Showing papers on "Electric field published in 1993"

Boson localization and correlated pinning of superconducting vortex arrays.

Abstract: A theory of vortex pinning in high-temperature superconductors by correlated disorder in the form of twin boundaries, grain boundaries, and columnar defects is described. Mapping vortex trajectories onto boson world lines leads to a ``superfluid'' flux liquid at high temperatures, as well as low-temperature ``Bose-glass'' and ``Mott-insulator'' phases, in which the flux lines are localized. Currents perpendicular to the average vortex direction act like an electric field applied to charged bosons, while currents parallel to the field act like an imaginary magnetic field in this approach. We discuss the equilibrium and dynamic properties of these phases, and propose a scaling theory for the flux-liquid to Bose-glass transition, at which the linear resistivity vanishes. Although the Bose-glass predictions share some features with vortex-glass behavior predicted for point disorder, the response to tilting the magnetic field in the two cases differs dramatically, thus allowing the two theories to be distinguished experimentally.

Measurements of Coulomb blockade with a noninvasive voltage probe.

Abstract: We have investigated the behavior of a laterally confined quantum dot in close proximity to a one-dimensional channel in a separate electrical circuit. When this channel is biased in the tunneling regime the resistance is very sensitive to electric fields, and therefore is sensitive to the potential variations on the dot when it is showing Coulomb blockade oscillations. This effect can be calibrated directly, allowing the Coulomb charging energy to be measured. We also found the activation energy of transport through the dot is much lower than expected.

Thin anodic oxide layers on aluminium and other valve metals: high field regime

Abstract: The formation kinetics and electric properties of anodic barrier oxides on Al, Hf, Nb, Ta, Ti, and Zr depend on the migration of ions, controlled by an electric field strength of up to 107 V cm−1. The high field model, based on ideas of Guntherschulze, Betz, Verwey, Mott, and Cabrera, explains only parts of the experiments. Contradictory models and investigations of the last 60 years are compared with new results, which enable a more detailed understanding of many aspects of the high field model, e.g. the shape of experimental transients, the time and field dependent concentration of mobile ions, the nature of the mobile species and the interaction of anion and cation transport, and the location of the rate determining step.

Simulation of the prompt energization and transport of radiation belt particles during the March 24, 1991 SSC

Abstract: We model the rapid (about 1 min) formation of a new electron radiation belt at L about or = 25 that resulted from the Storm Sudden Commencement (SSC) of March 24, 1991 as observed by the Combined Release and Radiation Effects Satellite (CRRES) satellite Guided by the observed electric and magnetic fields, we represent the time-dependent magnetospheric electric field during the SSC by an asymmetric bipolar pulse that is associated with the compression and relaxation of the Earth's magnetic field We follow the electrons using a relativistic guiding center code The test-particle simulations show that electrons with energies of a few MeV at L greater than 6 were energized up to 40 MeV and transported to L about or = 25 during a fraction of their drift period The energization process conserves the first adiabatic invariant and is enhanced due to resonance of the electron drift motion with the time-varying electric field Our simulation results, with an initial W(exp -8) energy flux spectra, reproduce the observed electron drift echoes and show that the interplanetary shock impacted the magnetosphere between 1500 and 1800 MLT

Surface-Enhanced Infrared Spectroscopy: The Origin of the Absorption Enhancement and Band Selection Rule in the Infrared Spectra of Molecules Adsorbed on Fine Metal Particles

Abstract: Infrared transmission spectra of molecules adsorbed on silver island films evaporated on CaF2 have been investigated. The spectra are remarkably simple compared with those of the molecules in the solid state (KBr pellets). Only the vibrational modes which give dipole changes perpendicular to the metal surface are infrared active. In addition, their intensities are about 200 times larger than those of the free molecule. These results can be fully accounted for if the electric field which excites the surface molecule is perpendicular to the local surface of the metal islands and is stronger than the incident electric field. The origin of the absorption enhancement and the surface selection rule is discussed theoretically by using a classical electromagnetic model.

Materials constants of KNbO3 relevant for electro‐ and acousto‐optics

Abstract: The elastic, piezoelectric, dielectric, elasto‐optic, and electro‐optic tensors have been determined by numerically evaluating the measurements published until now and using the additional measurements presented in this work. The dependence of the speed of two elastic waves on the applied electric field is measured yielding one electroelastic constant g13133=(95±10)NV−1 m−1. The complete set of parameters consisting of the low‐frequency clamped dielectric constants eSij, refractive indices, elastic stiffness constants at constant electric field CEijkl, piezoelectric coefficients eijk, elasto‐optic tensor at constant electric field pEijkl, and clamped electro‐optic coefficients rSijk is used to calculate effective electro‐optic coefficients and effective dielectric constants that have to be used in photorefractive experiments where the elastic deformations associated with a periodic space‐charge field have to be considered.

High-Density Nanosecond Charge Trapping in Thin Films of the Photoconductor ZnODEP.

Abstract: An electrooptical memory effect is observed with solid thin films of the photoconductor zinc-octakis(beta-decoxyethyl) porphyrin (ZnODEP) sandwiched between two optically transparent electrodes. Upon irradiation with the simultaneous application of an electric field, electron-hole pairs are generated and separated within the photoconductive layer. These electron-hole pairs become "frozen" within the films when the irradiation is interrupted. These trapped charges can be released by irradiation of the cell, resulting in a transient short-circuit photocurrent. No cross talk between adjacent memory elements separated by approximately 0.2 micrometer (a density of 3 gigabits per square centimeter) was detected. The charge storage system is robust and nonvolatile. The response time for the write-read beam is in the subnanosecond range, and no refreshing is required for long-term retention of trapped charges.

Formulation of the field-to-transmission line coupling equations in terms of magnetic excitation field

Abstract: Different formulations of the field-to-transmission-line coupling equations are reviewed and discussed. An equivalent formulation is derived in which the source terms (or forcing functions) are expressed in terms of the magnetic excitation field. This formulation is particularly useful for evaluating field-to-transmission-line coupling when the exciting field is determined experimentally, since only the measurement of the electric field-generally easier to measure than the electric field-is necessary. >

Standard probes for electromagnetic field measurements

Abstract: Discusses various standard antennas for measuring radio-frequency electric and magnetic fields. A theoretical analysis of each antenna's receiving characteristics is summarized and referenced. The standard probes described are an electrically short dipole, a resistively-loaded dipole, a half-wave dipole, an electrically small loop, and a resistively-loaded loop. A single-turn loop designed for simultaneous measurement of the electric and magnetic components of near-fields and other complex electromagnetic environments is also described. Each type of antenna demonstrates a different compromise between broadband frequency response and sensitivity. >

The influence of the strain‐induced electric field on the charge distribution in GaN‐AlN‐GaN structure

Abstract: We show that strongly pronounced piezoelectric properties play a key role in GaN‐AlN‐GaN semiconductor‐insulator‐semiconductor (SIS) and related structures. In sufficiently thin AlN layers, the lattice constant mismatch is accommodated by internal strains rather than by the formation of misfit dislocations. These lattice‐mismatch‐induced strains generate polarization fields. We demonstrate that, in a GaN‐AlN‐GaN SIS structure with the growth axis along a (0001) crystallographic direction, the strain‐induced electric fields can shift the flat band voltage and produce an accumulation region on one side and a depletion region on the other side of the AlN insulator. On which side of the insulator the accumulation region is produced depends on the type of atomic plane at the heterointerface (Ga or N). The surface charge density caused by the piezoeffect is on the order of 1012 cm−2. As a consequence of the asymmetry in the space charge distribution, the capacitance‐voltage (C‐V) characteristics of the SIS stru...

Optical, dielectric and polarization studies of the electric field-induced phase transition in Pb(Mg1/3Nb2/3)O3 [PMN]

Abstract: Single crystals of PMN have been studied between 10 and 300 K by means of dielectric, polarization and poling/depoling current measurements, associated with optical domain observations under bias field. Evidence for a first order electric field-induced phase transition, from the mean cubic phase to a macroscopically polar phase, has been disclosed by studying, as a function of temperature, the dielectric anomalies, the establishment of induced polarization, the onset of poling current and the appearance of the birefringence and phase boundary. By means of combined optical and domain switching analysis, the symmetry of the induced phase has been deduced to be trigonal 3m. The induced macropolarization Pind can be switched by a field of opposite polarity. The thermal depoling under a bias field takes place at a temperature depending on the field strength, whereas the zero-field depoling of an induced state occurs always at T do = 213 K, independently of the initial poling field. An electric field/t...

Dynamic Observation of Crack Propagation in Piezoelectric Multilayer Actuators

Abstract: Crack propagation in multilayer piezoelectric actuators made of Pb((Ni1/3Nb2/3), Ti, Zr)O3 ceramics with an interdigital electrode configuration was observed dynamically under an applied cyclic electric field using charge coupled device microscopy. The crack was observed only under a high electric field, and it healed under zero field. The crack was initiated at the internal electrode edge and propagated from the electrode edge in three directions. The electric-field-induced displacement was measured simultaneously with the propagation. The displacement of the multilayer actuator became gradually smaller and asymmetric with respect to the sign of the field with increased driving cycle.

Alignment of lamellar block copolymer microstructure in an electric field. 1. Alignment kinetics

Abstract: The kinetics of alignment of block copolymer microstructure in an electric field has been studied. The rate of alignment obeys a simple time-temperature-field superposition law when thermal history is well-controlled. Thermal history also strongly affects the alignment rate. It is proposed that observed trends can be understood in terms of mechanisms of defect movement and pinning

Effects of induced electric fields on finite neuronal structures: a simulation study

Abstract: An analysis is presented of magnetic stimulation of finite length neuronal structures using computer simulations. Models of finite neuronal structures in the presence of extrinsically applied electric fields indicate that excitation can be characterized by two driving functions: one due to field gradients and the other due to fields at the boundaries of neuronal structures. It is found that boundary field driving functions play an important role in governing excitation characteristics during magnetic stimulation. Simulations indicate that axons whose lengths are short compared to the spatial extent of the induced field are easier to excite than longer axons of the same diameter. Simulations also indicate that independent cellular dendritic processes are probably not excited during magnetic stimulation. Analysis of the temporal distribution of induced fields indicates that the temporal shape of the stimulus waveform modulates excitation thresholds and propagation of action potentials. >

A Review of Thunderstorm Electrification Processes

Abstract: Recent developments in the area of thunderstorm electrification processes are reviewed. These processes have two main divisions; (a) convective, in which particles charged by ion capture are moved by convection currents to strengthen the electric field in the cloud, and (b) processes involving charge transfer during particle interactions, following which oppositely charged particles move apart in the updraft to form the observed charge centers. Type-b processes are further subdivided into inductive (relying on the preexistence of an electric field) and noninductive charge-transfer mechanisms. Field and laboratory evidence points to the importance of interactions between particles of the ice phase, in the presence of liquid water droplets, in separating electric charge in thunderstorms. Recent experimental studies have investigated the dependence of charge transfer on the size and relative velocity of the interacting particles and have determined the dependence of the sign of the charge transfer o...

Electrostatic sample-tip interactions in the scanning tunneling microscope.

Abstract: Local surface photovoltage (SPV) measurements were used to measure how the electric field of a scanning tunneling microscope tip perturbs the electronic band structure at Si(001), Si(111)-(7\ifmmode\times\else\texttimes\fi{}7), and H-terminated Si(111) surfaces. The results demonstrate that tip-induced band bending is important under typical STM conditions even on surfaces whose surface Fermi levels are nominally ``pinned.'' Spatially resolved measurements of band bending as a function of sample bias show that atomic-scale contrast in SPV images can result from local variations in the ability of the surface states under the tip to screen external electric fields.

Electron transport properties of Si/SiGe heterostructures: Measurements and device implications

Abstract: We report electron transport properties of modulation‐doped Si/SiGe at 300 and 77 K. Record mobilities of 2830 and 18 000 cm2/V s at 300 and 77 K, respectively, have been measured. Depending on the spacer layer thickness, the sheet resistance of the Si channel is in the range of 2000–10 000 Ω/⧠ at 300 K and 450–700 Ω/⧠ at 77 K. The low field electron drift velocity is 2–3 (5–10) times higher than the corresponding velocity measured in Si/SiO2 structures at 300 K (77 K). The saturation velocity is measured to be only 5% higher than in bulk Si, at both 300 and 77 K, but appears at a lower electric field. The effect of the enhanced transport properties in modulation‐doped Si/SiGe on device design and performance is investigated.

Decay of equatorial ring current ions and associated aeronomical consequences

Abstract: The decay of the major ion species which constitute the ring current is studied by solving the time evolution of their distribution functions during the recovery phase of a moderate geomagnetic storm. In this work, only equatorially mirroring particles are considered. Particles are assumed to move subject to E x B and gradient drifts. They also experience loses along their drift paths. Two loss mechanisms are considered: charge exchange with neutral hydrogen atoms and Coulomb collisions with thermal plasma in the plasmasphere. Thermal plasma densities are calculated with a plasmaspheric model employing a time-dependent convection electric field model. The drift-loss model successfully reproduces a number of important and observable features in the distribution function. Charge exchange is found to be the major loss mechanism for the ring current ions; however the important effects of Coulomb collisions on both the ring current and thermal populations are also presented. The model predicts the formation of a low-energy (less than 500 eV) ion population as a result of energy degradation caused by Coulomb collision of the ring current ions with the plasmaspheric electrons; this population may be one source of the low-energy ions observed during active and quiet periods in the inner magnetosphere. The energy transferred to plasmaspheric electrons through Coulomb collisions with ring current ions is believed to be the energy source for the electron temperature enhancement and the associated 6300 A (stable auroral red (SAR) arc) emission in the subauroral region. The calculated energy deposition rate is sufficient to produce a subauroral electron temperature enhancement and SAR arc emissions that are consistent with observations of these quantities during moderate magnetic activity levels.

A two‐dimensional fluid model for an argon rf discharge

Abstract: A fluid model for an argon rf discharge in a cylindrical discharge chamber is presented. The model contains the particle balances for electrons and ions and the electron energy balance. A nonzero autobias voltage is obtained by imposing the condition that the time‐averaged current toward the powered and grounded electrode is zero. Particle densities and ionization profiles peak strongly in front of the smaller, powered electrode. There electric fields are stronger and the electron current density is higher, resulting in more ohmic heating and therefore higher ionization rates. The radial uniformity of the plasma in front of the powered electrode gives a homogeneous ion flux toward this electrode. The asymmetric character of the profiles of the cylindrical geometry is in clear contrast with the essentially one‐dimensional infinite parallel‐plate geometry, which is fully symmetric with respect to the center of the discharge and has a zero dc autobias voltage. A comparison with results of a one‐dimensional model shows that the average ion density, the average ion flux, and the average ionization rate in the cylindrical reactor are comparable to those in a parallel‐plate reactor. The numerical treatment of the time evolution of the transport equations and Poisson’s equation needs an implicit method to avoid numerical instabilities. The resulting system of discretized equations is solved by a multigrid technique. The spatial discretization uses the Sharfetter–Gummel scheme.

Atoms in ultra-intense laser fields

Abstract: The interaction of an atom with an ultra-intense radiation field is characterized by the involvement of many photons in absorptions and emissions. Of course the word 'intense' has to be compared with some atomic reference: if the induced transition coupling between bound states exceeds inherent widths, then the dressed atom Rabi oscillations which dominate the atomic evolution are typical intense field effects, even though laser intensities may actually be quite modest. When laser intensities exceed IOi3 Wcm-', and infrared frequencies are employed, tben free electrons are dressed by interaction energies which exceed the photon energies. Non-perturbative continuum processes such as above- threshold ionization then occur, combined with the emission of very high order harmonics ofthe pump laser frequency. At higher laser intensities, the optical electric field can exceed the Coulombic binding electric field and allow aver-the-barrier ionization. which defines a new regime of high intensity physics. In this region (or at higher intensities) the atomic electron charge cloud oscillates in the laser field with large amplitude excursions from the nucleus, during which time it is unable to absorb further photons. This stabilization regime is predicted to persist until the electron dressing energy approaches the restmass energy when wholly unexplored regions remain 10 be investigated. In this topical review, we examine theorelied models of atoms dressed by intense fields. We review the breakdown of lowest-order perturbation theory and those 'essential states' methods adopted to include Rabi frequencies, Stark shifts, induced widths and continuum dressing. Newer methods more suitable for super-svong fields are described, such as Floquet and Valkov methods and the direct numerical integration ofthe Schr6dinger equation. Such methods are used to provide completely non-perturbative strong field descriptions of atomic dynamics. We conclude with a brief examination of the relativistic effects expected to be important when new high intensity ultrashort pulse lasers currently under development are employed in strong field physics.

Particle and energy confinement bifurcation in tokamaks

Abstract: A bifurcation in the particle and energy confinement properties of a tokamak plasma, with properties similar to the experimentally observed L‐mode to H‐mode transition, is shown to follow from a simple model for the transport. The basic assumptions are that the edge turbulence is suppressed by sheared E×B flow, and the radial ion pressure gradient is approximately balanced by the radial electric field. The particle and thermal diffusivities are assumed to be given by simple nonlinear functions of the radial electric field shear, which is related to the density and pressure gradients. The steady‐state density and pressure profiles are found to have large gradient regions near the plasma edge when the product of edge energy flux and particle flux exceeds a threshold value, which corresponds to the H‐mode threshold. The particle and energy confinement times are significantly larger when this threshold is exceeded. The confinement times exhibit hysteresis, corresponding to a dependence of quasiequilibrium states on the time history. The width of the edge layer where the gradients are large is determined mainly by the particle source profile, which is assumed to be concentrated near the plasma edge. This edge layer width increases slowly with increasing edge heat flux or heating power, as observed experimentally.

Electromagnetic fields in a radio-frequency induction plasma

Abstract: The electromagnetic fields which drive a radio‐frequency induction plasma are both modeled and measured. The plasma source consists of a planar, square coil separated from a low pressure plasma chamber by a 2.54‐cm‐thick quartz window. A small loop antenna, which is sealed in a pyrex tube, is immersed in the discharge to determine the magnitude and direction of the rf magnetic field. The measured B field is primarily radial and axial. Typical rf field strengths vary from 2 to 7 G for rf powers of 0.1–1 kW. The radial B field decays exponentially in the axial direction. The skin depth of the electromagnetic field is 1.6–3.6 cm which is consistent with Langmuir probe measured ion densities (typically 3×1011 cm−3) in argon. Invoking Maxwell’s equations to deduce the rf electric field from the measured B field, we find the E field to be primarily azimuthal. Peak field strengths increase from 100 V/m at 100 W to 200 V/m at 600 W where they saturate for higher powers. Finally, we present a 3D finite element solution for the fields produced by this plasma source which employs a cold, collisionless plasma model to relate the relative plasma permittivity er to the electron plasma frequency, ωpe, using er=1−(ωpe/ω)2. The measured fields support this numerical solution.

Nonlinear optical rectification in parabolic quantum wells with an applied electric field

Abstract: Optical rectification in parabolic quantum wells, with an applied electric field, due to resonant inter-subband transitions is analyzed using a compact density-matrix approach. The large dipolar matrix elements obtained in such structures are partly due to the small effective masses of the host materials and are interpreted in terms of the participation of the whole band structure in the optical transitions. The other origin of the large optical rectification coefficient lies in the possibility of tuning independently the frequency ${\mathrm{\ensuremath{\omega}}}_{0}$ of the parabolic potential and the applied electric field F.