Showing papers on "Field (physics) published in 1993"

Electron transfer reactions in chemistry. Theory and experiment

Abstract: Since the late 1940s, the field of electron transfer processes has grown enormously in chemistry, electrochemistry, and biology. The development of the field, experimentally and theoretically, as well as its relation to the study of the other kinds of chemical reactions, presents an intriguing history in which many threads have been brought together. In this article, some history, recent trends, and my own involvement are described.

Effect of a nonzero temperature on quantum critical points in itinerant fermion systems

Abstract: I reexamine the work of Hertz on quantum phase transitions in itinerant fermion systems. I determine when it is permissible to integrate out the fermions and analyze the critical phenomena via an effective bosonic theory in which only fluctuations of the ordering field are explicitly retained. By solving appropriate scaling equations I obtain the different regimes of behavior of the correlation length and free energy in the disordered phase of the effective bosonic theory. The results in many cases differ from those of Hertz, but make contact with more recent work on the dilute Bose gas. I briefly discuss the relevance of the results to heavy-fermion materials.

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.

Quantum theory of field-quadrature measurements.

Abstract: The quantum effects on a cavity mode of the electromagnetic field caused by measuring one of its quadrature components is analyzed. We consider three measurement schemes: an intracavity quantum-nondemolition coupling to another mode, simple homodyne detection, and balanced homodyne detection. It is shown that, for suitable initial conditions, the first scheme has an effect which approaches that of a projective collapse of the state vector for long measurement times. However, the two homodyne schemes (which are shown to be equivalent for large local-oscillator amplitudes) do not approximate a projective measurement in any limit. In particular, it is shown that homodyne measurement cannot produce a squeezed state from a classical initial state. All three schemes are analyzed in terms of ‘‘quantum trajectories’’ which link measurement theory with stochastic quantum-jump processes.

Driving a quantum system with the output field from another driven quantum system.

Abstract: Quantum Langevin equations and a master equation are derived for a two-atom system in which the first atom is driven by coherent field, and the fluorescent light used to drive a second atom. We show that the light beams from both atoms are antibunched, and that they are mutually anticorrelated.

A finite-difference, time-domain solution for three-dimensional electromagnetic modeling

Abstract: We have developed a finite-difference solution for three-dimensional (3-D) transient electromagnetic problems. The solution steps Maxwell's equations in time using a staggered-grid technique. The time-stepping uses a modified version of the Du Fort-Frankel method which is explicit and always stable. Both conductivity and magnetic permeability can be functions of space, and the model geometry can be arbitrarily complicated. The solution provides both electric and magnetic field responses throughout the earth. Because it solves the coupled, first-order Maxwell's equations, the solution avoids approximating spatial derivatives of physical properties, and thus overcomes many related numerical difficulties. Moreover, since the divergence-free condition for the magnetic field is incorporated explicitly, the solution provides accurate results for the magnetic field at late times.An inhomogeneous Dirichlet boundary condition is imposed at the surface of the earth, while a homogeneous Dirichlet condition is employed along the subsurface boundaries. Numerical dispersion is alleviated by using an adaptive algorithm that uses a fourth-order difference method at early times and a second-order method at other times. Numerical checks against analytical, integral-equation, and spectral differential-difference solutions show that the solution provides accurate results.Execution time for a typical model is about 3.5 hours on an IBM 3090/600S computer for computing the field to 10 ms. That model contains 100 X 100 X 50 grid points representing about three million unknowns and possesses one vertical plane of symmetry, with the smallest grid spacing at 10 m and the highest resistivity at 100 Omega . m. The execution time indicates that the solution is computer intensive, but it is valuable in providing much-needed insight about TEM responses in complicated 3-D situations.

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.

Nearly incompressible fluids. II - Magnetohydrodynamics, turbulence, and waves

Abstract: The theory of nearly incompressible (NI) fluid dynamics developed previously for hydrodynamics is extended to magnetohydrodynamics (MHD). On the basis of a singular expansion technique, modified systems of fluid equations are derived for which the effects of compressibility are admitted only weakly in terms of the different possible incompressible solutions (thus ‘‘nearly incompressible MHD’’). NI MHD represents the interface between the compressible and incompressible magnetofluid descriptions in the subsonic regime. The theory developed here does not hold in the presence of very large thermal, gravitational, or field gradients. It is found that there exist three distinct NI descriptions corresponding to each of the three possible plasma beta (β ≡ the ratio of thermal to magnetic pressure) regimes (β≪1, β∼1, β≫1). In the β≫1 regime, the compressible MHD description converges in the low Mach number limit to the equations of classical incompressible three‐dimensional (3‐D) MHD. However, for the remaining plasma beta regimes, the imposition of a large dc magnetic field forces the equations of fully compressible 3‐D MHD to converge to the equations of 2‐D incompressible MHD in the low Mach number limit. The ‘‘collapse in dimensionality’’ corresponding to the different plasma beta regimes clarifies the distinction between the 3‐D and 2‐D incompressible MHD descriptions (and also that of 21/2‐D incompressible MHD). The collapse in dimensionality that occurs as a result of a decreased plasma beta can carry over to the weakly compressible corrections. For a β∼1 plasma, Alfven waves propagate parallel to the applied magnetic field (reminiscent of reduced MHD), while for a β≪1 magnetofluid, quasi‐1‐D long‐wavelength acoustic modes propagate parallel to the applied magnetic field. The detailed theory of weakly compressible corrections to the various incompressible MHD descriptions is presented and the implications for the solar wind emphasized.

Three-dimensional magnetotelluric modeling using difference equations­ Theory and comparisons to integral equation solutions

Abstract: We have developed an algorithm for computing the magnetotelluric response of three‐dimensional (3-D) earth models. It is a difference equation algorithm that is based on the integral forms of Maxwell’s equations rather than the differential forms. This formulation does not require approximating derivatives of earth properties or electromagnetic fields, as happens when using the second‐order vector diffusion equation. Rather, one must determine how averages are to be computed. Side boundary values for the H fields are obtained from putting two‐dimensional (2-D) slices of the model into a larger‐scale 2-D model and solving for the fields at the 3-D boundary positions. To solve the 3-D system of equations, we propagate an impedance matrix, which relates all the horizontal E fields in a layer to all the horizontal H fields in that same layer, up through the earth model. Applying a plane‐wave source condition and the side boundary H field values allows us to solve for the unknown fields within the model. The r...

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. >

On the generation of the large-scale and turbulent magnetic fields in solar-type stars

Abstract: It is thought that the large-scale solar-cycle magnetic field is generated in a thin region at the interface of the radiative core (RC) and solar convection zone (SCZ). We show that the bulk of the SCZ virogoursly generates a small-scale turbulent magnetic field. Rotation, while not essential, increases the generation rate of this field.

Inductance measurements of HTSC films with high critical currents

Abstract: Non-destructive inductance measurements of critical currents of thin superconducting YBa 2 Cu 3 O x films were carried out. In the framework of the critical state model, an exact analytical solution for current and field patterns on thin superconducting disks possessing pinning and oriented perpendicular to the external magnetic field was found. The problem was solved for both constant and periodically varying external fields. The solution allows one to describe the states of the film in weak field when there are wide regions free of vortices. The theoretical results were used to restore the temperature dependence of critical current density of HTSC films from their diamagnetic response.

Field measurements of the roughness of fault surfaces

Abstract: We recorded the height of a granitic fault surface as a function of position along one-dimensional profiles. We show that the profiles exhibit an "anisotropic" scaling invariance: self-affinity. The difference between the maximum and the minimum height, and the standard deviation of the height, over a length L are propor- tional to L½, where (  0.84. Other properties such as the Return Probability distribution or the Power Spectrum of the profile com- fort this result. This self-affne property is in good agreement with recent works on artificial fractured surfaces. Previous studies at field scale are consistent with this concept.

Testing the isotropic boundary algorithm method to evaluate the magnetic field configuration in the tail

Abstract: Simultaneous measurements of the low-altitude energetic particle flux by NOAA spacecraft and the geostationary magnetic field by GOES 2 spacecraft are used to test the recently proposed isotropic boundary algorithm (IBA) method to evaluate the instantaneous magnetospheric configuration. According to the IBA method, the equatorward boundary of the isotropic proton precipitation, in brief the isotropic boundary (IB), corresponds to the boundary separating adiabatic and chaotic regimes of particle motion in the tail current sheet and is controlled by the properties of the equatorial magnetic field. In this study we confirm some of the fundamental features of the IBA method. First, we show that the low-altitude IB position of 30- to 300-keV protons is strongly controlled by the equatorial magnetic field in the tail. (The corresponding correlation coefficient exceeds 0.9.) Second, the MLT dependence of the nightside IB latitude is in good agreement with that computed using magnetospheric models. Third, the observed magnetic field and the field predicted by the IBA method using the measured IB position have similar values and are well correlated with a correlation coefficient of at least 0.84 for the main components and a standard deviation of only about 10% of the dynamic range of these components. This shows that the threshold condition separating the two particle motion regimes is fulfilled in the proximity of the IB field line. We argue that the remaining inconsistencies between the calculated and observed magnetic fields are mainly due to the fact that the available magnetospheric models seem to underestimate the amount of tailward stretching of both the tail field lines during active conditions as well as field lines starting from the dayside. In view of its good capabilities to remotely determine the instantaneous magnetic field, we expect that the IBA method will find wide applications in the mapping of magnetic field lines and in testing of existing and new magnetospheric models.

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...

Elementary topology of two-dimensional turbulence from a Lagrangian viewpoint and single-particle dispersion

Abstract: We discuss a series of numerical experiments on the dispersion of neutrally buoyant particles in two-dimensional turbulent flows. The topology of two-dimensional turbulence is parametrized in terms of the relative dominance of deformation or rotation; this leads to a segmentation of the turbulent field into hyperbolic and elliptic domains. We show that some of the characteristic structural domains of two-dimensional turbulent flows, namely coherent structures and circulation cells, generate particle traps and peculiar accelerations which induce several complex properties of the particle dispersion processes at intermediate times. In general, passive particles are progressively pushed from the coherent structures and tend to concentrate in highly hyperbolic regions in the proximity of the isolines of zero vorticity. For large dispersion times, the background turbulent field is a privileged domain of particle richness; there is however a permanent particle exchange between the background field and the energetic circulation cells which surround the coherent structures. At intermediate times, an anomalous dispersion regime may appear, depending upon the relative weight of the different topological domains active in two-dimensional turbulence. The use of appropriate conditional averages allows the basic topology of two-dimensional turbulence to be characterized from a Lagrangian point of view. In particular, an intermediate dispersion law is quite robust and it can be observed under very general circumstances.

Analysis of antireflection-structured surfaces with continuous one-dimensional surface profiles.

Abstract: A novel thoretical treatment of antireflection-structured surfaces possessing general one-dimensional continuous profiles is presented. Closed-form solutions for the field reflection coefficients of these antireflection-structured surfaces are obtained through the use of effective medium theory and tapered transmission-line theory. Two specific surface profiles (sinusoidal and triangular) are analyzed in detail. Both the sinusoidal and triangular profiles are found to exhibit low reflectances over a broad range of angles and wavelengths. Results obtained with effective medium theory and transmission-line theory are compared with results obtained through the application of rigorous coupled-wave analysis.

Subpicosecond carrier transport in GaAs surface-space-charge fields.

Abstract: Above-band-gap pulsed optical excitation of electron-hole pairs within the surface-space-charge region of semiconductors alters the surface-space-charge field via free-carrier transport. We report on the direct observation of this ultrafast transient screening and the associated charge-carrier transport by applying reflective electro-optic sampling (REOS) with subpicosecond time resolution to (100)-oriented GaAs surfaces. The REOS measurements performed under different initial surface field conditions and various optical excitation densities are compared to numerical simulations of hot-carrier transport, including the calculation of the optical response. The simulations, which are based on a simple drift-diffusion model for optically excited electron-hole pairs, are in quantitative agreement with the experiment. The strength and sign of the static built-in field can be determined and the carrier drift velocities can be derived on a subpicosecond time scale.

Phase-field and sharp-interface alloy models.

Abstract: We consider a phase-field model of a binary mixture or alloy which has a phase boundary. The model identifies all macroscopic parameters and the interface thickness e. In the limit as e approaches zero, an alternative two-phase alloy solidification model (with a sharp interface) is obtained. For small concentrations, we recover the classical sharp-interface problems, the theory of which is reviewed

Vortex rings impinging on walls : axisymmetric and three-dimensional simulations

Abstract: Accurate numerical simulations of vortex rings impinging on flat boundaries revealed the same features observed in experiments. The results for the impact with a free-slip wall compared very well with previous numerical simulations that used spectral methods, and were also in qualitative agreement with experiments. The present simulation is mainly devoted to studying the more realistic case of rings interacting with a no-slip wall, experimentally studied by Walker et al. (1987). All the Reynolds numbers studied showed a very good agreement between experiments and simulations, and, at Rev > 1000 the ejection of a new ring from the wall was seen. Axisymmetric simulations demonstrated that vortex pairing is the physical mechanism producing the ejection of the new ring. Three-dimensional simulations were also performed to investigate the effects of azimuthal instabilities. These simulations have confirmed that high-wavenumber instabilities originate in the compression phase of the secondary ring within the primary one. The large instability of the secondary ring has been explained by analysis of the rate-of-strain tensor and vorticity alignment. The differences between passive scalars and the vorticity field have been also investigated.

Electrokinetic behaviour of colloidal particles in travelling electric fields: studies using yeast cells

Abstract: Suspensions of yeast cells (Saccharomyces cerevisiae) were used as model systems to investigate the electrokinetic behaviour of colloidal particles subjected to travelling electric fields generated using microelectrodes. Measurements were made over the frequency range 1 kHz to 10 MHz and for suspending medium conductivities in the range 6-260 mS m-1. A theoretical model is developed to provide a good description of the dependence of the observed translational motion, termed travelling-wave dielectrophoresis (TWD), on the dielectric properties of the particle and suspending medium, on the size of the particle, and on the magnitude and frequency of the applied field. Unlike conventional dielectrophoresis, the TWD effect is found to be related to the imaginary, rather than to the real, component of the induced dipole moment. Dielectrophoresis and electrorotation measurements were made to provide a further understanding of the observed effects and to support the theoretical model.

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.

Optical Control of Molecular Dynamics -- Liouville-Space Theory

Abstract: We lay some theoretical foundations to deal with the experimental realities faced in controlling molecular dynamics with tailored light fields: the nonideality of the light, the mixed rather than pure quantum-state nature of matter, and environmental and solvent effects. The optimal control of molecular dynamics using light fields is formulated in terms of the density matrix in Liouville space, generalizing existing wave-function-based formulations. This formulation allows the inclusion of mixed states, so that thermal and other nonpure quantum states of matter can be treated, as well as reduced descriptions useful in studying dense gas and condensed phases. In addition, it allows for general constraints and arbitrary coherent and partially coherent radiation fields and provides a unified picture for quantum, semiclassical, and classical molecular dynamics. For weak fields, the calculation simplifies and is given in terms of a molecular response function which itself does not depend upon the field. The solution of an eigenequation then directly gives the globally optimal field and the yield with respect to the target. As a demonstration, we explicitly consider a two electronic surface displaced harmonic oscillator molecular system in a Brownian oscillator solvent at finite temperatures, including nuclear and electronic solvation effects. Numerical illustrations are presented for the quantum control of thermal samples using phase-locked and random-phase light fields in the presence of solvent.

Weakly Non-Linear Gaussian Fluctuations and the Edgeworth Expansion

Abstract: We calculate the cosmological evolution of the 1-point probability distribution function (PDF), using an analytic approximation that combines gravitational perturbation theory with the Edgeworth expansion of the PDF. Our method applies directly to a smoothed mass density field or to the divergence of a smoothed peculiar velocity field, provided that rms fluctuations are small compared to unity on the smoothing scale, and that the primordial fluctuations that seed the growth of structure are Gaussian. We use this `Edgeworth approximation' to compute the evolution of $ $ and $ $; these measures are similar to the skewness and kurtosis of the density field, but they are less sensitive to tails of the probability distribution, so they may be more accurately estimated from surveys of limited volume. We compare our analytic calculations to cosmological N-body simulations in order to assess their range of validity. When $\sigma \ll 1$, the numerical simulations and perturbation theory agree precisely, demonstrating that the N-body method can yield accurate results in the regime of weakly non-linear clustering. We show analytically that `biased' galaxy formation preserves the relation $ \propto ^2$ predicted by second-order perturbation theory, provided that the galaxy density is a local function of the underlying mass density.