Showing papers on "Field (physics) published in 1994"

Interpreting magnetic fields of the brain: minimum norm estimates

Abstract: The authors have applied estimation theory to the problem of determining primary current distributions from measured neuromagnetic fields. In this procedure, essentially nothing is assumed about the source currents, except that they are spatially restricted to a certain region. Simulation experiments show that the results can describe the structure of the current flow fairly well. By increasing the number of measurements, the estimate can be made more localised. The current distributions may be also used as an interpolation and an extrapolation for the measured field patterns.

Direct evaluation of domain‐wall and intrinsic contributions to the dielectric and piezoelectric response and their temperature dependence on lead zirconate‐titanate ceramics

Abstract: By making use of the fact that domain‐wall motions do not produce volumetric changes, an experimental method is introduced to directly and quantitatively determine the domain‐wall and intrinsic contributions to the piezoelectric and dielectric responses of a ferroelectric material. Utilizing this method, the contributions from the domain walls and intrinsic part as well as their temperature dependence for lead zirconate‐titanate (PZT) 52/48 and PZT‐500 ceramics are evaluated. The data show that at temperatures below 300 K, the large change in the dielectric and piezoelectric constants with temperature is due to the change in the domain‐wall activities in the materials. The results confirm that most of the dielectric and piezoelectric responses at room temperature for the materials studied is from the domain‐wall contributions. The data also indicate that in PZT‐500, both 180° wall and non‐180° walls are possibly active under a weak external driving field.

Magnetization and transport currents in thin superconducting films.

Abstract: The critical-state behavior of an infinitely long type-II superconducting thin-film strip is theoretically analyzed for an arbitrary sequence of applied transport currents and perpendicular magnetic fields. Included are solutions for applied field only, transport current only, transport current applied to a sample initially in the remanent critical state, ac applied field, ac transport current, and simultaneously applied field and transport current. The results are compared side by side with corresponding solutions for the more famililar slab geometry; there are striking differences in behavior.

Interfaces in crystalline materials

Abstract: A brief review of the theoretical state of the art in the field of semiconductor interfaces is presented. Itis shown that the important factor controlling the different semiconductor barrier heights is the densityof states associated with the semiconductor dangling-bonds. Passivated semiconductor surfaces presentsaturated dangling-bonds and have modified barrier heights. Results for hydrogen-passivated GaAs(11O)-surfaces are presented; it is shown that the Schottky-barrier height formed by the deposition of a K-layeris sustantially changed by the hydrogen-passivation. 1 INTRODUCTION Interfaces of crystalline materials are at the heart of different devices, and their understanding is basic toan appropriate design of many microelectronic systems. In this regard, semiconductors and their interfaceswith other semiconductors or metals have received the major attention1 , and we shall concentrate ourdiscussion in this paper on analyzing their basic electronic properties and their relation to the formationof different barriers.There are, basically, two kinds of semiconductor contacts: metal- semiconductor2'3 and semiconductor-semiconductor interfaces4. A great deal of the recent research in this field has been addressed to under-standing how their different barrier heights depend on the properties of the crystals forming the contact5.Fig.1 shows the main parameters defining each barrier: (a) for a metal semiconductor interface,

Stochastic approach to noise modeling for free turbulent flows

Abstract: A new approach to noise modeling for free turbulent flows is presented. The equations governing the sound field are obtained in two steps. The first step consists of treating the mean and turbulent components of the flow while the acoustic perturbations are neglected. In the second step, a set of equations is derived for the acoustic variables. On the left-hand side of this system, one finds the linearized Euler equations, whereas the right-hand side exhibits source terms related to the turbulent fluctuations and their interactions with the mean flow. These terms are modeled using a stochastic description of the three-dimensional turbulent motion. This is achieved by synthesizing the velocity field at each point in space and for all times with a collection of discrete Fourier modes. The synthesized field posesses the suitable one- and two-point statistical moments and a reasonable temporal power spectral density. The linearized Euler equations including a stochastic description of noise sources are solved numerically with a scheme based on a fractional step treatment. Each one-dimensional problem is solved with a weak formulation. A set of calculations are carried out for a simple freejet. Comparisons between calculations and experiments indicate that a spatial filtering of the source terms is required to obtain the expected level in the far field. Realistic pressure signals, power spectral densities, and sound field patterns are obtained. It is indicated that the stochastic noise generation and radiation (SNGR) approach may be applied to more complex flows because the numerical codes used to calculate the mean flowfield and the wave propagation are not specific of jet configurations. The limitations of the present model lie in the statistical properties of the synthetic turbulent field and in the use of an axisymmetric modeling of the acoustic propagation.

Teleportation of an atomic state between two cavities using nonlocal microwave fields

Abstract: Implementing the ideas of Bennett et al. [Phys. Rev. Lett. 70, 1895 (1993)], we present an experimentally feasible scheme for the teleportation of an unknown atomic state between two high-Q cavities containing a nonlocal quantum superposition of microwave field states. This experiment provides alternative tests of quantum nonlocality involving high-order atomic correlations.

Complex wave-field reconstruction using phase-space tomography.

Abstract: A method is proposed for the experimental determination of the amplitude and phase structure of a quasimonochromatic wave field in a plane normal to its propagation direction. The wave field may represent either a scalar electromagnetic (EM) field or the quantum mechanical (QM) wave function of a matter wave. For coherent EM fields or pure QM states, the method uniquely reconstructs the complex wave fields. For partially coherent EM fields or mixed QM states, it reconstructs the two-point correlation function or density matrix. The experiment uses only intensity measurements and refractive optics (lenses), and the data analysis algorithm is noniterative and requires no deconvolution.

Complex wave-field reconstruction by using phase-space tomography

Abstract: A new class of phase-retrieval methods for 2-D fields is introduced. Phase-space tomography can be used for the experimental determination of the amplitude and phase structure of a quasi-monochromatic wave field in a plane normal to its propagation direction. The wave field ψ(r) may represent either a scalar electromagnetic (EM) field or the quantum-mechanical (QM) wave function of a matter wave. The complex wave field may be coherent or partially coherent, in which case the method reconstructs the two-point spatial correlation function, Γ(r, r′) = ⟨ψ(r)ψ*(r′)⟩. (In the QM case, the analogous quantity is the density matrix.) The experiment uses only intensity measurements and refractive optics (lenses), and the data-analysis algorithm is noniterative and requires no deconvolution.

Quantum mechanical computations on very large molecular systems: the local self-consistent field method

Abstract: Quantum chemical computations on a subset of a large molecule can be performed, at the neglect of diatomic differential overlap (NDDO) level, without further approximation provided that the atomic orbitals of the frontier atoms are replaced by parametrized orthogonal hybrid orbitals. The electrostatic interaction with the rest of the molecule, treated classically by the usual molecular mechanical approximations, is included into the self-consistent field (SCF) equations. The first and second derivatives of energy are obtained analytically, allowing the search for energy minima and transition states as well as the resolution of Newton equations in molecular dynamics simulations

Quantum-noise reduction using a cavity with a movable mirror

Abstract: Because of radiation pressure, an optical cavity with harmonically bound mirrors has an intensity-dependent length and behaves as an effective Kerr medium. We determine the quantum fluctuations of the field reflected by such a cavity, taking into account the input field fluctuations and the mirror Brownian motion. In the regions of parameter space close to bistability turning points, we obtain a significant quantum-noise reduction effect.

Scanning near-field optical microscopy

Abstract: Scanning Near-field Optical Microscopy (SNOM) allows the investigation of optical properties on subwavelength scales. During the past few years, more and more attention has been given to this technique that shows enormous potential for imaging, sensing and modification at near-molecular resolution. This article describes the technique and reviews recent progress in the field.

Turbulent Mixing of a Passive Scalar

Abstract: The small scale properties of a passive scalar mixed by a turbulent velocity field show departures from Komogorov theory at least as prominent as for the velocity field1. While one might have imagined that the scalar merely inherits the intermittency of the velocity field itself, a series of recent numerical experiments with a Gaussian but multiscale velocity field argue otherwise.2 A trivial but multiscale velocity produces scalar statistics in some cases quantitatively similar to those obtained in laboratory experiments.3

The Feynman path integral approach to atomic interferometry: A tutorial

Abstract: Many problems of current interest in atomic interferometry lend themselves to a path integral treatment. We present a practical guide to solving such problems, taking as examples the gravitational experiments of Kasevich and Chu, and the atomic equivalents of the Sagnac and Aharonov-Bohm effects. Atomic interferometry is a new and rapidly-developing field of research, concerned with physical phenomena in which the wave-nature of neutral atoms plays an important role ill. The wide variety of internal degrees of freedom of an atom opens up new possibilities for investigation which do not exist in the more traditional types of interferometry using photons, electrons and neutrons. The development of atomic interferometry has been aided by recent technical advances, particularly in the manipulation of atoms. New mechanisms for slowing, deflecting, cooling and trapping atoms allow control of both their position and momentum. Also important has been the birth of "atomic optics", a range of mechanisms providing the equivalent of mirrors, beamsplitters and lenses for atoms. Recently it has been pointed out that certain high-resolution spectroscopy techniques which avoid the Doppler effect amount to realizing an atomic interferometer (2). These methods have since been adapted to measure inertial fields (due to rotation and gravitation) by atomic interferometry. The situation encountered in atomic interferometry experiments is often close to the classical limit. When this is the case a path integral approach to the analysis is very appropriate since it reduces to a calculation of integrals along classical paths. Further simplifications can be made if the Lagrangian is quadratic, as is true for a particle in a gravitational field or a rotating (*) The Laboratoire Kastler Brossel is associated with the CNRS and the Universit4 Pierre et Marie

On the structure of an electrostatic spray of monodisperse droplets

Abstract: An experimental study has been performed on the structure of an electrostatic spray of monodisperse droplets. Such a spray is established when a liquid with sufficient electric conductivity and moderate surface tension, in the present case heptane doped with an antistatic additive, is fed through a small metal tube maintained at several kilovolts relative to a ground electrode a few centimeters away. The liquid meniscus at the outlet of the capillary takes a conical shape under the action of the electric field, with a thin jet emerging from the cone tip. This jet breaks up into charged droplets that disperse into a fine spray. Flash shadowgraph of the breakup region showed that the jet initially breaks into droplets of bimodal size distribution by varicose wave instabilities. The spray monodispersity is established farther downstream by a segregation process of electrostatic and inertial nature that confines the bulk of the mass flow rate (97%) and 85% of the total current in a core of nearly monodisperse primary droplets, with the remainder in a shroud of satellites. Droplet size, axial velocity, and concentration were measured throughout the spray by phase Doppler anemometry (PDA). The complementary use of these measurements permitted the determination of the electric field via the spray momentum equation.It was found that droplets are ejected from the jet at a relatively high velocity in a region characterized by a very intense electric field. They maintain this velocity farther downstream because of inertia, even though the field is precipitously decreasing, and ultimately decelerate under the action of the drag force and a progressively weaker electrostatic force. Velocity and concentration fields were shown to be self‐similar. Comparison between the external field, due to the potential difference applied between the electrodes, and the space charge field shows that the droplet axial motion is driven primarily by the external field, whereas the droplet radial motion and, consequently, the jet lateral spreading, is controlled primarily by the space charge field. The latter is typically at least one order of magnitude smaller than the external one, except at off‐axis locations near the breakup region of the spray, where the two fields can be comparable. The droplet charge distribution was also determined via the spray momentum equation and the simultaneous measurements of droplet size and velocity in a region where droplets experience negligible acceleration. The charge distribution was found to be narrow, with a ratio of standard deviation over mean of 0.15.

The soil ionization gradient associated with discharge of high currents into concentrated electrodes

Abstract: It has been known for many years that the grounding resistance of a concentrated electrode drops when it is subjected to a high current charge. This helps reduce the ground potential rise. The degree of the resistance depends on the magnitude of the ionization gradient of the soil E/sub o/. Based on both a theoretical analysis and a critical review of the large number of available measurements, this paper recommends that E/sub o/ be taken equal to 300 kV/m for typical soils. This is significantly less than the 1000 kV/m value used by some authors. Graphs are also given describing the behaviour of the rod electrodes which are used in many field installations. >

Wave-field phase singularities: The sign principle

Abstract: Phase singularities (topological charges, dislocations, defects, vortices, etc.), which may be either positive or negative in sign, are found in many different types of wave fields. We show that on every zero crossing of the real or imaginary part of the wave field, adjacent singularities must be of opposite sign. We also show that this ``sign principle,'' which is unaffected by boundaries, leads to the surprising result that for a given set of zero crossings, fixing the sign of any given singularity automatically fixes the signs of all other singularities in the wave field. We show further how the sign of the first singularity created during the evolution of a wave field determines the sign of all subsequent singularities and that this first singularity places additional constraints on the future development of the wave function. We show also that the sign principle constrains how contours of equal phase may thread through the wave field from one singularity to another. We illustrate these various principles using a computer simulation that generates a random Gaussian wave field.

Realization and verification of three-dimensional conformal radiotherapy with modulated fields

Abstract: Purpose : We describe the experimental demonstration of the delivery of a three-dimensional conformal radiotherapy dose distribution using in-field modulation of nine fixed-gantry fields. Methods and Materials : Two-dimensional in-field modulation profiles, varying from field to field, were realized by quasi-dynamic multilaf collimation using the prototype of a commercially available multileaf collimator installed on a medical linear accelerator. The profiles were calculated to deliver an optimal dose distribution for a patient with a prostate carcinoma. The target volume surface was invaginated and bifurcated. The calculated dose distribution was delivered to a homogeneous polystyrene phantom consisting of 1 cm thick slices that were cut to match the patient's outer contour. Seven therapy verification films were placed between the phantom slices. Results : Analysis of the films revealed a degree of conformation of the high-dose region to the target shape that would not be possible with unmodulated conformal therapy. However, small observed spatial displacements of the dose distribution confirm the need for very accurate positioning. Conclusions : It is feasible to deliver clinically relevant, three-dimensional dose distributions that conform to in-vaginated and bifurcated target volumes using fields modulated by multileaf collimators.

Thin superconductors in a perpendicular magnetic ac field: General formulation and strip geometry.

Abstract: The sheet current, electric field, and penetrating magnetic field in response to an applied perpendicular ac magnetic field are calculated for a thin type-II superconducting strip characterized completely by its sheet resistivity, which may be either nonlinear and frequency independent or linear, complex, and frequency dependent. The general formulation is given for the linear or nonlinear response of a strip and a circular disk in perpendicular time-varying magnetic field. An elegant and rapid numerical method is presented which solves this, in general, nonlinear one-dimensional integrodifferential equation with high precision on a personal computer and which accounts for the facts that the integral kernel has a logarithmic singularity and the sheet current for nearly ideal shielding (occurring at short times or high frequencies or for strong pinning of flux lines) has a one-over-square-root singularity near the specimen edges. As examples the linear Ohmic response of the strip to a sudden change of the applied field and to an ac field are given; Ohmic response is realized during flux flow or thermally activated flux flow. The complex magnetic susceptibility and the ac losses of the Ohmic strip are computed and approximated by simple expressions. This work completes the calculation of dissipation peaks in vibrating superconductors caused by various diffusion modes of the flux lines.

Vibrational Raman optical activity calculations using London atomic orbitals

Abstract: Ab initio calculations of Raman differential intensities are presented at the self-consistent field (SCF) level of theory. The electric dipole–electric dipole, electric dipole–magnetic dipole and electric dipole–electric quadrupole polarizability tensors are calculated at the frequency of the incident light, using SCF linear response theory. London atomic orbitals are employed, imposing gauge origin invariance on the calculations. Calculations have been carried out in the harmonic approximation for CFHDT and methyloxirane.

Impulsive excitation of coherent vibrational motion ground surface dynamics induced by intense short pulses

Abstract: A framework for understanding impulsively photoinduced vibrational coherent motion on the ground electronic surface is presented. In particular strong resonant excitation to a directly dissociative electronic surface is considered. Three distinct approaches are employed. A two surface Fourier wavepacket method explicitly including the field explores this process in isolated molecules. A coordinate dependent two‐level system is employed to develop a novel analytical approximation to the photoinduced quantum dynamics. The negligible computational requirements make it a powerful interactive tool for reconstructing the impulsive photoexcitation stage. Its analytical nature provides closed form expressions for the photoinduced changes in the material. Finally the full simulation of the process including the solvent effects is carried out by a numerical propagation of the density operator. In all three techniques the excitation field is treated to all orders, allowing an analysis of current experiments using strong fields, resulting in substantial photoconversion. The emerging picture is that the impulsive excitation carves a coherent dynamical ‘‘hole’’ out of the ground surface density. A rigorous definition of the dynamical ‘‘hole’’ is constructed and used to define a measure of its coherence. In particular all photoinduced time dependence in the system can be directly related to the dynamical ‘‘hole.’’ All three techniques are used to simulate the pump probe experiment on the symmetric stretch mode of I3−, including electronic and vibrational dephasing.

A Theory of D-E Hysteresis Loop Based on the Avrami Model

Abstract: The D - E hysteresis loop of ferroelectrics is theoretically studied on the basis of the extended Avrami theory. If the sideway velocity depends only on the instant value of the applied field, the volume fraction of the reversed area is expressed as q ( E )=1-exp (- f - d Φ( E )), where f and d are, respectively, the frequency of the applied field and the growth dimension of domains, and Φ is a function of E . This result is obtained irrespective of the waveform of the applied field and the field dependence of the sideway velocity, if the nucleation event is deterministic. For the stochastic nucleation due to thermal fluctuation, on the other hand, the above result is modified as q ( E )=1-exp (- f -( d +1) Φ( E )). The effect of the delay of wall motion is also discussed.

Contour Advection with Surgery: A Technique for Investigating Finescale Structure in Tracer Transport

Abstract: We present a trajectory technique, contour advection with surgery (CAS), for tracing the evolution of material contours in a specified (including observed) evolving flow. CAS uses the algorithms developed by Dritschel for contour dynamics/surgery to trace the evolution of specified contours. The contours are represented by a series of particles, which are advected by a specified, gridded, wind distribution. The resolution of the contours is preserved by continually adjusting the number of particles, and finescale features are produced that are not present in the input data (and cannot easily be generated using standard trajectory techniques). The reliability, and dependence on the spatial and temporal resolution of the wind field, of the CAS procedure is examined by comparisons with high-resolution numerical data (from contour dynamics calculations and from a general circulation model), and with routine stratospheric analyses. These comparisons show that the large-scale motions dominate the deformation field and that CAS can accurately reproduce small scales from low-resolution wind fields. The CAS technique therefore enables examination of atmospheric tracer transport at previously unattainable resolution.

A unified theory of dielectrophoresis and travelling wave dielectrophoresis

Abstract: We show for the first time that a particle in a non-uniform AC electric field experiences a dielectrophoretic force arising from spatial non-uniformities of the magnitude and phase of the field interacting, respectively, with the in-phase and out-of-phase components of the induced dipole moment.

Metastable lifetimes in a kinetic Ising model: Dependence on field and system size.

Abstract: The lifetimes of metastable states in kinetic Ising ferromagnets are studied by droplet theory and Monte Carlo simulation, in order to determine their dependences on applied field and system size. For a wide range of fields, the dominant field dependence is universal for local dynamics and has the form of an exponential in the inverse field, modified by universal and nonuniversal multiplicative power-law prefactors. Quantitative droplet-theory predictions for these dependences are numerically verified, and small deviations from the predictions are shown to depend nonuniversally on the details of the dynamics. We identify four distinct field intervals in which the field dependence and statistical properties of the lifetimes are markedly different. The field marking the crossover between the weak-field regime, in which the decay is dominated by a single droplet, and the intermediate-field regime, in which it is dominated by a finite density of droplets, vanishes logarithmically with system size. As a consequence, the slow decay characteristic of the former regime may be observable in systems that are macroscopic as far as their equilibrium properties are concerned.

Electron acceleration by kinetic Alfvén waves

Abstract: As Alfven waves with finite extent perpendicular to the magnetic field propagate from the magnetosphere to the ionosphere, there is a region of parallel electric field in the “wave front” of the propagating wave. For short perpendicular wavelengths this parallel electric field can be large enough to accelerate electrons to auroral energies. This problem is solved for the case of uniform plasma density and background magnetic field. The parallel electric field solution is then applied to a background Maxwellian plasma to study the effects of the acceleration due to this field on the electron distribution function. Two effects are found: (1) the relatively modest acceleration of the bulk of the background electrons and (2) Fermi-like resonant acceleration of a small component of the electrons up to velocities of the order of twice the Alfven speed. Although both effects always occur, the response of the background electrons is a sensitive function of the magnitude, wavelength, and timescale associated with the driving perpendicular electric field. In particular, the latter effect does not produce a significant signature for all conditions. However, for reasonable values of perpendicular electric field magnitude and scale size, and plasma parameters appropriate for auroral field lines at altitudes around 7000 km near where the Alfven speed peaks, the effect can be significant.