Showing papers on "Field (physics) published in 1982"
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TL;DR: In this article, a finite-difference time-domain (FD-TD) method is proposed to compute the near total fields within a rectangular volume which fully encloses the object and then an electromagnetic-field equivalence principle is invoked at a virtual surface of this rectangular volume to transform the tangential near scattered fields to the far field.
Abstract: The finite-difference time-domain (FD-TD) method is proposed as a means of accurately computing electromagnetic scattering by arbitrary-shaped extremely complex metal or dielectric objects excited by an external plane wave. In the proposed method, one first uses the FD-TD method to compute the near total fields within a rectangular volume which fully encloses the object. Then, an electromagnetic-field equivalence principle is invoked at a virtual surface of this rectangular volume to transform the tangential near scattered fields to the far field. To verify the feasibility of this method, the surface currents, near scattered fields, far scattered fields, and radar cross section of two canonical two-dimensional objects are presented. For these cases, it is shown that the FD-TD method provides magnitude of current and field predictions which are within ± 2.5 percent and further phase values within ± 30 of values predicted by the method of moments ( MOM) at virtually every point including in shadow regions.
513 citations
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TL;DR: In this paper, a momentum-conservation approximation was obtained for the scattering rates and momentum-relaxation rates of an electron in a quasi-two-dimensional quantum well interacting with acoustic, optical and intervalley modes via deformation potential and with longitudinal optical modes via the polar interaction.
Abstract: Approximate analytic expressions are obtained for the scattering rates and momentum-relaxation rates of an electron in a quasi-two-dimensional quantum well interacting with acoustic, optical and intervalley modes via the deformation potential and with longitudinal optical modes via the polar interaction. These analytic expressions are obtained using a momentum-conservation approximation. The threshold for optical phonon emission, unlike the case in the bulk, is abrupt. All scattering rates are energy-independent and are inversely proportional to L, the thickness of the well. The momentum-relaxation rate associated with the absorption of polar optical phonons, on the other hand, proves to be proportional to L. These properties are shown to lead to a negative differential resistance for pure polar mode scattering, and to the existence of a runaway field for deformation-potential scattering. The self-energy associated with the emission of polar optical phonons at absolute zero is shown to be divergent unless the polar interaction is screened, and some consequences of this for laser and other optical processes are pointed out. The description of scattering by perturbation theory breaks down in very narrow wells.
387 citations
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TL;DR: In this article, a description of MHD turbulence at low magnetic Reynolds number and large interaction parameter is proposed, in which attention is focussed on the role of insulating walls perpendicular to a uniform applied magnetic field.
Abstract: A description of MHD turbulence at low magnetic Reynolds number and large interaction parameter is proposed, in which attention is focussed on the role of insulating walls perpendicular to a uniform applied magnetic field. The flow is divided in two regions: the thin Hartmann layers near the walls, and the bulk of the flow. In the latter region, a kind of electromagnetic diffusion along the magnetic field lines (a degenerate form of Alfv6n waves) is displayed, which elongates the turbulent eddies in the field direction, but is not sufficient to generate a two-dimensional dynamics. However the normal derivative of velocity must be zero (to leading order) at the boundaries of the bulk region (as at a free surface), so that when the length scale 1, perpendicular to the magnetic field is large enough, the corresponding eddies are necessarily two-dimensional. Furthermore, if I, is not larger than a second limit, the Hartmann braking effect is negligible and the dynamics of these eddies is described by the ordinary Navier-Stokes equations without electromagnetic forces. MHD then appears to offer a means of achieving experiments on two-dimensional turbulence, and of deducing velocity and vorticity from measurements of electric field.
381 citations
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TL;DR: In this paper, it was shown that a field satisfying the Yang-Mills equations in dimension 4 with a point singularity is gauge equivalent to a smooth field if the functional is finite.
Abstract: We show that a field satisfying the Yang-Mills equations in dimension 4 with a point singularity is gauge equivalent to a smooth field if the functional is finite. We obtain the result that every Yang-Mills field overR
4 with bounded functional (L
2 norm) may be obtained from a field onS
4=R
4∪{∞}. Hodge (or Coulomb) gauges are constructed for general small fields in arbitrary dimensions including 4.
373 citations
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TL;DR: In this article, the probability of post-ionization of field evaporated ions is predicted as a function of field strength for all elements of interest in field evaporation, based on a physically realistic model potential.
332 citations
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TL;DR: In this article, a Monte Carlo method for studying the steady-state behavior of charged species under the influence of a non-uniform electric field is described, using a null-collision technique based on determination of the increase in kinetic energy between two collisions, avoiding calculation of the time variations of the phase space coordinates of the charged species.
Abstract: A Monte Carlo method for studying the steady-state behaviour of charged species under the influence of a non-uniform electric field is described. This method, using a null-collision technique based on determination of the increase in kinetic energy between two collisions, avoids calculation of the time variations of the phase space coordinates of the charged species. The electron behaviour in the cathode region of a glow discharge has been analysed using this method; spatial variations of the energy and angular electron distribution functions, and of the macroscopic electron parameters are discussed.
286 citations
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TL;DR: In this article, an axiomatic framework for a general, rigorous theory of relativistic quantum fields on a class of manifolds, including Kruskal's extension of Schwarzschild space-time, was proposed.
264 citations
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TL;DR: In this paper, a spherical harmonic model of the earth's internal magnetic field of degree and order 23 is derived from selected MagSat data, and its power spectrum, computed with terms developed by Mauersberger (1956) and Lowes (1974), is found to exhibit a change of a slope at n = 14 which is interpreted as an indication that the core field dominates at values lower than 13 while the crust field dominates above a value of 15.
Abstract: A spherical harmonic model of the earth's internal magnetic field of degree and order 23 is derived from selected Magsat data, and its power spectrum, computed with terms developed by Mauersberger (1956) and Lowes (1974), is found to exhibit a change of a slope at n = 14 which is interpreted as an indication that the core field dominates at values lower than 13 while the crust field dominates above a value of 15. The representations of the two portions of the spectrum obtained can be used to establish order-of-magnitude inaccuracies due to both crustal fields and the inability to observe core field wavelengths beyond n = 13, at which point they are obscured by the crustal field, in core field models.
261 citations
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TL;DR: In this article, low-temperature photoluminescence (PL) measurements have been performed in narrow GaAs-${\mathrm{Ga}}_{1\ensuremath{-}x{Al}}_{x}\mathrm {As}$ quantum wells subject to an electric field perpendicular to the well plane, showing two peaks associated with exciton and free-electron-to-impurity recombination.
Abstract: Low-temperature photoluminescence (PL) measurements have been performed in narrow GaAs-${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ quantum wells subject to an electric field perpendicular to the well plane. At low fields the PL spectra show two peaks associated, respectively, with exciton and free-electron-to-impurity recombination. With increasing field the PL intensity decreases, with the excitonic structure decreasing at a much faster rate, and becomes completely quenched at a field of a few tens of kV/cm. This is accompanied by a shift in the peak position to lower energies. The results are interpreted as caused by the field-induced separation of carriers and modification of the quantum energies. Variational calculations performed for isolated, finite quantum wells explain qualitatively the experimental observations.
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TL;DR: In this article, a model describing C-V and I-V characteristics of modulation doped FET's is proposed, which takes into account the change in the Fermi energy with the gate voltage.
Abstract: A model describing C-V and I-V characteristics of modulation doped FET's is proposed. The model takes into account the change in the Fermi energy with the gate voltage. At high two dimensional electron concentrations, the equations of the model for the charge control by the gate voltage become similar to the equations of the charge control model where the thickness d of AlGaAs layer should be substituted by d + Δ d and Δ d is the effective width of the potential well (≃ 80 A). Another important prediction of the model is the existence of the "subthreshold" current. A very good quantitative agreement is obtained with our experimental I-V curves using the measured values of the low field mobility and the source resistance.
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TL;DR: In this article, the inverse problems were applied to three cases in microtectonics, where a collection of striations measured on pre-existing faults may be processed automatically and interpreted in terms of tectonic stress.
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TL;DR: In this paper, the effects of magnetostatic and repulsive particle interactions on the formation of agglomerates in a magnetic fluid were investigated using Monte Carlo techniques, and the dependence on particle size and applied field of the form of the agglomers was studied using a spatial distribution function which allows a quantitative distinction to be made between clusters and anisotropic chain structures.
Abstract: Monte Carlo techniques have been used to investigate the effects of magnetostatic and repulsive particle interactions on the formation of agglomerates in a magnetic fluid The dependence on particle size and applied field of the form of the agglomerates was studied using a spatial distribution function which allows a quantitative distinction to be made between clusters and anisotropic chain structures Magnetization curves have been calculated for magnetic particle sizes varying from 5 to 15 nm with and without magnetostatic interactions For the larger particle sizes, it was found that the initial susceptibility is reduced in the presence of interactions This is associated with the presence of pronounced agglomeration in zero field, where open clusters are formed As the applied field is increased the clusters break up to form long chains aligned in the field direction At intermediate particle sizes, there is evidence of magnetic field induced agglomeration leading to the formation of dimers and trimers preferentially aligned in the field direction The smallest particle size showed little evidence of ordering even in strong applied fields, since thermal disordering dominates the situation
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TL;DR: In this article, the charge density in α-Al2O3 has been refined with respect to X-ray structure factors [complete spheres for both Mo Kα and Ag Kα radiations with (sin θ/λ)max = 1.19 and 1.495 A-1 respectively] and electric-field gradient tensors at both atomic sites.
Abstract: The charge density in α-Al2O3 has been refined with respect to X-ray structure factors [complete spheres for both Mo Kα and Ag Kα radiations with (sin θ/λ)max = 1.19 and 1.495 A-1 respectively] and electric-field gradient tensors at both atomic sites, using Hirshfeld- type deformation functions. Atomic charges from a κ refinement are + 1.32 (5) for Al and -0.88 (8) e for O. The charge distribution of O is polarized towards the four Al neighbours, and metal-metal bonds are clearly absent. Quadrupole coupling constants and asymmetry parameters of the field-gradient tensors cannot be determined from the structure factors. They define the quadrupolar deformations near the atomic centers, and the X-ray data define the charge density between the atoms. The orientational parameters of the tensors and the signs of their largest eigenvalues can be qualitatively retrieved from the X-ray data. The refinement of anisotropic secondary-extinction parameters may, however, destroy this information. Refinement with respect to the field gradients affects mainly the quadrupolar deformation terms, and has little influence on the X-ray scale factor (i.e. monopolar terms) and computed electrostatic fields (i.e. dipolar terms). Properties of the charge density with different angular symmetries are thus only weakly correlated.
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TL;DR: In this paper, the Green's function was extended to the case that the field component along the line of sight is not generally normal to the flat boundary plane, and the normal component on a spherical surface is specified, together with the condition that the magnetic field becomes approximately radial on an outer spherical surface.
Abstract: The Green's function method to calculate potential magnetic field on the Sun, which was first established by Schmidt (1964) in the case that the field component normal to a flat boundary plane is specified, is extended to the following three cases: (a) The field component along the line of sight, which is not generally normal to the flat boundary plane, is specified; (b) the line of sight component on a spherical boundary surface is specified; (c) the normal component on a spherical surface is specified, together with the condition that the field becomes approximately radial on an outer spherical surface (the so-called source surface). Properties of these Green's functions are examined, and the applicability of these methods to solar magnetic data is discussed.
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TL;DR: In this paper, the ground state properties of the one-dimensional spin-s (12⩽s <∞) anisotropic XYZ antiferromagnet in a magnetic field of arbitrary direction were studied.
Abstract: This is a study of the ground-state properties of the one-dimensional spin-s (12⩽s<∞) anisotropic XYZ antiferromagnet in a magnetic field of arbitrary direction. It provides the first rigorous results for the general case of this model in non-zero field. By exact calculations we find the existence of an ellipsoidal surface h = hN in field space where the ground state is of the classical two-sublattice Neel type with non-zero antiferromagnetic long-range order. At hN there are no correlated quantum fluctuations. We give upper and lower bounds for the critical field hc where antiferromagnetic long-range order is suppressed by the field. The zero-temperature phase diagrams are discussed for a few representative cases.
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TL;DR: In this article, the photon statistics in some fully quantized models of the interaction of a two-level atom with a single-mode radiation field have been studied using the operator equations of motion.
Abstract: Photon statistics in some fully quantized models of the interaction of a two-level atom with a single-mode radiation field have been studied using the operator equations of motion. Expressions for the photon number distribution and the mean photon number are presented for various initial conditions. It is found that the mean photon number may show decays and revivals of coherence similar to those of the atomic inversion in the coherent-state Jaynes-Cummings model. Application of these models to the study of multiphoton laser, absorption, and emission processes is also discussed.
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TL;DR: In this paper, a third-order zonal harmonic model fitted to Voyager 1 and 2 observations is found to be capable of predicting the magnetic field characteristics at one encounter based on those observed at another, unlike models including dipole and quadrupole terms.
Abstract: An analysis of the magnetic field of Saturn is presented which takes into account both the Voyager 1 and 2 vector magnetic field observations. The analysis is based on the traditional spherical harmonic expansion of a scale potential to derive the magnetic field within 8 Saturn radii. A third-order zonal harmonic model fitted to Voyager 1 and 2 observations is found to be capable of predicting the magnetic field characteristics at one encounter based on those observed at another, unlike models including dipole and quadrupole terms only. The third-order model is noted to lead to significantly enhanced polar surface field intensities with respect to dipole models, and probably represents the axisymmetric part of a complex dynamo field.
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01 Jan 1982
TL;DR: In this paper, the authors describe asymptotic formulae of quantum electrodynamics and show that double-logarithmic corrections occur in cases of two kinds.
Abstract: This chapter describes asymptotic formulae of quantum electrodynamics. The asymptotic constancy of the total cross-section is a characteristic property of scattering processes whose diagrams can be cut across internal photon lines. It occurs even when more than two particles are present in the final state of the reaction. The double-logarithmic corrections occur in cases of two kinds. One kind includes scattering through a fixed finite angle. The cross-sections decrease in the asymptotic high-energy range. In such cases, the double-logarithmic corrections are associated with the infrared divergence. These cases include elastic scattering of electrons in an external Coulomb field. The other class of cases includes reaction cross-sections, which decrease with increasing energy for a given square of the momentum transfer, that is, for scattering angles, which asymptotically approach zero or π.
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TL;DR: In this paper, the vector potential and magnetic field of a circular arc segment of a current-carrying conductor of arbitrary cross section and arbitrary azimuthal length were derived, based on known functions such as Jacobian elliptic functions, complete and incomplete elliptic integrals of the first, second, and third kind.
Abstract: Analytical expressions for the components of the vector potential and magnetic field of a circular arc segment of a current-carrying conductor of rectangular cross section and arbitrary azimuthal length are derived. Components of the field gradient are also calculated. All the expressions developed consist of known functions such as Jacobian elliptic functions, complete and incomplete elliptic integrals of the first, second, and third kind, and thus permit a new timesaving efficient computation algorithm.
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TL;DR: In this article, an analytical solution of the Laplace equation in the air-gap is presented, combined with the classical finite element solution, and the results of field calculation obtained using such a method are more precise than those calculated when modelling the air gap with classical elements.
Abstract: In the design of electric machine working under unbalanced conditions or supplied by non sinusoidal current, a detailed knowledge of the magnetic field distribution is required to predict the machine performance. Under such conditions this field distribution must be calculated using a dynamic model for the machine air-gap: A particular element constituted by the uniform part of the air-gap (u.p.a.g.) allows such a dynamic model in the present paper. An analytical solution of the Laplace equation in the air-gap is done. The field distribution in a saturated machine can be obtained within this air-gap model, combined with the classical finite element solution. The results of field calculation obtained using such a method are more precise than those calculated when modelling the air-gap with classical elements. Examples are given in the paper, showing this fact for both the cases of static and dynamic calculations.
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TL;DR: In this article, it was shown that when a monochromatic laser couples a single ground level to two closely spaced excited levels, the system can be driven into a state in which quantum interference prevents any fluorescence from the excited levels regardless of the intensity of the exciting field.
Abstract: It is shown that when a monochromatic laser couples a single atomic ground level to two closely spaced excited levels the system can be driven into a state in which quantum interference prevents any fluorescence from the excited levels, regardless of the intensity of the exciting field. This steady-state interference occurs only at a particular excitation frequency which depends on the separation of the excited states and the relative size of the two transition dipole matrix elements. The results are derived from the density matrix equations of motion. It is shown that a correct description of the effect requires the inclusion of generalised Einstein A coefficients which are usually neglected in phenomenological damping theories. A dressed-state analysis is introduced to simplify the generalisation to atoms having more complex manifolds of excited states. Analogous interferences in multiphoton absorption and ionisation are also discussed briefly.
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TL;DR: In this paper, the authors studied magnetic field fluctuations measured by the two HELIOS spacecraft in the frequency range between 2.4×10−5 Hz and 1.3 × 10−2 Hz at distances from the sun between 0.29 AU and 0.4 AU.
Abstract: To obtain information on the temporal and spatial evolution of MHD waves and discontinuities in the solar wind, we studied by means of statistical methods magnetic field fluctuations measured by the two HELIOS spacecraft in the frequency range between 2.4×10−5 Hz and 1.3×10−2 Hz at distances from the sun between 0.29 AU and 1.0 AU. Some statistical properties as magnetic field variances show the fluctuations remaining very similar on their way out from the sun. A different picture of the fluctuations emerges from analyzing the spectral properties of the magnetic field: The slope of the power spectral densities (both the vector components and the magnitude of the magnetic field) as a function of frequency becomes steeper with increasing heliographic distance—the solar wind seems to act as a low-pass filter. The major changes occur within a heliographic distance up to about 0.4 AU. Field magnitude fluctuations fall off less rapidly with increasing radius than do fluctuations in the vector components. Cross spectral analysis between magnetic field components reveal the fluctuations as generally being field magnitude conserving.
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TL;DR: In this paper, simultaneous electric field, magnetic field, and electron flux measurements from three dawn-dusk passes of S3-2 were analyzed for signatures of sun-aligned arcs in the polar cap.
Abstract: Simultaneous electric field, magnetic field, and electron flux measurements from three dawn-dusk passes of S3-2 were analyzed for signatures of sun-aligned arcs in the polar cap. Two of the passes were over the summer and one over the winter polar cap. In all three cases the IMF had a strong northward component. During the pass over the winter polar cap the electric fields were highly irregular but tracked with nearly one-to-one correspondence to the transverse magnetic fluctuations. Upward Birkeland currents were embedded in regions of negative space charge. The carriers for the upward Birkeland currents were weakly field aligned fluxes of electrons with a temperature of a few hundred eV that had been accelerated through a potential drop of ∼1 kV. The two summer polar cap passes differed from each other in their large-scale electric field patterns. In one pass the electric field was irregular with the upward Birkeland currents in negative space charge regions. In the other summer hemisphere pass, convection was sunward in the central polar cap. Small-scale Birkeland current systems, possibly signatures of arclike structures, were found in regions of both sunward and antisunward convection. In this pass, variations in the electric and magnetic field did not always show the strong correlations found in the other two cases, due to the effects of electrojet activity near the flanks of the polar cap. The measurements are found to be consistent with a ‘static’ model for the electrical coupling between the ionoshere and magnetosphere with the generator at the magnetopause.
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TL;DR: In this article, it was shown that replica symmetry breaking occurs simultaneously with transverse spin-glass ordering as the temperature is reduced, and that the irreversibility onset will be strong in the transverse direction, but only weak longitudinally, with a crossover to strong longitudinal irreversible at a lower temperature.
Abstract: It is demonstrated that for a vector spin-glass in a magnetic field, replica-symmetry breaking, the theoretical indicator for magnetic irreversibility, occurs simultaneously with transverse spin-glass ordering as the temperature is reduced. It is argued that the irreversibility onset will be strong in the transverse direction, but only weak longitudinally, with a crossover to strong longitudinal irreversibility at a lower temperature.
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TL;DR: In this article, low threshold nematic mixtures are presented exhibiting very low dielectric crossover frequencies fc ≃ 1 kHz at 20°C and unusually large, low- as well as high-frequency anisotropies.
Abstract: Novel low threshold nematic mixtures are presented exhibiting very low dielectric crossover frequencies fc ≃ 1 kHz at 20°C and unusually large, low- as well as high-frequency dielectric anisotropie...
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TL;DR: In this paper, the method of dimensional regularization for quantum field theories with large (i.e., not infinitesimal) classical background fields is introduced and discussed in detail for the case of quantum fields with large background fields.
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TL;DR: In this paper, a simple molecular field model is presented for the prediction of magnetic transition temperatures of rare-earth (RE) compounds when crystalline electric field (CEF) splittings are significant.