Showing papers in "Physical Review A in 1990"
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TL;DR: The quantum Zero effect is the inhibition of transitions between quantum states by frequent measurements of the state by means of pulses of light in an rf transition between two ground-state hyperfine levels.
Abstract: The quantum Zero effect is the inhibition of transitions between quantum states by frequent measurements of the state. The inhibition arises because the measurement causes a collapse (reduction) of the wave function. If the time between measurements is short enough, the wave function usually collapses back to the initial state. We have observed this effect in an rf transition between two $^{9}$${\mathrm{Be}}^{+}$ ground-state hyperfine levels. The ions were confined in a Penning trap and laser cooled. Short pulses of light, applied at the same time as the rf field, made the measurements. If an ion was in one state, it scattered a few photons; if it was in the other, it scattered no photons. In the latter case the wave-function collapse was due to a null measurement. Good agreement was found with calculations.
977 citations
TL;DR: Stochastic differential equations describing the Markovian evolution of state vectors in the quantum Hilbert space are studied as possible expressions of a universal dynamical principle and the stochastic evolution is proved to induce continuous dynamical reduction of the state vector onto mutually orthogonal subspaces.
Abstract: Stochastic differential equations describing the Markovian evolution of state vectors in the quantum Hilbert space are studied as possible expressions of a universal dynamical principle. The general features of the considered class of equations as well as their dynamical consequences are investigated in detail. The stochastic evolution is proved to induce continuous dynamical reduction of the state vector onto mutually orthogonal subspaces. A specific choice, expressed in terms of creation and annihilation operators, of the operators defining the Markov process is then proved to be appropriate to describe continuous spontaneous localization of systems of identical particles. The dynamics obtained in such a way leaves practically unaffected the standard quantum evolution of microscopic systems and induces a very rapid suppression of coherence among macroscopically distinguishable states. The classical behavior of macroscopic objects as well as the reduction of the wave packet in a quantum measurement process can be consistently derived from the postulated universal dynamical principle.
972 citations
TL;DR: The growth of ramified metallic deposits by electrodeposition from dilute salt solutions and in a high electric field has been considered in the geometry of a thin rectangular cell and it is found that ramified growth is a direct consequence of the creation of a space charge upon anion depletion in the vicinity of the cathode.
Abstract: The growth of ramified metallic deposits by electrodeposition from dilute salt solutions and in a high electric field has been considered in the geometry of a thin rectangular cell. The equations governing ion motion in the case of a dilute electrolyte have been solved numerically and analytically in a one-dimensional (1D) and a 2D approximation. It is found that ramified growth is a direct consequence of the creation of a space charge upon anion depletion in the vicinity of the cathode. The front of the ramified deposit is predicted to advance at a speed just equal to the velocity of the anions in the applied electric field. The presence of this space charge ahead of the growing front is associated with a potential drop \ensuremath{\delta}V. Resolution of the equations in the 2D case shows that the dense-parallel morphology of the deposit also results quite naturally from the existence of a space charge in the vicinity of the filament tips. The average filament spacing and sidebranch tilting angle can be directly related to the values of \ensuremath{\delta}V and of the applied electric field. The mechanism giving rise to the space charge will apply as well to any physical system involving electric conduction with two types of carriers, if one of them exhibits blocking-contact characteristics.
925 citations
TL;DR: Rheological studies on the elastic behavior of two types of boehmite alumina gels, Catapal and Dispal powders, showed the strong-link behavior and the results are in good agreement with the theoretical predictions.
Abstract: The scaling behavior of the elastic properties of colloidal gels that are well above the gelation threshold is studied both theoretically and experimentally. A scaling theory was developed by considering the structure of the gel network as a collection of flocs, which are fractal objects closely packed throughout the sample. Two regimes are found based on the relative value of the elastic constant of the interfloc links to that of the flocs. In the strong-link (interfloc) regime, the elastic constant of the gels increases but the limit of linearity decreases with increasing particle concentration, whereas in the weak-link regime both the elastic constant and the limit of linearity increase with increasing particle concentration. Rheological studies on the elastic behavior of two types of boehmite alumina gels, Catapal and Dispal powders, were performed. Both types of gels in the concentration range studied showed the strong-link behavior and the results are in good agreement with the theoretical predictions. The value of the fractal dimension of the flocs D\ensuremath{\simeq}1.95, deduced from the rheological measurements, is in agreement with the value D\ensuremath{\simeq}2.04 deduced from the static light-scattering measurements on dilute suspensions. Therefore the scaling theory we developed also enables us to extract structural information from the rheological measurements.
767 citations
TL;DR: A description of quantum systems at the time interval between two successive measurements is presented, and how this approach leads to a new concept: a weak value of an observable.
Abstract: A description of quantum systems at the time interval between two successive measurements is presented. Two wave functions, the first preselected by the initial measurement and the second postselected by the final measurement, describe quantum systems at a single time. It is shown how this approach leads to a new concept: a weak value of an observable. Weak values represent novel characteristics of quantum systems between two measurements. They are outcomes of a standard measurement procedure that fulfills certain requirements of ``weakness.'' We call it weak measurement. Physical meaning, underlying mathematical structure, and prospects of practical usage of weak measurements are explored.
701 citations
TL;DR: In this article, the authors report on molecular-dynamics simulations of Lennard-Jones liquids sheared between two solid walls and show that the degree of slip is directly related to the amount of structure induced in the fluid by the periodic potential from the solid walls.
Abstract: We report on molecular-dynamics simulations of Lennard-Jones liquids sheared between two solid walls. The velocity fields, flow boundary conditions, and fluid structure were studied for a variety of wall and fluid properties. A broad spectrum of boundary conditions was observed including slip, no-slip, and locking. We show that the degree of slip is directly related to the amount of structure induced in the fluid by the periodic potential from the solid walls. For weak wall-fluid interactions there is little ordering and slip was observed. At large interactions, substantial epitaxial ordering was induced and the first one or two fluid layers became locked to the wall. This epitaxial ordering was enhanced when the wall and fluid densities were equal. For unequal densities, high-order commensurate structures formed in the first fluid layer creating slip within the fluid.
638 citations
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TL;DR: It is shown that adhering (or bound) vesicles can exhibit a large variety of different shapes and the notion of a contact angle governed by tension is found to be applicable only for a restricted subset of these shapes.
Abstract: A simple model for the adhesion of vesicles to interfaces and membranes is introduced and theoretically studied. It is shown that adhering (or bound) vesicles can exhibit a large variety of different shapes. The notion of a contact angle governed by tension is found to be applicable only for a restricted subset of these shapes. Furthermore, the vesicle undergoes a nontrivial adhesion transition from a free to a bound state. This transition is governed by the balance between the overall bending and adhesion energies, and occurs even in the absence of shape fluctuations.
467 citations
TL;DR: In this article, the authors describe a theoretical model that shows how stimulated Brillouin scattering (SBS) is initiated by thermally excited acoustic waves distributed within a SBS-active medium, which predicts how the SBS reflectivity, Stokes linewidth, and fluctuations in Stokes intensity depend upon the laser intensity and upon the physical properties of the medium.
Abstract: We describe a theoretical model that shows how stimulated Brillouin scattering (SBS) is initiated by thermally excited acoustic waves distributed within a Brillouin-active medium. This model predicts how the SBS reflectivity, Stokes linewidth, and fluctuations in Stokes intensity depend upon the laser intensity and upon the physical properties of the SBS medium. This model also leads to the prediction that the value of the single-pass gain (i.e., G=gIL) at the threshold for SBS is not a universal number, but depends upon the laser frequency and on the properties of the SBS medium. For typical organic liquids at room temperature, G is in the range 20--25.
427 citations
TL;DR: It is shown that the master curves for three completely different colloids, gold, silica, and polystyrene, are indistinguishable, which demonstrates that reaction-limited colloid aggregation is universal, independent of the detailed chemical nature of the colloid system.
Abstract: We study slow, or reaction-limited, colloid aggregation (RLCA) with both static and dynamic light scattering and develop a self-consistent interpretation of the results. Static light scattering is used to determine the fractal dimension of the clusters and the cutoff mass of the power-law cluster-mass distribution. Using this same cutoff cluster mass, we can predict the shape of the temporal autocorrelation function measured by dynamic light scattering. Good agreement with experiments is obtained provided the effects of rotational diffusion are included. In addition, we determine the ratio of the hydrodynamic radius to the radius of gyration of individual RLCA clusters and find \ensuremath{\beta}=1.0. A scaling method is used for the q-dependent first cumulants of the temporal autocorrelation functions to obtain a single master curve for data obtained at different times in the aggregation process. The shape of this master curve is very sensitive to several key features of the process of reaction-limited colloid aggregation. It allows us to unambiguously determine the exponent for the power-law cluster-mass distribution, \ensuremath{\tau}=1.5\ifmmode\pm\else\textpm\fi{}0.05. Furthermore, we show that the master curves for three completely different colloids, gold, silica, and polystyrene, are indistinguishable. In addition, the fractal dimensions of their RLCA clusters, as measured by static light scattering, are all ${d}_{f}$=2.1\ifmmode\pm\else\textpm\fi{}0.05, while the aggregation kinetics for each colloid are exponential. This demonstrates that reaction-limited colloid aggregation is universal, independent of the detailed chemical nature of the colloid system.
425 citations
TL;DR: The heat flux (Nusselt number) as a function of Rayleigh number is deduced from the presence of a mean flow and the nesting of the thermal boundary layer within the viscous one.
Abstract: The heat flux (Nusselt number) as a function of Rayleigh number, ${\mathit{N}}_{\mathrm{Nu}}$\ensuremath{\approxeq}0.3${\mathit{N}}_{\mathrm{Ra}}^{2/7}$, is deduced from the presence of a mean flow and the nesting of the thermal boundary layer within the viscous one. The numerical coefficients are obtained from those known empirically for turbulent boundary layers. The consistency of our assumptions as a function of Prandtl number limits this regime to (${10}^{7}$--${10}^{8}$)${\mathit{N}}_{\mathrm{Pr}}^{5/3}$\ensuremath{\lesssim}${\mathit{N}}_{\mathrm{Ra}}$\ensuremath{\lesssim} (${10}^{13}$--${10}^{15}$)${\mathit{N}}_{\mathrm{Pr}}^{4}$. The Bolgiano-Obukhov ${\mathit{k}}^{\mathrm{\ensuremath{-}}7/5}$ spectrum for the temperature fluctuations is inconsistent with a simple scaling treatment of the equations.
412 citations
TL;DR: In this paper, a classification theorem is given for superintegrable potentials with invariants that are quadratic polynomials in the canonical momenta, and a complete list of all potentials for which the Hamilton-Jacobi equation is separable in more than one coordinate system is given.
Abstract: Superintegrable Hamiltonians in three degrees of freedom possess more than three functionally independent globally defined and single-valued integrals of motion. Some familiar examples, such as the Kepler problem and the harmonic oscillator, have been known since the time of Laplace. Here, a classification theorem is given for superintegrable potentials with invariants that are quadratic polynomials in the canonical momenta. Such systems must possess separable solutions to the Hamilton-Jacobi equation in more than one coordinate system. There are 11 coordinate systems for which the Hamilton-Jacobi equation separates in ${\mathit{openR}}^{3}$. One coordinate system may be arbitrarily rotated or translated with respect to the other, yielding 66 distinct cases. In each case, the differential equations for separability in the two coordinates are integrated to give a complete list of all superintegrable potentials with four or five quadratic integrals. The tables---which may be consulted independently of the main body of the paper---list the distinct superintegrable potentials, the separating coordinates, and the isolating integrals of the motion. If there exist five isolating integrals, then all finite classical trajectories are closed; if only four, then the trajectories are restricted to two-dimensional surface. An extraordinary consequence of the work is the discovery of perturbations to both the Kepler problem and the harmonic oscillator that do not destroy the fragile degeneracy. The perturbed systems still have five isolating integrals of the motion.
TL;DR: It is shown how it is possible, by means of a small parametric perturbation of suitable frequency, to bring the system to a regular regime in the dynamics of a Duffing-Holmes oscillator.
Abstract: Starting from a chaotic regime in the dynamics of a Duffing-Holmes oscillator, we show how it is possible, by means of a small parametric perturbation of suitable frequency, to bring the system to a regular regime. This situation is studied from the analytic point of view using the Melnikov method and from the numerical point of view computing Lyapunov exponents. The corresponding bounds for the perturbation are compared. Noting that the time, measured along the original unperturbed separatrix, that elapses between two successive homoclinic intersections grows when we approach the resonance, we propose a possible scenario for this type of regularization of the dynamics.
TL;DR: The magnitudes of the single-ionization-peak cross sections decrease monotonically across rows of the periodic table from group IIIA (Al,Ga,In) to group VIIIA (Ar,Kr,Xe), varying much more than predicted by various empirical formulas and classical and quantum-mechanical theories.
Abstract: Absolute electron-impact cross sections have been measured from 0 to 200 eV for single ionization of 16 atoms (Mg, Fe, Cu, Ag, Al, Si, Ge, Sn, Pb, P, As, Sb, Bi, S, Se, and Te) with an estimated accuracy of \ifmmode\pm\else\textpm\fi{}10%. Combined with our recent measurements of He, Ne, Ar, Kr, Xe, F, Cl, Br, I, Ga, and In [Wetzel et al., Phys. Rev. A 35, 559 (1987); Hayes et al., ibid. 35, 578 (1987); Shul, Wetzel, and Freund, ibid. 39, 5588 (1989)], a set of 27 atomic single-ionization cross sections has now been measured with the same apparatus. In addition, cross sections are reported for double ionization of ten atoms and triple ionization of eight atoms. The measurements are made by crossing an electron beam with a 3-keV beam of neutral atoms, prepared by charge-transfer neutralization of a mass-selected ion beam. The critical measurement of absolute neutral beam flux is made with a calibrated pyroelectric crystal. The magnitudes of the single-ionization-peak cross sections decrease monotonically across rows of the periodic table from group IIIA (Al,Ga,In) to group VIIIA (Ar,Kr,Xe), varying much more than predicted by various empirical formulas and classical and quantum-mechanical theories.
TL;DR: In this article, a complete orthonormal set of operators that can describe states of finite energy is introduced, and a generalization of the single-mode normal-ordering theorem is proved.
Abstract: We formulate the quantum theory of optical wave propagation without recourse to cavity quantization. This approach avoids the introduction of a box-related mode spacing and enables us to use a continuum frequency space description. We introduce a complete orthonormal set of operators that can describe states of finite energy. The set is countable and the operators have all the usual properties of the single-mode frequency operators. With use of these operators a generalization of the single-mode normal-ordering theorem is proved. We discuss the inclusion of material dispersion and pulse propagation in an optical fiber. Finally, we consider the process of photodetection in free space, concluding with a discussion of homodyne detection with both local oscillator and signal fields pulsed.
TL;DR: A self-consistent fluid model of radio-frequency glow discharges has been used to analyze the existence of two different discharge regimes and of the transition between them, and a realistic description of the electron kinetics has been obtained by considering separately two electron groups representing, respectively, the tail and the bulk of the electrons distribution function.
Abstract: A self-consistent fluid model of radio-frequency glow discharges has been used to analyze the existence of two different discharge regimes and the transition between them The existence of these regimes had been previously established by Levitskii [Sov Phys Tech Phys 2, 887 (1958)] The self-sustaining and power-deposition mechanisms that characterize each of these regimes are drastically different In the first regime, termed as the ``wave-riding regime'' corresponding to low discharge power, most of the power deposition is due to bulk plasma electrons heated by the sheath expansions In the second regime termed as the ``secondary electron regime'' corresponding to higher discharge power, the discharge is sustained mainly by electrons emitted by the electrodes under ion bombardment and avalanching in the sheath regions The numerical results are in good agreement with previous experimental measurements by Godyak and Kanneh [IEEE Trans Plasma Sci PS-14, 112 (1986)] The results presented in this paper form the first self-consistent description of these different regimes and of the transition between them The validity domain of the model is restricted to pressure higher than a fraction of Torr and frequency less than a few tens of MHz The gas being considered is helium and the discharge power varies between 0 and 700 mW ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ The model is based on solutions of electron and ion fluid equations describing charged particle transport coupled with Poisson's equation for the electric field A realistic description of the electron kinetics has been obtained by considering separately two electron groups representing, respectively, the tail and the bulk of the electron distribution function The validity of the two-electron group fluid model has been checked with Monte Carlo simulations
TL;DR: The equivalence of the two approaches to quantum interference in the presence of an environment is proven and dephasing of electron interference by photon modes in coherent and thermal states and by electromagnetic fluctuations in metals is proven.
Abstract: The problem of quantum interference in the presence of an environment is considered by two approaches. One treats the problem from the point of view of the trace left by the interfering particle on its environment. The other regards the phase accumulation of the interfering waves as a statistical process, and explains the loss of interference in terms of uncertainty in the relative phase. The equivalence of the two approaches is proven for the general case. The two approaches are applied to dephasing of electron interference by photon modes in coherent and thermal states, and to dephasing by electromagnetic fluctuations in metals.
TL;DR: A simplified version of the free-energy density functional for the inhomogeneous hard-sphere fluid mixture recently derived by Rosenfeld generates a triplet-direct-correlation function for the one-component fluid, which is in good agreement with Monte Carlo simulation results.
Abstract: We propose a simplified version of the free-energy density functional for the inhomogeneous hard-sphere fluid mixture recently derived by Rosenfeld [Phys. Rev. Lett. 63, 980 (1989)]. This new functional, which requires four distinct weight functions, generates a triplet-direct-correlation function for the one-component fluid, which is in good agreement with Monte Carlo simulation results. It also performs well in describing the density profile of the hard-sphere fluid in contact with hard and soft walls. Yet, it is not suitable for studying the freezing transition.
TL;DR: In this paper, a method to determine the resolvent of a quantum system coupled to a harmonic-oscillator bath is derived by extending the continued fraction theory of a Gaussian-Markovian bath that has been presented by Tanimura and Kubo.
Abstract: A method to determine the resolvent of a quantum system coupled to a harmonic-oscillator bath is derived by extending the continued-fraction theory of a Gaussian-Markovian bath that has been presented by Tanimura and Kubo [J. Phys. Soc. Jpn. 58, 101 (1989)]. The results are expressed in terms of continued fractions and apply to an oscillator bath with a general spectral density, corresponding to colored noise, at various temperatures. Exact values of the resolvent can be calculated for arbitrary strength of the system-bath interaction by making use of the convergence properties of the continued fractions. For the weak-interaction case these results agree with the quantum master equation. The physical meaning of the results is also discussed by a diagrammatic method. As an application, the result of the Gaussian-Markovian system is extended to the case of the low-temperature bath. Correlated (unfactorized) initial conditions are also discussed.
TL;DR: In this paper, the photon number distribution is shown to display unusual oscillations which are interpreted as interference in phase space, analogous to Franck-Condon oscillations in molecular spectra, and the possibility of detecting these oscillations is discussed, through the photodetection counting statistics of the displaced number states.
Abstract: Recent developments in quantum optics have led to new proposals to generate number states of the electromagnetic field using conditioned measurement techniques or the properties of atom-field interactions in microwave cavities in the micromaser. The number-state field prepared in such a way may be transformed by the action of a displacement operator; for the microwave micromaser state this could be implemented by the action of a classical current that drives the cavity field. We evaluate some properties of such displaced number states, especially their description in phase space. The photon number distribution is shown to display unusual oscillations, which are interpreted as interference in phase space, analogous to Franck-Condon oscillations in molecular spectra. The possibility of detecting these oscillations is discussed, through the photodetection counting statistics of the displaced number states. We show that the displaced-number-state quantum features are relatively robust when dissipation of the field energy is included.
TL;DR: In this article, a nonlinear one-dimensional theory is developed that describes some important aspects of intense laser-plasma interactions, including nonlinear plasma wake-field generation, relativistic optical guiding, coherent harmonic radiation production, as well as other related phenomena.
Abstract: A nonlinear one-dimensional theory is developed that describes some important aspects of intense laser-plasma interactions. The self-consistent laser-plasma analysis includes nonlinear plasma wake-field generation, relativistic optical guiding, coherent harmonic radiation production, as well as other related phenomena. Relativistic optical guiding is found to be most effective for long laser pulses having slow rise times. Short laser pulses are shown to be weakly guided. Coherent harmonic generation using a linearly polarized laser is found to be most efficient for short laser pulses and can be enhanced by the presence of large amplitude plasma wake fields. Aspects of particle acceleration by laser pulses as well as possible methods for upshifting the frequency of laser pulses are also discussed.
TL;DR: Two-dimensional reaction-diffusion equations with simple reaction kinetics are used to study the dynamics of spiral waves in excitable media and it is shown that this transition occurs via a supercritical Hopf bifurcation and there is no frequency locking within the quasiperiodic regime.
Abstract: Two-dimensional reaction-diffusion equations with simple reaction kinetics are used to study the dynamics of spiral waves in excitable media. Detailed numerical results are presented for the transition from simple (periodic) rotation to compound (quasiperiodic) rotation of spiral waves. It is shown that this transition occurs via a supercritical Hopf bifurcation and that there is no frequency locking within the quasiperiodic regime.
TL;DR: The found relations between the sheath characteristics show that the rf sheath capacitance and the equivalent sheath resistance, corresponding to ion acceleration losses, are practically independent of the rF voltage and the discharge current.
Abstract: A self-consistent dynamic model for rf sheaths in the frequency range between the ion and electron plasma frequencies is developed and solved for arbitrary collision parameters and arbitrary rf sheath voltages. For floating dc sheaths with no rf voltage, and for collisionless and highly collisional rf sheaths at high rf voltages, the obtained solutions for the dc sheath voltage and the capacitive sheath width converge to the known limits. However, for rf voltages of tens and hundreds of volts, usually encountered in rf discharge applications, our results differ from those obtained using high voltage approximations and are in good agreement with the experiment. The found relations between the sheath characteristics show that the rf sheath capacitance and the equivalent sheath resistance, corresponding to ion acceleration losses, are practically independent of the rf voltage and the discharge current.
TL;DR: A formalism is developed to obtain two different types of nearest-neighbor probability density functions and closely related quantities, such as their associated cumulative distributions and conditional pair distributions, for many-body systems of {ital D}-dimensional spheres.
Abstract: The probability of finding a nearest neighbor at some given distance from a reference point in a many-body system of interacting particles is of importance in a host of problems in the physical as well as biological sciences. We develop a formalism to obtain two different types of nearest-neighbor probability density functions ({ital void} and {ital particle} probability densities) and closely related quantities, such as their associated cumulative distributions and conditional pair distributions, for many-body systems of {ital D}-dimensional spheres. For the special case of impenetrable (hard) spheres, we compute low-density expansions of each of these quantities and obtain analytical expressions for them that are accurate for a wide range of sphere concentrations. Using these results, we are able to calculate the mean nearest-neighbor distance for distributions of {ital D}-dimensional impenetrable spheres. Our theoretical results are found to be in excellent agreement with computer-simulation data.
TL;DR: This paper addresses the question: How is the complex eigenvalue changed when the cavity is perturbed by a small change of dielectric constant, and generalizes the work of Zeldovich for scalar fields in one dimension, expressing the change of frequency in terms of matrix elements involving the unperturbed eigenfunctions, so that the problem is reduced to quadrature.
Abstract: Laser cavities are open systems, in that energy can leak to the outside via output coupling. The ``normal modes'' are therefore quasinormal modes, with eigenvalues that are complex and eigenfunctions that extend outside the cavity, such that any normalization integral is dominated by the region outside; in short, such systems are non-Hermitian. This paper addresses the question: How is the complex eigenvalue (i.e., the mode frequency) changed when the cavity is perturbed by a small change of dielectric constant? The usual time-independent perturbation theory fails because of non-Hermiticity. By generalizing the work of Zeldovich [Sov. Phys.---JETP 12, 542 (1961)] for scalar fields in one dimension, we express the change of frequency in terms of matrix elements involving the unperturbed eigenfunctions, so that the problem is reduced to quadrature. We then apply the formalism to shape perturbations of a dielectric microdroplet, and give analytic formulas for the frequency shifts of the morphology-dependent resonances. These results are, surprisingly, independent of the radial wave function, so that all integrals can be performed and explicit algebraic expressions are given for axially symmetric perturbations.
TL;DR: In this article, the authors explore three methods to account for fast, nonequilibrium electrons: the single-beam method, the multibeam method, and particle (Monte Carlo) simulations.
Abstract: Mathematical models of dc glow discharges sustained by electrons emitted by the cathode and accelerated into the cathode fall must take into account the highly nonequilibrium nature of these fast electrons. However, the electric field profile through the discharge is determined mainly by the distribution of ions and slow electrons. In this paper we explore three methods to account for fast, nonequilibrium electrons: the single-beam method, the multibeam method, and particle (Monte Carlo) simulations. Ions and cold electrons are treated using equations of change assuming collisionally dominated motion (i.e., drift and diffusion), and the self-consistent electric field is determined by solving these equations simultaneously with Poisson's equation. Creation rates for ions and slow electrons are obtained from the fast-electron models. Simulation results indicate that, although the single-beam model is qualitatively correct, it is hampered by its sensitivity to assumptions in the numerical approach, and its tendency to predict negative voltage-current characteristics at low pressures and high voltages, which are not evident in results from the higher-order multibeam model. Although an improvement over the single-beam model, comparison with experimental optical-emission measurements reveals that the multibeam model predicts excitation profiles that extend too far into the discharge. Accurate comparisons are possible with particle simulations, which incorporate angular scattering of fast electrons.
TL;DR: In this paper, a modification of Di\`osi's model is proposed, which overcomes the difficulties and retains partially its appealing features, allowing the derivation from microdynamics of wave-packet reduction, and accounts for the emergence of definite macroscopic properties for macroobjects.
Abstract: A continuous-reduction model implying the dynamical suppression of linear superpositions of macroscopically distinguishable states, presented recently by Di\`osi [Phys. Rev. A 40, 1165 (1989)], is investigated. The model exhibits appealing features; in particular, it relates reduction to gravity and contains no constants besides Newton's gravitational constant G. It turns out, however, that the model is not fully consistent. A slight modification of this model is proposed, which overcomes the difficulties and retains partially its appealing features. The resulting model deals with systems containing distinguishable or identical constituents, allows the derivation from microdynamics of wave-packet reduction, and accounts for the emergence of definite macroscopic properties for macro-objects. Reduction is related to gravity in the same way as in Di\`osi's model, but a fundamental length must be introduced to avoid inconsistencies.
TL;DR: In this paper, the evolution of a Raman loss compensated soliton in an optical fiber was studied using a singular perturbation expansion, and the analytical results agreed quite well with the numerical results of Mollenauer, Gordon, and Islam.
Abstract: Using a singular perturbation expansion, we study the evolution of a Raman loss compensated soliton in an optical fiber. Our analytical results agree quite well with the numerical results of Mollenauer, Gordon, and Islam [IEEE J. Quantum Electron. QE-22, 157 (1986)]. However, there are some differences in that our theory predicts an additional structure that was only partially seen in the numerical calculations. Our analytical results do give a quite good qualitative and quantitative check of the numerical results.
TL;DR: In this article, it was shown that the microscopic origin of the time dependence can be understood in terms of coherent exciton-exciton interactions, and that this behavior is more general and should be seen in numerous dense media where strong nonlinear interactions of polarizations occur.
Abstract: Time-resolved four-wave-mixing experiments are usually interpreted in terms of noninteracting two-level systems in order to obtain information on the polarization dephasing time ${\mathit{T}}_{2}$. Recent experiments involving excitonic resonances in semiconductor quantum wells (including results presented in this paper) show striking qualitative deviations from this simple picture. In particular, an exponential tail is observed at low excitation for negative time delays. At high excitation, the four-wave-mixing signal is found to evolve into two distinct temporal maxima. We demonstrate that the microscopic origin of this time dependence can be understood in terms of coherent exciton-exciton interactions. We show in fact that this behavior is more general and should be seen in numerous dense media where strong nonlinear interactions of polarizations occur. In addition to presenting rigorous numerical results, we analyze two simple situations in which such interactions exist: dielectric media with local-field effects and the anharmonic oscillator. We derive analytical expressions for their time-dependent four-wave-mixing response and discuss the physical origin of these new nonlinear-optical effects.
TL;DR: In this article, the stationary states of the kinetic Ising model described by the Glauber stochastic dynamics and subject to a time-dependent oscillating external field were analyzed within a mean-field approach.
Abstract: We analyze within a mean-field approach the stationary states of the kinetic Ising model described by the Glauber stochastic dynamics and subject to a time-dependent oscillating external field We have found that the magnetization of the system oscillates in time around a certain value that is zero at high temperatures or large field amplitudes and nonzero at low temperatures and small field amplitudes The transition from one regime to the other, which corresponds to a spontaneous symmetry breaking, is found to be continuous for sufficiently small values of the field amplitudes For higher values the transition becomes discontinuous and the system exhibits a dynamical tricritical point
TL;DR: In this paper, the authors present a systematic experimental study of the dissociation and ionization of molecular hydrogen in intense laser fields at three different wavelengths: 1.064 µm, 532 µm and 355 µm.
Abstract: We present a systematic experimental study of the dissociation and ionization of molecular hydrogen in intense laser fields at three different wavelengths: 1.064 \ensuremath{\mu}m, 532 nm, and 355 nm. The light intensity ranges from ${10}^{13}$ W/${\mathrm{cm}}^{2}$ to ${10}^{15}$ W/${\mathrm{cm}}^{2}$. Measurements include electron spectra and angular distributions, kinetic-energy spectra and angular distributions of ion fragments, and ion dissociation fractions. We report a number of interesting and novel phenomena, including the following: above-threshold ionization to various vibrational states in the molecular ion, production of very-high-energy electrons from ionization at 1.064 \ensuremath{\mu}m, enhanced dissociation of the ${\mathrm{H}}_{2}^{+}$ molecular ion via the mechanism of bond softening, above-threshold dissociation, and evidence for the alignment of the molecule in a high-intensity laser field.