Presence Of External Field Research Articles

Co double-layer nanostructures on Pt(111) studied by spin-polarized scanning tunnelling microscopy

Spin-resolved scanning tunnelling microscopy measurements were performed for Co double-layer nanostructures on Pt(1 1 1). High-resolution spectroscopy shows that the electronic structure is highly inhomogeneous due to dislocations induced by the lattice mismatch between Pt and Co. Spin-polarized measurements allow a magnetic state dependent determination of out-of-plane magnetized islands. In the presence of external fields we have observed two different types of Co double-layer islands which exhibit a surprisingly large coercivity depending on their preparation.

The Influence of Electric Field on the Global and Local Reactivity Descriptors: Reactivity and Stability of Weakly Bonded Complexes

The response of the global and local reactivity density-based descriptors (chemical potential, hardness, softness, Fukui function, and local softness) in the presence of external electric field has been studied for some of the simple prototype molecular systems. In addition to the analysis on the reactivity of these systems, the influence of the electric field on the interaction energy of the complexes formed by these systems has also been studied using the recently proposed semiquantitative model based on the local hard-soft acid-base principle. By using the inverse relationship between the global hardness and softness parameters, a simple relationship is obtained for the variation of hardness in terms of the Fukui function under the external electric field. It is shown that the increase in the hardness values for a particular system in the presence of external field does not necessarily imply that the reactivity of the system would be deactivated or vice versa.

Ordering transition and demixing of a binary mixture of thick and thin rodlike molecules in the presence of an external field

The effect of an external field (electric/magnetic) on the phase behavior of the binary mixture of very long thick and thin rodlike particles is studied. Both the thick and thin particles possess positive but different susceptibility anisotropics (Delta alpha). The difference in the extent of interaction between the external field and the two species is varied by means of a coupling parameter (l = Delta alpha(thick)/Delta alpha(thin)). Isotropic-nematic phase transition and demixing phase transitions taking place both in the isotropic and nematic phases are examined as a function of field strength on the level of the second virial theory of Onsager in the range of 0 < l <1. The approximate sixth order Legendre polynomial expansion method is used to represent the excluded volume interaction between the rodlike particles. It is found that the isotropic phase becomes weakly nematic (paranematic) in the presence of external field and the field orients both components in the direction of the field even if the field does not have direct interaction with the thick component (l = 0). Analytical expressions are derived for the external field induced order parameters and birefringence. The increasing field destabilizes both types of demixing transitions (isotropic-isotropic and nematic-nematic) and the paranematic-nematic phase transition. Moreover it induces closed loop immiscibility, and upper and lower critical points terminating the paranematic-nematic phase coexistence may occur for low values of the coupling parameter. It is interesting that while the phase boundaries of the paranematic-paranematic demixing and the paranematic-nematic transitions are very sensitive to the value of the coupling parameter at low pressures, the paranematic-nematic and nematic-nematic phase boundaries are practically independent of the coupling parameter at high pressures.

The dynamics of activated surface diffusion

An elementary process occurring on surfaces is diffusion. The dynamics is simplest whenthe concentration of adsorbates is sufficiently small that interaction between adsorbatescan be ignored. But even for this tracer diffusion process, much remains to beuncovered. Here, we present the interplay between experimental measurement of tracerdiffusion and its theoretical interpretation, which leads to good estimates of theinteraction of the adparticle with the surface. We show how the results fromthree different experimental techniques—field ion microscopy, scanning tunnellingmicroscopy and quasielastic helium atom scattering—can be interpreted. Usingthe generalized Langevin equation as a model for the diffusion dynamics, weshow how the turnover theory for activated diffusion may be used to describe themeasured time evolution of the adparticle distribution on the surface. The differentactivation energy measured for hopping over single or double lattice lengths is shownto come from the added energy loss to the surface, as the particle moves overthe longer path. We discuss some of the issues which are not yet clear; theseinclude quantum effects, such as the quantum suppression of diffusion, vibrationallyassisted diffusion, multidimensional effects and diffusion in the presence of externalfields.

Thermal activation in Permalloy nanorectangles at room temperature

The effect of thermal activation in planar Permalloy nanorectangles of thickness L z = 2.5 nm , width L y = 20 nm and different in plane aspect ratios ( r = L x / L y ) has been analyzed assuming uniform magnetization in the sample at room temperature. The relaxation times were computed by integrating the Langevin equation in presence of external fields that reduce the energy barrier separating the two energy minima. From these data we obtain the pre-exponential factor in the Arrhenius formula for each aspect ratio. That allows us to estimate the relaxation time in the absence of external field. A minimum aspect ratio of r = 3.25 is required for a lifetime larger than 100 years, which is a commonly accepted stability criterion.

The condition of mechanical equilibrium on the surface of a nonuniform thin film

The condition of internal mechanical equilibrium of a curved surface layer is derived, and its application to practically important cases of incomplete formation of the surface layer (as is sometimes the case, for example, in thin films) is considered. The notion of a local disjoining pressure is introduced, and the equilibrium condition for a variable-thickness thin film is obtained; this condition is valid both in the absence and in the presence of external fields. The cases of a wedge-shaped film, cylindrical film, spherical film, and transition zone of a wetting film are analyzed.

External Fields and Color Confinement

U(1), SU(2), and SU(3) lattice gauge theories in presence of external fields are investigated both in (3+1) and (2+1) dimensions. The free energy of gauge systems has been measured. While the phase transition in compact U(1) is not influenced by the strength of an external constant magnetic field, the deconfinement temperature for SU(2) and SU(3) gauge systems in a constant abelian chromomagnetic field decreases when the strength of the applied field increases. The dependence of the deconfinement temperature on the strength of an external constant chromomagnetic field seems to be a peculiar feature of non abelian gauge theories.

Exact lattice calculations of dispersion coefficients in the presence of external fields and obstacles.

We present a study of the field-dependent dispersion coefficient of point-like particles in various 2D overdamped systems with obstructions (periodic, percolating, and trapping distributions of obstacles). These calculations profit from the synthesis of a newly proposed Monte Carlo algorithm--the first such algorithm that correctly reproduces the free dispersion coefficient in the presence of finite external fields--and an asymptotically exact calculation technique. The resulting method efficiently produces algebraic and numerical results without the need to actually perform Monte Carlo simulations. When compared to such simulations, our exact method features a negligible computational cost and exponentially small errors. Utilizing the power of this numerical method, we engage in comprehensive parametric analysis of several model systems, revealing very subtle effects that would otherwise be swamped by statistical errors or incur prohibitive computational costs. The unified framework presented here serves as a template for further applications of lattice random-walk models of biased diffusion.

Hydrogen bonding and molecular mobility in liquid water in external electromagnetic fields

Nonequilibrium molecular dynamics simulations of water have been performed in the isothermal–isobaric ensemble in the presence of external electromagnetic fields of varying intensity in the microwave to far-infrared frequency range, using a rigid/polarizable and a flexible/nonpolarizable potential model, from 260 to 400 K. Significant alterations in molecular mobility and hydrogen bonding patterns were found vis-à-vis zero-field conditions. In addition, the influence of the isothermal–isobaric ensemble on these observations was gauged by means of comparison with pure Newtonian simulation findings in the presence of external fields, and the former results were in reasonable accord with the latter.

Equilibrium Problems for Potentials with External Fields

We investigate the problem on the minimum of energy over fairly general (generally speaking, noncompact) classes of real-valued Radon measures associated with a system of sets in a locally compact space in the presence of external fields. The classes of admissible measures are determined by a certain normalization or by a normalization and a certain majorant measure σ. In both cases, we establish sufficient conditions for the existence of minimizing measures and prove that, under fairly general assumptions, these conditions are also necessary. We show that, for sufficiently large σ, there is a close correlation between the facts of unsolvability (or solvability) of both variational problems considered.

Real and imaginary mass generation in the presence of external fields and axions

Small fluctuations around a constant electric or constant magnetic field F are analyzed in a theory with pseudo scalar [phi] with a coupling g[phi]F~F. It is found that a magnetic external field leads to mass generation for the small perturbations, while an electric field suffers from a tachyonic mass generation in the case in which the field strength is higher than a critical value (related to the pseudo scalar mass). The vacuum energy can be exactly evaluated and it is found that an imaginary part is present when the external electric field exceeds its critical value.

Quantum mechanical equations of motion of particles and spin in polarized media

Quantum mechanical equations of motion are obtained for particles and spin in media with polarized electrons in the presence of external fields. The motion of electrons and their spins is governed by the exchange interaction, while the motion of positrons and their spin is governed by the annihilation interaction. For particles with spin S ≥ 1, second-order terms in spin are taken into account. The equations obtained can be applied to describe the motion of particles and spin both in magnetic and nonmagnetic media.

A geometrical derivation of the Dirac equation

We give a geometrical derivation of the Dirac equation by considering a spin-1/2 particle travelling with the speed of light in a cubic spacetime lattice. The mass of the particle acts to flip the multi-component wavefunction at the lattice sites. Starting with a difference equation for the case of one spatial and one time dimensions, we generalize the approach to higher dimensions. Interactions with external electromagnetic and gravitational fields are also considered. One logical interpretation is that only at the lattice sites is the spin-1/2 particle aware of its mass and the presence of external fields.

Reversible Decoherence of a Mesoscopic Superposition Drivenby a Time-Dependent External Field

By using the normal-ordering technique in the coherent staterepresentation, we discuss the dynamical evolution of the coherenceof a Schrödinger cat state in a high Q cavity coupled to anotherresonator in the presence of external fields. During the progressionof the coherence, we pay particular attention to the effect of theexternal fields, as well as to the initial phase of the Schrödingercat state.

Instanton and Monopole in External Chromomagnetic Fields

We study properties of instanton and monopole in an external chromomagnetic field. Generally, the 't Hooft ansatz is no longer a solution of the Yang-Mills field equation in the presence of external fields. Therefore, we investigate a stabilized instanton solution with minimal total Yang-Mills action in a nontrivial topological sector. With this aim, we consider numerical minimization of the action with respect to the global color orientation, the anisotropic scale transformation and the local gauge-like transformation starting from a simple superposed gauge field of the 't Hooft ansatz and the external color field. Here, the external color field is, for simplicity, chosen to be a constant Abelian magnetic field along a certain direction. Then, the 4-dimensional rotational symmetry O(4) of the instanton solution is reduced to two 2-dimensional rotational symmetries $O(2)\times O(2)$ due to the effect of a homogeneous external field. In the space $\mib{R}^{3}$ at fixed $t$, we find a quadrupole deformation of this instanton solution. In the presence of a magnetic field $\vec{H}$, a prolate deformation occurs along the direction of $\vec{H}$. Contrastingly, in the presence of an electric field $\vec{E}$ an oblate deformation occurs along the direction of $\vec{E}$. We further discuss the local correlation between the instanton and the monopole in the external field in the maximally Abelian gauge. The external field affects the appearance of the monopole trajectory around the instanton. In fact, a monopole and anti-monopole pair appears around the instanton center, and this monopole loop seems to partially screen the external field.

Monopoles, confinement and deconfinement in lattice compact QED in (2+1)D with external fields

Finite temperature compact electrodynamics in (2+1) dimensions is studied in the presence of external electromagnetic fields. The deconfinement temperature is found to be insensitive to the external fields. This result corroborates our observation that external fields create additional small—size magnetic dipoles from the vacuum which do not spoil the confining properties of the model at low temperature. However, the Polyakov loop is not an order parameter of confinement. It can vanish in deconfinement in the presence of external field. This does not mean the restoration of confinement for certain external field fluxes. As a next step in the study of (2+1)D QED, the influence of monopoles on the photon propagator is studied. First results are presented showing this connection in the confining phase (without external field).

The Complete Condition of Mechanical Equilibrium at a Curved Nonspherical Surface

A simple procedure for deriving the complete condition of mechanical equilibrium at a curved nonspherical surface, valid both in the absence and in the presence of external fields, is described. The condition is written in the vector form, no less compact than the Laplace formula, and one of its components generalizes this formula.

Dielectric properties of La-modified antiferroelectric PbZrO 3 thin films

Lead zirconate is a room temperature antiferroelectric material, which exhibits double polarization hysteresis characteristic in the presence of external field. A transformation from antiferroelectric to paraelectric behavior through a ferroelectric phase has been exhibited upon addition of La-to pure lead zirconate thin films, which were confirmed from dielectric, hysteresis and pyroelectric and micro Raman analyses. Thin films of pure and La-modified lead zirconate were processed using A-site vacancy formula Pb (l−1.5 x) La X ZrO 3 by pulsed excimer laser ablation technique. Pyroelectric coefficient increased gradually from 55 to 190 μC cm −2 K −1 with the addition of 0 to 9 mole% of La to pure lead zirconate. The transition temperature has been reduced to room temperature (∼34 °C) with addition of a 9 mole% of La-to lead zirconate. Presence of antiferroelectricity in pure lead zirconate thin films is due to antiparallel shifts of Pb-ions in (110) direction and reduction of transition temperature is attributed to creation of A-site vacancies would have caused enhancing of ferro-and paraelectric behavior with La-addition.

Electro-optic Behavior of Opal-LC Photonic Crystals

In this paper, we describe the electro-optic effects of photonic crystals made of a synthetic opal filled with a nematic liquid crystal(LC). By applying an external electric field, a shift in the Bragg reflection peak position(stop band) and a field-induced change in its peak reflectivity are observed. These significant surface alignment effects of the opal-LC composite are discussed in a similar manner for Freederick-type transitions of LC within a confined geometry in the presence of external fields.

Energy Levels in GaAs Low-Dimensional Systems: Effects of Electric and Magnetic Fields

We investigate the electronic structure of cylindrical pills of GaAs low-dimensional systems, under the action of a magnetic field applied in the axial direction and an electric field with varying orientations. Effects of the competition between the magnetic radial confinement and the electric-field-induced polarization on the e1–hh1 transition energy are studied in detail. The results show that the energies of electron and hole states depend sensitively on the dot radius and the relative direction of the electric field to the magnetic field. These aspects must be taken into account in the understanding of optical phenomena related to the energy spectrum of carriers in the presence of external fields and the design of materials and devices in low-dimensional systems.