Local Field Configuration Research Articles

Localization of elastic deformation in strongly anisotropic, porous, linear materials with periodic microstructures: Exact solutions and dilute expansions

Exact solutions are derived for the problem of a two-dimensional, infinitely anisotropic, linear-elastic medium containing a periodic lattice of voids. The matrix material possesses either one infinitely soft, or one infinitely hard loading direction, which induces localized (singular) field configurations. The effective elastic moduli are computed as functions of the porosity in each case. Their dilute expansions feature half-integer powers of the porosity, which can be correlated to the localized field patterns. Statistical characterizations of the fields, such as their first moments and their histograms are provided, with particular emphasis on the singularities of the latter. The behavior of the system near the void close-packing fraction is also investigated. The results of this work shed light on corresponding results for strongly non-linear porous media, which have been obtained recently by means of the “second-order” homogenization method, and where the dilute estimates also exhibit fractional powers of the porosity.

Dielectrophoretic tweezers using sharp probe electrode

The pick and place ability, precise positioning and high spatial resolution of end-effector can be defined as the functional requirements of dielectrophoretic tweezers. The tweezers can manipulate objects in any direction with the strength of dielectrophoretic force. A localized and 3D movable electric field configuration is proposed here and analyzed for the functional requirements of dielectrophoretic tweezers. To achieve a steeply focused field, an electrochemical machining method was developed for a sharp probe electrode, and the developed dielectrophoretic tweezers was used to manipulate cells and beads.

Local field distribution and configuration of CO molecules adsorbed on the nanostructure platinum surface

This paper shows that the local electric field distribution near the nanostructure metallic surface is obtained by solving the Laplace equation, and furthermore, the configuration of CO molecules adsorbed on a Pt nanoparticle surface is obtained by using Monte Carlo simulation. It is found that the uneven local electric field distribution induced by the nanostructure surface can influence the configuration of carbon monoxide (CO) molecules by a force, which drags the adsorbates to the poles of the nanoparticles. This result, together with our results obtained before, may explain the experimental results that the nanostructure metallic surface can lead to abnormal phenomena such as anti-absorption infrared effects.

Spontaneous skyrmion ground states in magnetic metals

Since the 1950s, Heisenberg and others have addressed the problem of how to explain the appearance of countable particles in continuous fields. Stable localized field configurations were searched for an ingredient for a general field theory of elementary particles, but the majority of nonlinear field models were unable to predict them. As an exception, Skyrme succeeded in describing nuclear particles as localized states, so-called 'skyrmions'. Skyrmions are a characteristic of nonlinear continuum models ranging from microscopic to cosmological scales. Skyrmionic states have been found under non-equilibrium conditions, or when stabilized by external fields or the proliferation of topological defects. Examples are Turing patterns in classical liquids, spin textures in quantum Hall magnets, or the blue phases in liquid crystals. However, it has generally been assumed that skyrmions cannot form spontaneous ground states, such as ferromagnetic or antiferromagnetic order, in magnetic materials. Here, we show theoretically that this assumption is wrong and that skyrmion textures may form spontaneously in condensed-matter systems with chiral interactions without the assistance of external fields or the proliferation of defects. We show this within a phenomenological continuum model based on a few material-specific parameters that can be determined experimentally. Our model has a condition not considered before: we allow for softened amplitude variations of the magnetization, characteristic of, for instance, metallic magnets. Our model implies that spontaneous skyrmion lattice ground states may exist generally in a large number of materials, notably at surfaces and in thin films, as well as in bulk compounds, where a lack of space inversion symmetry leads to chiral interactions.

Q-BALLS CONSTRUCTED OF SPINORS IN LAGRANGIANS WITH SU(2) SYMMETRY

In the present work we investigate the existence and stability properties of q-balls which consist of a couple of scalar fields, forming an SU (2) doublet in a Lagrangian with a global SU (2) symmetry. We find that these spinors can form a localized and stable field configuration, if they rotate in their internal SU (2) space. We find the energy and charge of the soliton in both thin and thick-wall approximation and we prove its stability against decaying to free particles. We also find the asymptotic forms of the scalar and gauge field and the energy and charge of the configuration when the SU (2) symmetry is local. The only assumption is the smallness of the coupling constant g. Using numerical methods we prove the stability of the q-ball in the local case.

Long-lived oscillons from asymmetric bubbles: Existence and stability

The possibility that extremely long-lived, time-dependent, and localized field configurations (``oscillons'') arise during the collapse of asymmetrical bubbles in 2+1 dimensional phi^4 models is investigated. It is found that oscillons can develop from a large spectrum of elliptically deformed bubbles. Moreover, we provide numerical evidence that such oscillons are: a) circularly symmetric; and b) linearly stable against small arbitrary radial and angular perturbations. The latter is based on a dynamical approach designed to investigate the stability of nonintegrable time-dependent configurations that is capable of probing slowly-growing instabilities not seen through the usual ``spectral'' method.

Energetic neutral atom imaging at low altitudes from the Swedish microsatellite Astrid: Observations at low (≤10 keV) energies

Intense (≤106 cm−2 sr−1 s−1) fluxes of upflowing energetic neutral atoms (ENAs) from the polar cap have been observed in the energy range ∼0.1–10 keV (hydrogen assumed) by the ENA imager PIPPI (Prelude In Planetary Particle Imaging) onboard the Astrid satellite at 1000 km altitude. If a source altitude of 400 km is assumed, the ENA emissions maps to magnetic latitudes 70°–85°extending from dusk over nightside. There were no measurements in the midnight sector below approximately 80°. It is shown that the emissions cannot originate from an isotropically emitting source, which rules out any optical emissions. Local charged particle detection is ruled out on the basis of no correlation with local electron measurements and local magnetic field configuration. The ENA emissions can be explained by a conically pitch angle distributed ion source emitting ENAs in the 110°–140°pitch angle interval (source altitude 400 km). It is suggested, with support from low altitude rocket observations that the ENAs are produced by low altitude (400–600 km) ion conies of several hundreds eV charge exchanging with the upper atmosphere.

Metastable ringlike semitopological solitons.

We show the existence of new stable ringlike localized scalar field configurations whose stability is due to a combination of topological and nontopological charges. In that sense these defects may be called semitopological. These rings are Noether charged and also carry Noether current (they are superconducting). They are local minima of the energy in scalar field theories with an unbroken U(1) global symmetry. We obtain numerical solutions of the field configuration corresponding to large rings and derive virial theorems demonstrating their stability. We also derive the minimum energy field configurations in 3D and simulate the evolution of a finite size Q ring on a three dimensional lattice.

Chern–Simons structure of solitons in the model of the discrete (2+1)D nonlinear Schrödinger equation

The structure of solitons in the model of gauged nonlinear Schrödinger equation on (2+1)D lattice and their contribution to the value of the number of particles are considered. It is shown that in the region of small values of the Chern–Simons coefficient k, there exists a universal attraction between localized field configurations. The universal topological character of the Chern–Simons correlations is determined by the degree k of the braiding of the excitation world lines. The attraction due to the Chern–Simons correlations has been obtained under assumption of the discretization of the temporal variable. For k=2, this phenomenon may be a dynamic origin of the semion pairing in planar systems.

Long-lived localized field configurations in small lattices: application to oscillons

Long-lived localized field configurations such as breathers, oscillons, or more complex objects naturally arise in the context of a wide range of nonlinear models in different numbers of spatial dimensions. We present a numerical method, which we call the adiabatic damping method, designed to study such configurations in small lattices. Using three-dimensional oscillons in straight phi(4) models as an example, we show that the method accurately (to one part in 10(5) or better) reproduces results obtained with static or dynamically expanding lattices, dramatically cutting down in integration time. We further present results for two-dimensional oscillons, whose lifetimes would be prohibitively long to study with conventional methods.

Magnetic field and plasma properties associated with pressure pulses and magnetic reconnection at the dayside magnetopause

Most of the models for the formation of magnetic flux transfer events (FTEs) at the dayside magnetopause (MP) are based on magnetic reconnection. However, it has been suggested that a wavy magnetopause motion caused by solar wind pressure variations might provide an alternative explanation. Using two‐dimensional MHD simulations, we study the dynamics and the local field and plasma signatures which are caused by the interaction of enhanced pressure regions with a current layer. The results are compared with two‐dimensional magnetic reconnection processes. The magnetopause motion due to the enhanced pressure agrees qualitatively with the suggested model. The earthward or sunward displacement of the magnetopause is driven by the interaction of a fast mode wave with the current sheet. However, the variation of the magnetic field component normal to the current layer BN is largely monopolar and for the considered cases typically smaller than for the reconnection cases. In addition, the magnetic field signatures depend strongly on the propagation direction of the boundary wave relative to the local magnetic field configuration. The normal magnetic field signature reverses sign across the magnetopause for large magnetic shear and propagation along the direction of antiparallel field components. In the reconnection case, the bipolar BN signature has the same polarity on both sides of the current sheet. Differences between magnetic reconnection and pressure driven boundary motions are manifold and systematic differences are summarized which can be used to test statistical ensembles of such events.

Estimate of quantum corrections to the mass of the chiral soliton in the Nambu-Jona-Lasinio model

The Bethe-Salpeter equation for pion fluctuations off the chiral soliton in the Nambu-Jona-Lasinio model is constructed. By Goldstone's theorem this equation has rotational and translational zero-modes because the classical soliton is a localized stationary field configuration which violates rotational and translational invariance. Furthermore, the proper normalization of the fluctuating eigenmodes is obtained. Second quantization of the pion fluctuations off the chiral soliton provides an energy functional of the pion fluctuations which formally coincides with that of a harmonic oscillator. The corresponding quantum corrections to the soliton mass together with the semi-classical cranking prescription yield reasonable predictions for the masses of the nucleon and the Δ-resonance when the constituent quark mass is chosen to be about 400 MeV. These calculations are, to some extent, hampered by the non-confining character of the Nambu-Jona-Lasinio model. Comments on the 1/ N C counting scheme are added.

Topological quasi-charges in the Einstein theory of teleparallelism and particle combinatorics

A Riemann space with absolute parallelism is considered. Geometry is given by the field of n-frames h μ a . It is shown that for a localized field configuration h one can define a topological charge — an element of the homotopy group Gm= πm(SOm), m=n−1. The most interesting case of m=6, G6=0, is considered in detail. Even though in this case there is no topological charge, for configurations symmetrical in κ dimensions, one can define a quasi-charge — an element of the group G6−κ. The groups G3, G4, G5 may correspond to elementary particles (quasiparticles) of the photon, electron, and neutrino types, accordingly. The combinatorics emerging from this scheme is compared with the observation.

Impedance methods for surface state electrons

A numerical procedure is given to determine the static charge configuration and electrical impedance of a circular pool of electrons on the surface of liquid helium. The response for an ac Corbino probe is obtained for arbitrary frequency. By fitting the results to audiofrequency capacitance-conductance data the local charge density and electric field configuration are found for arbitrary confining potentials. The locations of2D plasma modes are also in excellent agreement with those obtained numerically. Analytic forms are derived for uniform charge density configurations.

Dc Detection of Spin Resonance in Thin Metallic Films

Ohm's law in strongly magnetic, though not necessarily ferromagnetic materials, has leading terms in the presence of a magnetization M and an induction B of the form E=ρJ−R0J×B−R1J×M+Δρ/M2(J·M)M+…,which represent the ordinary and extraordinary Hall effects and a magnetoresistance. The M-dependent terms, in particular, describe interactions due to polarized scattering centers and to the left-right asymmetry of polarized mobile charge carriers.1 An electromagnetic wave interacting with such a material will, under resonance conditions, cause a precession of M, while at the same time inducing an eddy current density J. According to Eq. (1), the presence of two such terms at microwave frequencies introduces local dc electric fields related to the time average of the various products. These fields will be largest near spin resonance. Such fields are observable only in films thin compared to the electromagnetic skin depth,2 and they may be used either to study spin resonance in very thin films or to determine their transport properties at microwave frequencies. In particular, for a configuration in which the static field is in the film plane and the electromagnetic wave is at normal incidence 〈J×B〉 has no contribution in the film plane; such an experiment allows a separation of R0 and R1 even in materials which show no magnetic saturation. The detailed electromagnetic theory of this configuration has been worked out3 for an isotropic material and experiments have been carried out on nickel films to test the various features of the theory, and to determine the usefulness of the method in studying high-frequency magnetic and conduction properties in thin films.4 The experimental arrangement consists of placing the sample close to a wave guide or cavity wall, but insulated from it, and measuring the dc voltages, either steady or modulated, induced by the microwaves. The major precaution necessary for a quantitative study is that the microwave field pattern within the film must be well known, so that sample geometry, its electrical boundary conditions, and placement of detection wires is very critical. The experimental results reproduce all the qualitative features of the theory, with regard to sample thickness, local field configuration, power, and magnetic field orientation. The agreement with the theory is better than reported by Seavey5 for Permalloy films, most likely because of differences in local field configuration, and by Heinz and Silber6 for a ferrite, whose experiment did not permit separation of all parameters. A quantitative analysis of the data7 gives consistent values of the magnetic resonance parameters g, M0, and τ. It also requires that the films be described at microwave frequencies by a complex conductivity which is generally smaller than at low frequencies. Within the over-all accuracy of the analysis—about 15%—the magnetoresistance Δρ is the same as at low frequencies, and the increase of R1 is related to that of ρ, so that no dispersion of these constants is observed. The same method has been applied to thin films of gadolinium, and while the signals are much smaller, the dc effects clearly persist in the paramagnetic region above room temperature.8 Quantitative analysis of the data has been impeded because the power levels necessary to observe the signals cause considerable heating of the sample. Since both the magnetic and galvanomagnetic properties of gadolinium are very temperature sensitive, a careful determination of the local temperature within the film during the microsecond long pulse of microwaves is required. The data obtained so far indicate that the g value of our gadolinium films is higher than in bulk, and that the extraordinary Hall constant appears to be larger than in bulk material.

Electromagnetic Theory of dc Effects in Ferromagnetic Resonance

Effects of magnetoresistance and extraordinary Hall effect on the propagation of electromagnetic waves through very thin sheets of ferromagnetic conductors are studied. These contributions to the conductivity produce electric fields varying like products of current and magnetization and introduce nonlinear interactions, which are particularly large in the neighborhood of ferromagnetic resonance. Detailed first-order dc electric fields are obtained for the configuration of a very thin ferromagnetic sheet very close to a conducting wall. The results predict signals strictly proportional to incident microwave power, and mostly inversely proportional to film thickness, becoming independent of thickness for the thinnest films. Their magnetic field or frequency dependence gives typical resonance curves modulated by slowly varying amplitudes. The maxima and zeros of these amplitudes are very sensitive to the local field configuration. Once this latter is well known, however, these effects can be employed to study conveniently the high-frequency conduction and magnetic properties of the thinnest films.