of the westward propagating wave. This current system resulting from the temporal development of the instability produces the so-called Pi 2 pulsations, at the ionospheric level. The closure of this current system in the equatorial region leads to a current antiparallel to the tail current, and therefore to its reduction or cancellation. This reduction/cancellation of the tail current restores the dipole magnetic field (dipolarization) and generates a large westward directed induced electric field (injection). Hence, dipolarization and injection are the consequences of the instability. Finally, it is suggested that the westward traveling surge observed simultaneously by all-sky cameras, close to the magnetic field of GEOS 2, is the image of the instability in the equatorial region transmitted to the upper atmosphere by precipitating electrons.

Cold field emission from CVD diamond films observed in emission electron microscopy

Abstract: Thick (100 μm) chemical vapour deposited (CVD) polycrystalline diamond films have been observed to emit electrons with an intensity sufficient to form an image in the accelerating field of an emission microscope without external excitation (> 3 MV/m). The individual crystallites are of the order of 1–10 μm. The combined crystallite diameter and the electric field strength in these ‘self-emitting’ films are far below those typical for cold field emitters.

Constraints on biological effects of weak extremely-low-frequency electromagnetic fields.

Abstract: Concerns have been raised over the possibility that extremely-low-frequency (ELF) electromagnetic fields are carcinogenic and leukegenic. An examination of the physical interaction of such fields with the body shows that such interactions are too weak to have a significant effect on human biology at the cell level. Because of the high electrical conductivity of tissues, the coupling of external electric fields in air to tissue in the body is such that the effects of the internal fields on cells is smaller than thermal noise. Static magnetic fields smaller than the earth's field of 50 \ensuremath{\mu}T and varying fields weaker than the 4-\ensuremath{\mu}T 60-Hz fields that are equivalent in effect to that from walking in the earth's field, cannot be expected to generate significant biological effects. Moreover, the interactions of such weak fields at the cell level are also small compared to thermal noise. These conclusions would be modified by 60-Hz cell resonances. But such resonances are shown to be incompatible with cell characteristics and the requirement from equipartition that the mean resonance energy must be kT. Hence, any biological effects of weak ELF fields on the cellular level must be found outside of the scope of conventional physics.

Dynamics of band electrons in electric and magnetic fields: rigorous justification of the effective Hamiltonians

Abstract: Results concerning the rigorous justification of the effective Hamiltonians for band electrons in the presence of weak homogeneous electric and magnetic fields are reviewed In the electric-field case the existence, in the sense of spectral concentration, of the Stark-Wannier resonances is proved In the magnetic-field case, the existence of exponentially localized magnetic Wannier functions is established As a consequence the Peierls-Onsager effective Hamiltonian is obtained

Transverse laser patterns. I. Phase singularity crystals

Abstract: We analyze the interaction and the competition of a set of transverse cavity modes, which belong to a frequency-degenerate family. The laser turns out to be able to realize several different stationary spatial patterns, which differ in the transverse configuration of the intensity or of the field and are met by varying the values of the control parameters. A striking feature that emerges in almost all steady-state patterns is the presence of dark points, in which both the real and the imaginary part of the electric field vanish and such that, if one covers a closed loop around one of these points, the field phase changes by a multiple of 2\ensuremath{\pi}, which corresponds to the topological charge of the point. We show in detail the analogy of these phase singularities to the vortex structures well known in such fields as, for example, hydrodynamics, superconductivity, and superfluidity. In our case, at steady state, these singularities are arranged in the form of regular crystals, nd the equiphase lines of the field exhibit a notable similarity to the field lines of the electrostatic field generated by a corresponding set of point charges. We analyze in detail the patterns that emerge in the cases 2p+l=2 and 2p+l=3, where p and l are the radial and angular modal indices, respectively, and we compare the results with the experimental observations obtained from a ${\mathrm{Na}}_{2}$ laser. The observed patterns agree in detail with those found by theory; in particular, they exhibit the predicted phase singularities in each pattern. The transitions from one pattern to another, that one observes under variation of the control parameters, basically agree with those predicted by theory.

Force constant and effective mass of 90° domain walls in ferroelectric ceramics

Abstract: Domain wall contributions to the dielectric, piezoelectric, and elastic properties of tetragonal ferroelectric ceramics, as discussed extensively in the past, are calculated. A simple model shows that the motion of 90° domain walls causes a shear deformation and an approximately homogeneous electric field in the grain. The elastic and electric field energies involved allow the calculation of the force constant for the domain wall displacement by external fields. The displacements agree with experimental results. In a moderate electric field the displacement is a small fraction of the lattice cell only. By averaging over the orientational distributions of all grains the contributions of the 90° domain walls to the material properties are calculated for unpolarized and for polarized ceramics and agree with experimental results. The effective mass, which has to be attributed to the domain walls is the mass of the whole grain reduced by the factor S0 (spontaneous deformation), is independent of the domain wid...

Electric-field-induced second-harmonic generation with reduced absorption in atomic hydrogen.

Abstract: We show that dc-electric-field coupling of the 2s and 2p states in atomic hydrogen leads to resonantly enhanced second-order susceptibility with reduced absorption at the second-harmonic wavelength, and exact phase matching at the center of the Stark-split components.

Laser-induced electric breakdown in water

Abstract: Dielectric breakdown induced in water by Nd: YAG laser pulses is considered experimentally and theoretically. The effect appears to be due to electron avalanche ionization. The aspects of this process considered here are the following: (i) The dependence of the breakdown probability on the laser field. At high fields, electron interaction with molecular (Raman) vibrations or with collective molecular motions occurs. (ii) Bragg scattering, which contributes to keeping the electron motion in phase with the optical field. (iii) The role of the electron mobility, which contributes to stabilizing the process. (iv) The generation of the electrons that start the avalanche in relation to different laser-pulse durations and irradiances.

Charge yield for cobalt-60 and 10-keV X-ray irradiations of MOS devices

Abstract: The radiation response of MOS devices exposed to /sup 60/Co and low-energy ( approximately 10 keV) X-ray irradiation is evaluated as a function of electric field during exposure. Improved charge yield estimates are obtained for /sup 60/Co irradiations at fields below 1 MV/cm by matching voltage shifts due to oxide-trap and interface-trap charge to an E/sup -0.55/ electric field dependence. Combining these improved charge yield estimates and calculated dose enhancement factors, the relative response of X-ray to /sup 60/Co irradiations is accurately predicted for oxide electric fields from 0.03 MV/cm to 5.0 MV/cm. The ability to predict the relative response to X-ray to /sup 60/Co irradiations-should speed acceptance of X-ray testers as a hardness assurance tool. >

Effects of an electric field in atomic manipulations

Abstract: Some recent scanning-tunneling-microscopy (STM) experiments on atomic-scale manipulation of material surfaces are explained in terms of high-temperature field evaporation, directional walks in an applied field gradient, and the formation of liquid-metal cone, etc. The evaporation fields for metals and the charge states of ions formed by an electric field in the STM configuration are shown to depend on the tip-to-sample distance. The possibility of field evaporating as negative ions is also considered. We find this possibility exists especially for alkali metals and adsorbed atoms of large electron affinity and noble gases. Favorable physical conditions for atomic manipulations are also discussed. In addition, a method for creating a thermally stable top surface layer with an atomic pattern of embedded foreign atoms of one's design, using electric-field manipulations and atomic replacement, is described.

Pair production in a strong electric field.

Abstract: We investigate the mechanism of pair creation in scalar QED from spatially homogeneous strong electric fields in 1+1 dimensions. Solution of the semiclassical field equations shows particle creation followed by plasma oscillations. We compare our results with a model based on a relativistic Boltzmann-Vlasov equation with a pair-creation source term related to the Schwinger mechanism. The time evolution of the electric field and the current obtained from the Boltzmann-Vlasov model is surprisingly similar to that found in the semiclassical calculation.

Trap‐enhanced electric fields in semi‐insulators: The role of electrical and optical carrier injection

Abstract: We report extremely large field enhancement near the anode of an electrically biased metal/semi‐insulator/metal structure. The large anode field results from a trap‐enhanced space‐charge region and is large enough to cause injection of holes at the anode. Our numerical simulations confirm this interpretation and show that for typical semi‐insulating GaAs, large trap‐enhanced fields (TEF) are to be expected. The TEF effect, contrary to that observed in doped materials, is enhanced by optical injection of carriers near the anode, and can be exploited for the efficient generation of ultrafast THz radiation.

Effect of an electric field on block copolymer microstructure

Abstract: SAXS measurements showed that macroscopic alignment of microstructure in a symmetric styrene-methyl methacrylate diblock copolymer can be induced by application of an electric field while cooling through the order-disorder transition.

Influence of electric fields on pinning in YBa2Cu3O7- delta films.

Abstract: Experimental evidence is provided that the pinning force and the critical current density of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} films can be controlled by electric fields. This novel effect is attributed to field-induced changes of the density of mobile charge carriers in the superconductor.

Electric field soundings through thunderstorms

Abstract: Twelve balloon soundings of the electric field in thunderstorms are reported. The maximum magnitude of E in the storms averaged 96 +/-28 kV/m, with the largest being 146 kV/m. The maximum was usually observed between vertically adjacent regions of opposite charge. Using a 1D approximation to Gauss' law, four to ten charge regions in the storms are inferred. The magnitude of the density in the charge regions varied between 0.2 and 13 nC/cu m. The vertical extent of the charge regions ranged from 130 to 2100 m. None of the present 12 storms had charge distributions that fit the long-accepted model of Simpson et al. (1937, 1941) of a lower positive charge, a main negative charge, and an upper positive charge. In addition to regions similar to the Simpson model, the present storms had screening layers at the upper and lower cloud boundaries and extra charge regions, usually in the lower part of the cloud.

Electromigration Induced Step Bunching on Si Surfaces – How Does it Depend on the Temperature and Heating Current Direction?

Abstract: Recently reported data for an influence of the heating current direction on the micromorphology of Si surfaces have been analyzed on the basis of the hypothesis for electromigration of Si adatoms. The results provide a reasonable explanation of the observed step bunching on the (111) Si surface, which occurs either at moderate or at high temperature depending on the direction (step up or step down) of the heating current. The confrontation with the experimental data results in a negative effective charge of the Si adatoms. The formation of major and minor domains on (001) Si vicinal surfaces under low temperature annealing is also interpreted in accordance with the obtained theoretical results. The force acting on the Si adatoms is estimated to be 0.8×10-16 N and can be created by an electric field of 5 V/cm if the effective charge of an adatom is equal to the elementary electric charge.

Near‐field optics: Microscopy with nanometer‐size fields

Abstract: The electromagnetic fields that can build up around metallic or dielectric pointed tips are of increasing interest in context with the new scanning probe microscopies (tunneling, near‐field optics, Coulomb and van der Waals forces etc.). The paper presents exact solutions of Laplace’s equations for the tip/sample geometry. For suitable media, plasmons are found whose electric fields are highly localized in the gap region. We believe that the field enhancement associated with such tip plasmons is instrumental for inelastic tunneling and light emission during scanning tunneling microscopy.

Electric field effect in high Tc superconducting ultrathin YBa2Cu3O7−x films

Abstract: A multilayer high Tc superconducting field‐effect transistor‐like structure was made from ultrathin YBa2Cu3O7−x films. An epitaxially grown dielectric SrTiO3 insulation layer, which had a forward bias breakdown voltage of about 20 V, allowed an electric field induced change in the channel layer of 1.25×1013 carrier/cm2 per volt of the gate voltage. A significant modulation of the normal state and superconducting properties was observed in samples with YBa2Cu3O7−x channel layers of a few unit cells thick. By applying gate voltage of different polarities, Tc was both suppressed and enhanced by ∼1 K. The resistance was modulated by as much as 20% in the normal state and by over 1500% near the zero resistance temperature.

On the possibility of orienting rotationally cooled polar molecules in an electric field

Abstract: It has long been thought that only symmetric top molecules (or equivalent) can be appreciably oriented in an electric field. This assessment is unduly pessimistic. In molecular beams produced by supersonic expansion, the rotational temperature can be made very low (∼ 1 K). For many diatomic, linear, or asymmetric top molecules, quite substantial orientation can thereby be attained for a large fraction of the beam at accessible field strengths (∼ 100 kV/cm or less). We present model calculations and an experimental design to evaluate the method by observing the fluorescence of photofragments from oriented molecules. Nomograms are provided from which the orientation can be estimated for linear molecules from the dipole moment, rotational constant, rotational temperature, and field strength.

Fields radiated by electrostatic discharges

Abstract: Electrostatic discharge (ESD) fields are examined both analytically and experimentally. Measurements indicate that the electric fields can be quite significant (>or=150 V/m at a distance at 1.5 m, for example) for short periods of time (a few nanoseconds), particularly for relatively low-voltage events ( >

Inkjet printer with an electric curtain force

Abstract: An inkjet printer which comprises a nozzle having an exit opening; an electric field curtain forming unit for causing an electric field curtain force or a travelling-wave electric field curtain to act on inking material located in the vicinity of the exit opening of the nozzle; a charge injecting unit disposed in the vicinity of the exit opening of the nozzle for injecting an electric charge into the inking material; and an image control unit operable in response to image information to control the charge injecting unit to vary the amount of electric charge to be injected into the inking material. The inking material located in the vicinity of the nozzle can be expelled outwardly from the nozzle by the action of the electric field curtain force in a quantity dependent on the amount of the electric charge injected by the charge injecting unit.

Model for second-harmonic generation in glass optical fibers based on asymmetric photoelectron emission from defect sites.

Abstract: We present a self-consistent calculation of anomalous second-harmonic generation in glass optical fibers. Quantum interference between multiphoton absorption processes leads to asymmetric photoelectric emission from defects, creating a spatially periodic space-charge electric field. The second harmonic is found to grow exponentially along the fiber, then saturate to a maximum value proportional to the square of the fundamental intensity. The predicted conversion efficiency is in reasonable agreement with experiments.

Ultrasonic probe system

Abstract: An ultrasonic probe system is disclosed, which is designed to allow connection of a DC power supply capable of applying a voltage higher than the coercive electric field of each of a plurality of piezoelectric layers thereto, and includes a polarization turn over circuit means for, when the DC power supply is driven, turning over the polarity of the DC power supply so as to direct electric fields of every two adjacent layers constituting the piezoelectric layers in substantially opposite directions or electric fields of all the layers in the same direction. When the polarization turn over circuit means turns over the polarity of a voltage to be applied to direct electric fields of every two adjacent layers of the piezoelectric layers in substantially opposite directions or electric fields of all the layers in the same direction, the polarization turn over circuit means performs control to apply the voltage during a blanking time of an operating time of the system, thereby performing conversion of a resonance frequency, and selectively generating ultrasonic waves having a plurality of different frequencies.

Exciton Green's-function approach to optical absorption in a quantum well with an applied electric field.

Abstract: An exciton Green's function is derived and used to calculate the polarization-dependent optical absorption in a semiconductor quantum well with an applied electric field. With use of the exciton (or Coulomb) Green's-function approach, the optical-absorption coefficient due to the bound and continuum states of excitons can be obtained simultaneously and this approach also takes into account the coupling between different subband pairs. This is in contrast with the conventional approach in which the 1s exciton bound state is calculated variationally and the continuum states are calculated simply using the Sommerfeld enhancement factor from the pure two-dimensional case without the correct quantum size effect. Also, the coupling between different subband pairs is usually neglected. We compare the numerical results of the Green's-function method with those of the commonly used variational method and find that the variational method overestimates the oscillator strength by 20% for the 1s bound state and by 50% for the continuum, although the 1s bound-state energy can be quite accurate. The numerical results using the exciton Green's function are compared with experimental data and found to be in very good agreement.

Flashover in wide-band-gap high-purity insulators : methodology and mechanisms

Abstract: The electron beam of a scanning electron microscope (SEM) is used to charge an unmetallized insulator (Al2O3, Y2O3, SiO2) in vacuum. The charging is found to be stable in time after the e‐beam is switched off. The SEM is also used to measure the implanted charge by measuring the resulting electrostatic potential. The distribution of potential around the trapped charges is determined by the classical laws of electrostatics. The electrostatic energy stored in the polarized dielectric can thus be determined. The implanted charge can be removed by the introduction of carriers into the polarized sample by flooding the surface with electron beams of varying energies. Slow or rapid relaxation occurs depending on the operating conditions of the flood gun (energy, intensity, etc.) which introduces the electrons. When the relaxation kinetics are slow, the electrostatic charge decreases slowly as a function of time. On the other hand, a rapid relaxation of the dielectric leads to the appearance of a high‐density pla...

Electric field effect on superconducting YBa2Cu3O7?? films

Abstract: Experimental evidence for a significant electric field effect in thin superconducting films of YBa2Cu3O7−δ is presented. MISFET-type structures have been developed which allow the application of electric fields larger than 4×106 V/cm across insulating SrTiO3 barriers into thin cpitaxial YBa2Cu3O7−δ channel layers. The results demonstrate that with these structures the electrical resistivity aboveT c (R=0) and the density of free carriers in the YBa2Cu3O7−δ films can be modified by 1–2% with gate voltages smaller than 50 V.

Ferroelectric Polarization Switching in Nylon 11

Abstract: : Cold drawn nylon 11 films, which were prepared by stretching the melt quenched films to a draw ratio of 2.8:1 at room temperature, were found to exhibit an electric displacement vs. electric field hysteresis loop. The results confirmed that the nylon 11 film exhibited ferroelectric behavior at room temperature or lower. The coercive fields were 65, 98, 125, 160 and 215 MV/m at 20, 0, -20, -40 and -60 deg C and the remanent polarization at -20 deg C (where there was considerably less dc conduction) was 56 mC/m2. Switching of the polarization was almost completed within 20 ms.