Showing papers on "Field (physics) published in 1998"

A Bond Path: A Universal Indicator of Bonded Interactions

Abstract: The quantum mechanics of proper open systems yields the physics that governs the local behavior of the electron density, ρ(r). The Ehrenfest force F(r) acting on an element of ρ(r) and the virial field ν(r) that determine its potential energy are obtained from equations of motion for the electronic momentum and virial operators, respectively. Each is represented by a “dressed” density, a distribution in real space that results from replacing the property in question for a single electron with a corresponding density that describes its average interaction with all of the remaining particles in the system. All bond paths, lines of maximum density linking neighboring nuclei in a system in stable electrostatic equilibrium, have a common physical origin in terms of F(r) and ν(r), regardless of the nature of the interaction. Each is homeomorphically mirrored by a virial path, a line of maximally negative potential energy density linking the same nuclei. The presence of a bond path and its associated virial path...

Discrete Spatial Optical Solitons in Waveguide Arrays

Abstract: We report the observation of discrete spatial optical solitons in an array of 41 waveguides. Light was coupled to the central waveguide. At low power, the propagating field spreads as it couples to more waveguides. When sufficient power was injected, the field was localized close to the input waveguides and its distribution was successfully described by the discrete nonlinear Schr\"odinger equation.

Optimized δ expansion for relativistic nuclear models

Abstract: The optimized δ-expansion is a nonperturbative approach for field theoretic models which combines the techniques of perturbation theory and the variational principle This technique is discussed in the λφ4 model and then implemented in the Walecka model for the equation of state of nuclear matter The results obtained with the δ expansion are compared with those obtained with the traditional mean field, relativistic Hartree and Hartree-Fock approximations

Critical exponents of the N-vector model

Abstract: Recently the series for two renormalization group functions (corresponding to the anomalous dimensions of the fields and ) of the three-dimensional field theory have been extended to next order (seven loops) by Murray and Nickel. We examine the influence of these additional terms on the estimates of critical exponents of the N-vector model, using some new ideas in the context of the Borel summation techniques. The estimates have slightly changed, but remain within the errors of the previous evaluation. Exponents such as (related to the field anomalous dimension), which were poorly determined in the previous evaluation of Le Guillou-Zinn-Justin, have seen their apparent errors significantly decrease. More importantly, perhaps, summation errors are better determined. The change in exponents affects the recently determined ratios of amplitudes and we report the corresponding new values. Finally, because an error has been discovered in the last order of the published expansions (order ), we have also re-analysed the determination of exponents from the -expansion. The conclusion is that the general agreement between -expansion and three-dimensional series has improved with respect to Le Guillou-Zinn-Justin.

Langevin-dynamics study of the dynamical properties of small magnetic particles

Abstract: The stochastic Landau-Lifshitz-Gilbert equation of motion for a classical magnetic moment is numerically solved (properly observing the customary interpretation of it as a Stratonovich stochastic differential equation), in order to study the dynamics of magnetic nanoparticles. The corresponding Langevin-dynamics approach allows for the study of the fluctuating trajectories of individual magnetic moments, where we have encountered remarkable phenomena in the overbarrier rotation process, such as crossing-back or multiple crossing of the potential barrier, rooted in the gyromagnetic nature of the system. Concerning averaged quantities, we study the linear dynamic response of the archetypal ensemble of noninteracting classical magnetic moments with axially symmetric magnetic anisotropy. The results are compared with different analytical expressions used to model the relaxation of nanoparticle ensembles, assessing their accuracy. It has been found that, among a number of heuristic expressions for the linear dynamic susceptibility, only the simple formula proposed by Shliomis and Stepanov matches the coarse features of the susceptibility reasonably. By comparing the numerical results with the asymptotic formula of Storonkin {Sov. Phys. Crystallogr. 30, 489 (1985) [Kristallografiya 30, 841 (1985)]}, the effects of the intra-potential-well relaxation modes on the low-temperature longitudinal dynamic response have been assessed, showing their relatively small reflection in the susceptibility curves but their dramatic influence on the phase shifts. Comparison of the numerical results with the exact zero-damping expression for the transverse susceptibility by Garanin, Ishchenko, and Panina {Theor. Math. Phys. (USSR) 82, 169 (1990) [Teor. Mat. Fiz. 82, 242 (1990)]}, reveals a sizable contribution of the spread of the precession frequencies of the magnetic moment in the anisotropy field to the dynamic response at intermediate-to-high temperatures.

Determination of piezoelectric fields in strained GaInN quantum wells using the quantum-confined Stark effect

Abstract: We have identified piezoelectric fields in strained GaInN/GaN quantum well p-i-n structures using the quantum-confined Stark effect. The photoluminescence peak of the quantum wells showed a blueshift with increasing applied reverse voltages. This blueshift is due to the cancellation of the piezoelectric field by the reverse bias field. We determined that the piezoelectric field points from the growth surface to the substrate and its magnitude is 1.2 MV/cm for Ga0.84In0.16N/GaN quantum wells on sapphire substrate. In addition, from the direction of the field, the growth orientation of our nitride epilayers can be determined to be (0001), corresponding to the Ga face.

Representation of electromagnetic fields over arbitrary surfaces by a finite and nonredundant number of samples

Abstract: It is shown that the electromagnetic (EM) field, radiated or scattered by bounded sources, can be accurately represented over a substantially arbitrary surface by a finite number of samples even when the observation domain is unbounded. The number of required samples is nonredundant and essentially coincident with the number of degrees of freedom of the field. This result relies on the extraction of a proper phase factor from the field expression and on the use of appropriate coordinates to parameterize the domain. It is demonstrated that the number of degrees of freedom is independent of the observation domain and depends only on the source geometry. The case of spheroidal sources and observation domains with rotational symmetry is analyzed in detail and the particular cases of spherical and planar sources are explicitly considered. For these geometries, precise and fast sampling algorithms of central type are presented, which allow an efficient recovery of EM fields from a nonredundant finite number of samples. Such algorithms are stable with respect to random errors affecting the data.

Ultrahigh‐Intensity Lasers: Physics of the Extreme on a Tabletop

Abstract: Over the past ten years, laser intensities have increased by more than four orders of magnitude to reach enormous intensities of 1020 W/cm2. The field strength at these intensities is on the order of a teravolt per centimeter, or a hundred times the Coulombic field binding the ground state electron in the hydrogen atom. The electrons driven by such a field are relativistic, with an oscillatory energy of 10 MeV. At these intensities, the light pressure, P = I/c, is extreme, on the order of giga‐ to terabars. The laser interacting with matter—solid, gas, plasma—generates high‐order harmonics of the incident beam up to the 3 nm wavelength range, energetic ions or electrons with mega‐electron‐volt energies (figure 1), gigagauss magnetic fields and violent accelerations of 1021 g (g is Earth's gravity). Finally, the interaction of an ultraintense beam with superrelativistic particles can produce fields approaching the critical field in which an electron gains in one Compton wavelength an energy equal to twice ...

Assisted inflation

Abstract: In inflationary scenarios with more than one scalar field, inflation may proceed even if each of the individual fields has a potential too steep for that field to sustain inflation on its own We show that scalar fields with exponential potentials evolve so as to act cooperatively to assist inflation, by finding solutions in which the energy densities of the different scalar fields evolve in fixed proportion Such scaling solutions exist for an arbitrary number of scalar fields, with different slopes for the exponential potentials, and we show that these solutions are the unique late-time attractors for the evolution We determine the density perturbation spectrum produced by such a period of inflation, and show that with multiple scalar fields the spectrum is closer to the scale-invariant than the spectrum that any of the fields would generate individually

Underlying physics of the thermochemical e model in describing low-field time-dependent dielectric breakdown in sio2 thin films

Abstract: The underlying physics behind the success of the thermochemical E model in describing time-dependent dielectric breakdown (TDDB) in SiO2 thin films is presented. Weak bonding states can be broken by thermal means due to the strong dipolar coupling of intrinsic defect states with the local electric field in the dielectric. This dipole-field coupling serves to lower the activation energy required for thermal bond-breakage and accelerates the dielectric degradation process. A temperature-independent field acceleration parameter γ and a field-independent activation energy ΔH can result when different types of disturbed bonding states are mixed during TDDB testing of SiO2 thin films. While γ for each defect type alone has the expected 1/T dependence and ΔH shows a linear decrease with electric field, a nearly temperature-independent γ and a field-independent ΔH can result when two or more types of disturbed bonding states are mixed. The good agreement between long-term TDDB data and the thermochemical model su...

Strong coupling phenomena in quantum microcavity structures

Abstract: The physics of strong coupling phenomena in semiconductor quantum microcavities is reviewed This is a relatively new field with most important developments having occurred in the last 5 years We describe how such microcavities enable both electronic and photonic properties of semiconductors, and the interaction between them, to be controlled in the same structure The resulting coupled exciton-photon eigenstates, cavity polaritons, have many interesting properties including very low mass for small in-plane wavevectors, and can be studied easily and directly in optical experiments, unlike exciton-polaritons in bulk semiconductors A wealth of new optical phenomena has been reported in this field in the last few years This review describes the most important of these phenomena We discuss the reasons why polaritons have fundamentally different properties in microcavities as compared with those in bulk materials or quantum wells We explain the factors which control the strength of the exciton-photon coupling and how the resulting optical spectra can be modelled We then emphasize, in the main body of the review, the particularly important results of reflectivity experiments at both normal and oblique angles of incidence, the effects of external electric and magnetic fields, the results of coherent Raman scattering experiments, the effects of disorder on microcavity spectra, including the observation of motional narrowing over the exciton disorder potential, studies of coupled microcavities, and photoluminescence and time-resolved phenomena

New models of Jupiter's magnetic field constrained by the Io flux tube footprint

Abstract: Spherical harmonic models of the planetary magnetic field of Jupiter are obtained from in situ magnetic field measurements and remote observations of the position of the foot of the Io flux tube in Jupiter's ionosphere. The Io flux tube (IFT) footprint locates the ionospheric footprint of field lines traced from Io's orbital radial distance in the equator plane (5.9 Jovian radii). The IFT footprint is a valuable constraint on magnetic field models, providing “ground truth” information in a region close to the planet and thus far not sampled by spacecraft. The magnetic field is represented using a spherical harmonic expansion of degree and order 4 for the planetary (“internal”) field and an explicit model of the magnetodisc for the field (“external”) due to distributed currents. Models fitting Voyager 1 and Pioneer 11 magnetometer observations and the IFT footprint are obtained by partial solution of the underdetermined inverse problem using generalized inverse techniques. Dipole, quadrupole, octupole, and a subset of higher-degree and higher-order spherical harmonic coefficients are determined and compared with earlier models.

Modification of the Landau-Lifshitz equation in the presence of a spin-polarized current in colossal- and giant-magnetoresistive materials

Abstract: We derive a continuum equation for the magnetization of a conducting ferromagnet in the presence of a spin-polarized current. Current effects enter in the form of a topological term in the Landau-Lifshitz equation. In the stationary situation the problem maps onto the motion of a classical charged particle in the field of a magnetic monopole. The spatial dependence of the magnetization is calculated for a one-dimensional geometry and suggestions for experimental observation are made. We also consider time-dependent solutions and predict a spin-wave instability for large currents.

Effects of weak magnetic fields on free radical recombination reactions

Abstract: The radical pair mechanism is used to elucidate how applied magnetic fields that are weaker in strength than typical hyperfine interactions can influence the yields and kinetics of recombination reactions of free radicals in solution. The so-called low field effect is shown to arise from coherent superpositions of degenerate electron-nuclear spin states in a spin-correlated radical pair in zero field. A weak applied magnetic field causes these (zero-quantum) coherences to oscillate, leading to coherent interconversion of singlet and triplet electronic states of the radical pair and hence changes in the yields of recombination products and of the free radicals that escape into solution. For singlet geminate radical pairs, the low field effect leads to a boost in the concentration of free radicals, which may be relevant in the context of in vivo biological effects of electromagnetic fields. Using analytical approaches in limiting cases, the maximum possible low field effects are calculated for a variety of ...

A Time-Dependent Three-Dimensional Magnetohydrodynamic Model of the Coronal Mass Ejection

Abstract: We present a theoretical magnetohydrodynamic (MHD) model describing the time-dependent expulsion of a three-dimensional coronal mass ejection (CME) out of the solar corona. The model relates the white-light appearance of the CME to its internal magnetic field, which takes the form of a closed bubble, filled with a partly anchored, twisted magnetic flux rope, and embedded in an otherwise open background field. The model is constructed by solving in closed form the time-dependent ideal MHD equations for a γ = 4/3 polytrope making use of a similarity assumption and the application of a mathematical stretching transformation in order to treat a complex field geometry with three-dimensional variations. The density distribution frozen into the expanding CME magnetic field is obtained. The scattered white light integrated along the line of sight shows the conspicuous three features often associated with CMEs as observed with white-light coronagraphs: a surrounding high-density region, an internal low-density cavity, and a high-density core. We also show how the orientation of this three-dimensional structure relative to the line of sight can give rise to a variety of different geometric appearances in white light. These images generated from a CME model in a realistic geometry offer an opportunity to directly compare theoretical predictions on CME shapes with observations of CMEs in white light. The mathematical methods used in the model construction have general application and are described in the Appendices.

Instant Preheating

Abstract: We describe a new efficient mechanism of reheating. Immediately after rolling down the rapidly moving inflaton field $\phi$ produces particles $\chi$, which may be either bosons or fermions. This is a nonperturbative process which occurs almost instantly; no oscillations or parametric resonance is required. The effective masses of the $\chi$ particles may be very small at the moment when they are produced, but they ``fatten'' when the field $\phi$ increases. When the particles $\chi$ become sufficiently heavy, they rapidly decay to other, lighter particles. This leads to an almost instantaneous reheating accompanied by the production of particles with masses which may be as large as $10^{17}-10^{18}$ GeV. This mechanism works in the usual inflationary models where $V(\phi)$ has a minimum, where it takes only a half of a single oscillation of the inflaton field $\phi$, but it is especially efficient in models with effective potentials slowly decreasing at large $\phi$ as in the theory of quintessence.

Primordial hypermagnetic fields and triangle anomaly

Abstract: The high-temperature plasma above the electroweak scale $\ensuremath{\sim}100\mathrm{GeV}$ may have contained a primordial hypercharge magnetic field whose anomalous coupling to the fermions induces a transformation of the hypermagnetic energy density into fermionic number. In order to describe this process, we generalize the ordinary magnetohydrodynamical equations to the anomalous case. We show that a not completely homogeneous hypermagnetic background induces fermion-number fluctuations, which can be expressed in terms of a generic hypermagnetic field configuration. We argue that, depending upon the various particle physics parameters involved in our estimate (electron Yukawa coupling, strength of the electroweak phase transition) and upon the hypermagnetic energy spectrum, sizable matter-antimatter fluctuations can be generated in the plasma. These fluctuations may modify the predictions of the standard big bang nucleosynthesis (BBN). We derive constraints on the magnetic fields from the requirement that the homogeneous BBN is not changed. We analyze the influence of primordial magnetic fields on the electroweak phase transition and show that some specific configurations of the magnetic field may be converted into net baryon number at the electroweak scale.

Dynamics of a ferromagnetic domain wall: Avalanches, depinning transition, and the Barkhausen effect

Abstract: We study the dynamics of a ferromagnetic domain wall driven by an external magnetic field through a disordered medium. The avalanchelike motion of the domain walls between pinned configurations produces a noise known as the Barkhausen effect. We discuss experimental results on soft ferromagnetic materials, with reference to the domain structure and the sample geometry, and report Barkhausen noise measurements on Fe21Co64B15 amorphous alloy. We construct an equation of motion for a flexible domain wall, which displays a depinning transition as the field is increased. The long-range dipolar interactions are shown to set the upper critical dimension to dc53, which implies that mean-field exponents~with possible logarithmic correction! are expected to describe the Barkhausen effect. We introduce a mean-field infinite-range model and show that it is equivalent to a previously introduced single-degree-of-freedom model, known to reproduce several experimental results. We numerically simulate the equation in d53, confirming the theoretical predictions. We compute the avalanche distributions as a function of the field driving rate and the intensity of the demagnetizing field. The scaling exponents change linearly with the driving rate, while the cutoff of the distribution is determined by the demagnetizing field, in remarkable agreement with experiments.@S0163-1829~98!08833-X#

Transfer of angular spectrum and image formation in spontaneous parametric down-conversion

Abstract: We show that the two-photon state generated in the process of spontaneous parametric down-conversion in a thin crystal carries information about the angular spectrum of the pump beam. This information transfer allows one to control the transverse correlation properties of the down-converted fields by manipulating the pump field, with consequences for a broad class of experiments. The effect is demonstrated theoretically and experimentally, in connection with the formation of fourth-order images by the down-converted beams.

Effective Field Theory for Short-Range Forces

Abstract: The method of effective field theories (EFTs) is developed for the scattering of two particles at wavelengths which are large compared to the range of their interaction. It is shown that the renormalized EFT is equivalent to the effective range expansion, to a Schroedinger equation with a pseudo-potential, and to an energy expansion of a generic boundary condition at the origin. The roles of regulators and potentials are also discussed. These ideas are exemplified in a toy model.

The Magnetic Topology of Solar Eruptions

Abstract: We present an explanation for the well-known observation that complexity of the solar magnetic field is a necessary ingredient for strong activity such as large eruptive flares. Our model starts with the standard picture for the energy build up -- highly-sheared, newly-emerged magnetic field near the photospheric neutral line held down by overlying unsheared field. Previously, we proposed the key new idea that magnetic reconnection between the unsheared field and neighboring flux systems decreases the amount of overlying field and, thereby, allows the low-lying sheared flux to ``break out'' (Antiochos, DeVore and Klimchuk 1998, ApJ, submitted). In this paper we show that a bipolar active region does not have the necessary complexity for this process to occur, but a delta sunspot has the right topology for magnetic breakout. We discuss the implications of these results for observations from SOHO and TRACE.

Electromagnetically induced grating: Homogeneously broadened medium

Abstract: A strong coupling standing wave, interacting with three-level \ensuremath{\Lambda}-type (or ladder-type) atoms, can diffract a weak probe field (propagating along a direction normal to the standing wave) into high-order diffractions, a phenomenon which we name electromagnetically induced grating (EIG). We develop in this work a theory for studying EIG in a homogeneously broadened medium consisting of three-level \ensuremath{\Lambda}-type atoms. We show that by taking advantage of the absorption and dispersion properties of electromagnetically induced transparency one can create an atomic grating that can effectively diffract light into the first-order direction.

Fractional Statistics and Quantum Theory

Abstract: Fractional statistics in two dimensions quantum mechanics of anyons statistical mechanics of anyon gas fractional exclusion statistics introduction to Chern-Simons term anyon as soliton in field theories Chern-Simons field theories mean field approach to anyons and beyond anyons and fractional quantum Hall effect omitted topics.