Modified multicomponent band pass optical reflection filters at oblique incidence

Reflection and transmission characteristics of a modified long-wavelength cutoff filter using a three-component system with an angular dependence are studied. In this case, a layer of high refractive index is sandwiched between edge layers of equivalent refractive index with the base structure [G(0.5AB0.5A)]^N; then, an optimized structure of the form [(0.5AB0.5A)^N?X (0.5AB0.5A)^X (0.5AB0.5A)^N?X] is taken and its spectral properties are studied. In the case of a short-wavelength three-component cutoff filter, a layer of low refractive index is sandwiched between two edge layers of high equivalent refractive index with a base structure [G(0.5BA0.5D)]^N and its spectral characteristics are studied. Then, the structure is optimized [(0.5BA0.5B)^{N? X}(0.5BA0.5B)^X(0.5BA0.5B)^N?tX] and spectral characteristics of this structure are also studied. Spectral properties of band pass optical reflection filters of the type [G(XY)A] whose components are the proposed long-wavelength cutoff [(0.5AB0.5B)^{N? X}(0.5AB0.5A)^X(0.5BA0.5B)^N?X] and short-wavelength cutoff [(0.5BA0.5B)^N?X(0.5BA0.5B)^X(0.5BA0.5B)^N?X] filter structures are studied with angular dependences. No increase in the bandwidth with the least oscillatory components at the edges can be precisely formed by known classes of a two-component system at normal incidence. It is shown that the use of reflection and transmission filters with oblique bands yields an ideal polarizer. Angular dependences of the filter and the transmission and reflection band shapes on the parameters of a high-reflectance multilayer coating, such as the thickness, refractive indices, and the number of layers, are analyzed in detail.     

Pattern formation in the reaction with anisotropic lateral adsorbate-adsorbate interactions

We present Monte Carlo simulations of the formation of (1×2) islands in the case of the 2 A + B ₂ → 2 AB reaction occurring via the Langmuir-Hinshelwood mechanism on a square lattice under steady-state conditions. The model employed takes into account the effect of anisotropic lateral B-B interactions on the rates of B diffusion and elementary reaction events. The results obtained with qualitatively realistic ratio of the rate of elementary reaction steps indicate that the average island size depends on the details of diffusion and reaction dynamics in a similar way as in the earlier studied case of the simplest A + B reaction running via the Eley-Rideal mechanism. Received 4 January 2002 and Received in final form 2 April 2002 Published online 25 June 2002     

Resonant tunneling effect in one-dimensional twinned lattice photonic crystal under total reflection conditions

Under total reflection conditions, it typically seems as though light waves will be reflected completely on the interface; in actuality, the waves can penetrate the medium as evanescent waves. In this paper, we present a twinned lattice photonic crystal with a unit cell composed of AB layers and their mirror. We assume that the refractive index n ₀ of the input and output end is equal to n _B and larger than n _A . We first demonstrate the dependence of band structure on the incidence angle and normalized wavelength, in which the resonant tunneling bands are exposed. We then draw a comparison of bands between ABBA and AB. To conclude, we discuss the resonant tunneling effect in the twinned lattice photonic crystal under the total reflection conditions. As incidence angle increases, the resonant tunneling band ultimately vanishes completely.     

The mixed-spin ternary-alloy in the form of ABpC1−p on the Bethe lattice

The AB_pC_1−p type of mixed ferromagnetic-ferrimagnetic ternary-alloy with A (spin-3/2), B (spin-1) and C (spin-5/2) ions was studied on the Bethe lattice with the odd numbered shells containing only A ions, while the even numbered shells either containing B or C ions randomly. The phase diagrams were obtained on the (R=|J_AC|/J_AB,kT_c/J_AB) and (p, kT_c/J_AB) planes for given values of p and R, respectively, with the coordination numbers z=3, 4, 5 and 6. The explicit dependence of the phase diagrams on z and each shell of the Bethe lattice having only one type of ion lead to some differences when compared with the previous works. The model presents one or two compensation temperatures for appropriate values of the system parameters.     

Dynamic and thermodynamic consequences of adsorbate lateral interactions in surface reaction kinetics

The effects of lateral interactions on the two-dimensional distribution of adspecies on solid surfaces and their consequences for reaction kinetics are demonstrated for the bimolecular reactive systemA+BAB. The discussion concentrates on systems where one reactant,A, is stationary while the other,B, is freely diffusing and instantaneously relaxing. A modified Bethe-Peierls-type lattice gas approximation is formulated in order to account for the rapidly-equilibrating distribution ofB atoms. The approximation takes into account all nearest and next nearest neighbor interactions between the adspecies and the nonuniformity of the lattice available toB implied by the presence of immobileA's on the surface. This model is combined with a Monte Carlo simulation of the reactive events in order to calculate reaction rates, e.g., in temperature-programmed processes. The rates are compared with full Monte Carlo simulations (for all kinetic processes), showing good agreement between the two schemes, except at very high coverages, where very long range correlations in the system which are ignored in the lattice gas approximation must be taken into account.This paper is dedicated to Howard Reiss in celebration of his 66th birthday.     

Low-temperature oxidation of CoCu films with long-term irradiation by oxygen ion beams

The surface and surface layers of Co_xCu_100?x inhomogeneous thin films irradiated by an oxygen ion beam for a long time (to 100 min) are studied. The films are obtained by electrolytic deposition. With X-ray photoelectron spectroscopy and conversion electron Mössbauer spectroscopy, it is shown that the irradiation leads to the formation of an oxidized surface layer. The continuity and thickness of the layer depend on the roughness of the initial film. For a cobalt content of 8≤x≤20 at. %, the oxide layer is continuous and nonuniform in thickness, the mean thickness being estimated at several tens of nanometers. The interface between the layer and the underlying film is sharp. The films irradiated are smoother than the asdeposited ones. The formation of the oxide layer is treated in terms of a qualitative model.     

Core-level shifts in metallic compounds

The method of Mårtensson and Johansson to calculate core level shifts in metallic compounds has been extended to general AB compounds by introducing an environment factor ?_B^A. The core level shift is now expressed in the heats of formation of binary compounds weighted by the environment factors. The introduction of the factor ?_B^A also allows to predict the concentration dependence of the core level shifts in alloys. Observed core level shifts are in general agreement with predicted values.     

Properties of defect modes in one-dimensional symmetric defective photonic crystals

We theoretically investigate the properties of defect modes in one-dimensional symmetric defective photonic crystals. We consider three defective photonic crystal structures, air/[(AB)^NsA^α(BA)^Ns]^Np/air, air/[(AB)^NsAB^βA(BA)^Ns]^Np/air, and air/{[(AB)^NsAB^βA(BA)^Ns]B^γ}^Np−1[(AB)^NsAB^βA(BA)^Ns]/air, where A and B are respectively taken to be the high- and low-index dielectric materials. The first has a defect layer of A^α, the second has a composite defect, AB^βA, and the third has a interleaving defect B^γ. The effect of thickness on the defect mode is studied by varying the parameters α, β, and γ, respectively, for the above model structures. It is found that the positions and the number of defect modes can be significantly changed due to the change in the defect thickness. In addition, by increasing the repeated number N_p, we can have multiple defect modes, leading to a possible design of tunable multichannel filter.     

Grazing incidence diffraction of X-rays in semiconductor heterostructures: Application of the integral mode

The grazing incidence diffraction (GID) of X-rays enables to characterize thin subsurface layers in semiconductor heterostructures having a thickness smaller than 100 nm. The dynamical theory of X-ray diffraction is extended for the case of identical in plane lattice parameters at the heterointerface. Especially the variation in the specular diffracted (220) Bragg intensity measured with open detector (integral mode) is evaluated in dependence on the grazing angle Φ₀ of the primary beam with respect to the (001) surface. Using a parallel beam an oscillation behaviour occurs at the high angle side Θ _c < Φ₀≦0.5⁰(Θ _c is the angle of total external reflection) of the diffraction curveI(Φ₀) which can be related to the thickness of the perfect crystalline part of the epilayert _K . Having an incident beam divergence and a small difference in the effective refractive indices of the layer and the substrate the oscillations are almost leveled. They are further visible in case of a minute inclination of the (220) lattice plane with respect to the surface normal. In the interval 0 < Φ₀<Θ _c the slope of the integral curve depends on the thickness of the subsurface layert _A which does not contribute to the Bragg diffraction. The integral mode is sensitive for layers of about 0<t _A <15 nm and 15<t _K <80 nm. The proposed theory working principally for multilayer structures is presently suplicated to interpret GID curves ofA ^III B ^v heterostructures.     

Mixed spin-1 and spin-3/2 Ising system with two alternative layers of a honeycomb lattice within the effective-field theory

An effective-field theory with correlations is developed for a mixed spin-1 and spin-3/2 Ising system with two alternative layers of a honeycomb lattice. Spin-1 atoms and spin-3/2 atoms are distributed in alternative layers of a honeycomb lattice. We consider that the nearest-neighbor spins of each layer are coupled ferromagnetically and the interaction between the vertically aligned spins and adjacent spins are coupled either ferromagnetically or antiferromagnetically depending on the sign of the bilinear exchange interactions. We investigate the temperature dependence of the total magnetization to find the compensation points and to determine the type of compensation behavior. We present the phase diagrams in different planes for h=0, and the phase diagrams contain the paramagnetic, nonmagnetic and ferrimagnetic phases. The system also presents a tricritical behavior besides multicritical point (A), isolated critical point (C) and double critical end point (B) depending on the interaction parameters.     

Formation of short-period multilayer W/B4C compositions

The quantitative characteristics of the interlayer interaction in multilayer W/B₄C periodic compositions produced by magnetron sputtering are studied by small-angle X-ray diffraction using CuK _?? radiation and by electron microscopy of transverse cuts. It is found that approximately 0.85 nm of the tungsten layer thickness is consumed for the formation of mixed zones at layer boundaries. The mixed layers have a density of 13.4 ± 0.7 g/cm³ and contain tungsten in a bound chemical state. The effect of these mixed zones on the X-ray reflectivity of multilayer W/B₄C compositions is estimated. A method is proposed to determine the layer thickness at a small number of peaks in an X-ray diffraction pattern.     

Statistical Investigation of the Mean Size Distribution of AB2/B f Type Hyperbranched Polymer

A statistical method was applied to investigate the number of isomeric structures of hyperbranched polymers formed by the polymerization of an AB ₂ type moiety with B_f polyfunctional core moieties (abbreviation as AB₂/B _f type). By means of the kinetic model described by a Smoluchowski-like coagulation equation, the explicit expressions of z-average mean-square radii of gyration are evaluated as a function of the conversion of functional groups. The numerical calculation shows that the mean size of the AB ₂/B _f type hyperbranched polymers will increase with the increase of the conversion of A groups or the reaction time, furthermore increasing sharply at the end of the reaction. When the reaction is quantitatively completed, the z-average mean-square radii of gyration of AB ₂/B _f type hyperbranched polymers show a maximum. The results are similar to those for star branched polymers, rather than those of hyperbranched polymers made by the AB_f type monomers without cores; the former had finite values and the latter reached infinite values when the reaction was quantitatively completed. These theoretical analysis conclusions will be important for effectively controlling the mean sizes or the average degrees of hyperbranched polymers by varying the number of the B_f type polyfunctional core moieties in experimental research.     

Mössbauer study of Fe1 + xV2−xO4 spinels for the determination of cation distributions and magnetic structure

Mössbauer experiments were carried out in the temperature range of 80–500 K on the spinel series of Fe_{1 + x}V_2?xO₄. The determined cation distribution has Fe²⁺ and Fe³⁺ at both A and B spinel sublattice sites for t whole series except for x = 0. Charge hopping was observed at the B lattice sites. The present cation distribution reasonably predicts several experimentally observed constants such as the lattice constant, the oxygen parameter, the saturation magnetic moment and the Curie constant. The magnetic structure in best agreement with the experimental values shows the AA spins to be antiparallel for Fe, the BB spins to be parallel for identical ions and antiparallel for different ions, and shows a strongly antiparallel AB interaction between trivalent iron ions. Crystal distortions due to the Jahn-Teller effect were observed at 80 K for compositions with 0 ? x ? 0.75.     

The existence of two nematic phases caused by dimer recombination

A mixture of A and B monomers without shape anisotropy but that are capable of forming three types, viz., AA, BB, and AB mesogenic dimers is investigated. The possibility of a temperature transition from one nematic phase consisting mainly of AB dimer formed by monomers of different types into another nematic phase consisting mainly of AA and BB dimers formed by monomers of the same type is shown.     

Coherent potential approximation to order-disorder transition in binary alloys

A band theory of order-disorder transition in an AB alloy is studied using the locator formalism in the coherent potential approximation. The influence of the difference in the band widths of A and B metals on the transition temperature is discussed.     

Relationship between triple and pair correlations in an A y B 1 − y solid solution with a planar hexagonal lattice

An analytical relationship is found between triple correlations and nearest neighbor pair correlations in an A _y B _{1 ? y} solid solution with a planar hexagonal lattice (plane group p6mm). The range of variations in the triple correlation is determined as a function of the composition of the A _y B _{1 ? y} solid solution and the value of the pair correlation. It is assumed that periodically ordered planar hexagonal and square island nanostructures can be considered as solid solutions A _1/4□_3/4 with vacancy sites □ and sites occupied by nanoparticles A. In this case, an ordered distribution of nanoparticles over the planar-lattice sites occurs because the nearest neighbor pair correlation is negative and has a maximum magnitude.     

Conformal behavior of a single chain <Emphasis Type="Italic">AB</Emphasis> block copolymer with mobile <Emphasis Type="Italic">B</Emphasis>-blocks

Monte-Carlo computer modeling was used to investigate the conformal behavior of a single chain AB block copolymer with mobile hydrophobic B-blocks and hydrophilic A-links. The formation of a “tailed globule” was observed with an increase of the energy of attraction between hydrophobic links. A comparison between the collapsing of a chain with mobile blocks and the behavior of regular and random block copolymers of the same structure was conducted.     

A Comparison of GaN Epilayers with Multiple Buffer Layers and with a Single Buffer Layer Grown on Si（111） Studied by HRXRD and RBS/Channeling

Two hexagonal GaN epilayers （samples A and B） with multiple buffer layers and single buffer layer are grown on Si （111） by metal-organic vapour phase epitaxy （MOVPE）. From the results of Rutherford backscattering （RBS）/channeling and high resolution x-ray diffraction （HRXRD）, we obtain the lattice constant （a and c） of two GaN epilayers （aA = 0.3190 nm, cA = 0.5184 nm and aB = 0.3192 nm, CB = 0.5179 nm）, the crystal quality of two GaN epilayers （ ХminA=4.87%, ХminB =7.35% along 〈1-↑213〉 axis） and the tetragonal distortion eT of the two samples along depth （sample A is nearly fully relaxed, sample B is not relaxed enough）. Comparing the results with the two samples, it is indicated that sample A with multiple buffer layers have better crystal quality than sample B with a single buffer layer, and it is a good way to grow GaN epilayer on Si （111） substrates using multiple buffer layers to improve crystal quality and to reduce lattice mismatch.     

Measurement of the lattice parameter of sodium by neutron-back-scattering

We report high precision results (δa/a ≈ 3 x 10^-6) of the lattice parameter of high purity sodium (99.99%) in the temperature region from 32°C up to the melting point. Data of the crystal quality are also given: the half width of the angular mosaic spread amounts to about 4'. Preliminary results on vacancy formation energy and entropy, using literature data of volume expansion, are 0.35 ± 0.02 eV and (3.7 ± 0.5) k_B resp. for a single vacancy.     

Formation of the composition and topography of surface layers of Cu50Ni50 foils under laser irradiation

The influence of the laser radiation power density on the changes in the composition and mechanical properties of surface layers of Cu₅₀Ni₅₀ foils has been investigated using X-ray photoelectron spectroscopy, scanning probe microscopy, X-ray diffraction, and microhardness measurements. It has been found that, after laser irradiation, the redistribution of elements occurs in the surface layer with a thickness of ～30 nm on the irradiated side of the foil. It has been revealed that there are microdistortions in the crystal lattice of the alloy, microdeformations of grains, and variations in the microhardness of the irradiated surface. The mechanisms explaining the observed changes in the foils after laser irradiation have been proposed.