Analysis of diffracted fields with the extended theory of the boundary diffraction wave for impedance surfaces

Uniform diffracted fields from impedance surfaces are investigated by the extended theory of boundary diffraction wave (ETBDW). The new vector potential of the ETBDW is constructed by considering the pseudoimpedance boundary condition. The method is applied to the diffraction problem from an impedance half-plane. It is shown that the total fields from an impedance half-plane reduce to the case of a perfectly electric or magnetic conducting and opaque half-plane for special values of surface impedance. The total and diffracted fields are compared numerically with the exact solution for the impedance half-plane and modified theory of physical optics (MTPO) solution for an impedance wedge. The numerical results show that the field expressions are in very good agreement with the exact and MTPO solutions.     

Spin filters and supermirrors: a comparison study of two methods of high-precision neutron polarisation analysis

The precise knowledge of the neutron polarisation is needed in tests of the electroweak Standard Model using angular correlations in polarised neutron beta decay. We performed an experimental comparison study of two different methods of polarisation analysis of a cold neutron beam which are based on spin-dependent reflection, respectively, transmission. The compared devices are a supermirror analyser used in prior neutron decay studies and an opaque transmission spin filter of either polarised ³He or polarised protons. The results of the neutron polarisation measured with the supermirror analyser and with the spin filter coincided in three different experiments within 0.1–0.2%.     

A unified formalism using effective surface permittivity to study acoustic waves in various anisotropic and piezoelectric multilayers

A unified formalism is presented that uses the effective surface permittivity (ESP) to study surface acoustic waves (SAW) in layered substrates and guided waves in layered plates. Based on known mathematical tools, such as ordinary differential equation and transfer matrix, a generalized surface impedance (GSI) concept is developed and exploited to investigate the acoustic propagation in various anisotropic and piezoelectric layered structures. The ESP function, originally defined for the surface of a homogeneous and semi-infinite piezoelectric substrate, is extended to both the top surface of and an interface in a layered half space, as well as to either surface of a finite-thickness plate. General ESP expressions for all mentioned configurations are derived in terms of an equivalent GSI matrix. It is shown that, when using the appropriate GSI matrices, the same form of the ESP expressions applies no matter whether the structure is a homogeneous half space alone or coated with a layered plate or a layered plate alone. GSI matrices are explicitly given in terms of the bulk partial mode solutions for a substrate and via the transfer matrix for a plate. Modified GSI matrices for structures consisting of both a plate and a substrate are also specified. Analytical development is fully detailed to suit program implementation. To illustrate its versatility, the formalism is also applied to two-substrate configurations, allowing one to analyze guided waves in a plate sandwiched between and interfacial waves existing along the boundary of two different media. Numerical examples are given to illustrate the spectrum features that the ESP shows for various structures. Deduced ESP expressions allow one to locate directly all piezoelectrically active waves in any structure including at least one piezoelectric layer. Acoustic modes that are not piezoelectrically active and those in non-piezoelectric materials can be also obtained by using the intermediate results, such as derived GSI matrices.     

Interface waves along an anisotropic imperfect interface between anisotropic solids

First and second order asymptotic boundary conditions are introduced to model a thin anisotropic layer between two generally anisotropic solids. Such boundary conditions can be used to describe wave interaction with a solid-solid imperfect anisotropic interface. The wave solutions for the second order boundary conditions satisfy energy balance and give zero scattering from a homogeneous substrate/layer/substrate system. They couple the in-plane and out-of-plane stresses and displacements on the interface even for isotropic substrates. Interface imperfections are modeled by an interfacial multiphase orthotropic layer with effective elastic properties. This model determines the transfer matrix which includes interfacial stiffness and inertial and coupling terms. The present results are a generalization of previous work valid for either an isotropic viscoelastic layer or an orthotropic layer with a plane of symmetry coinciding with the wave incident plane. The problem of localization of interface waves is considered. It is shown that the conditions for the existence of such interface waves are less restrictive than those for Stoneley waves. The results are illustrated by calculation of the interface wave velocity as a function of normalized layer thickness and angle of propagation. The applicability of the asymptotic boundary conditions is analyzed by comparison with an exact solution for an interfacial anisotropic layer. It is shown that the asymptotic boundary conditions are applicable not only for small thickness-to-wavelength ratios, but for much broader frequency ranges than one might expect. The existence of symmetric and SH-type interface waves is also discussed.     

Modeling the passive microwave devices based on planar multilayer anisotropic structures with high-temperature superconductors

The electrodynamic properties of an arbitrary multilayer medium, including anisotropic layers and conductors of an arbitrary shape on one of the interfaces, are studied. The conductors represent thin layers of a high-temperature superconductor (HTSC). A system of integral equations for the electric field is solved in the spatial domain. The electrodynamic problem was solved by numerical methods to determine the surface current density by applying the Galerkin procedure and by solving the main matrix equation relative to coefficients of the current density expansion in a basis set of finite functions. The losses in HTSC layers are taken into account by using the concept of the equivalent surface impedance and the Leontovich boundary conditions. The anisotropy is taken into account in determining the Green dyad for a structure with an arbitrary number of anisotropic or isotropic layers. Correctness of the proposed model is confirmed by the results of calculations of the surface current density distribution.     

A novel undulator-based PGM beamline for circularly polarised synchrotron radiation at BESSY II

We present the design of a beamline for circularly polarised radiation from an elliptical double undulator to be built at BESSY II. The UE56 undulator of Sasaki type will emit simultaneously two angularly separated circularly polarised beams of opposite helicities. The undulator will, through emissions in first, third and fifth harmonics, provide circularly polarised radiation in the energy range 89–1328 eV. The beamline essentially consists of a plane grating monochromator (PGM) working with collimated light and employing only one set of optical elements for steering and monochromatising the two beams. A high energy resolution from 6000 to 13 000 can be achieved at a flux between about 10¹⁰ and 10¹² photon/s, respectively. The helicity of circular polarisation can be switched dynamically in the beamline at a high rate (>100 Hz) by use of a suitable chopper. The degree of circular polarisation of the source ranging from 78 to 100% is well preserved by the beamline. In the worst case — at the lowest energy — the degree of polarisation is reduced by 5%. In the higher energy range above 357 eV the input degree of circular polarisation can even be increased by the beamline.     

Multi-mode cylindrical surface wave excitation

In this paper, we study the solution of the 3-dimensional reduced wave equation above an infinite plane 2-dimensional surface wave supporting structure. The structure is simulated by a multiple impedance boundary condition that allows for the excitation of two different cylindrical surface wave modes. At first we investigate the problem of a simple source which is then generalized to multi-pole sources. For the cases considered, we are able to give an explicit solution that exhibits the character of the surface wave field. Furthermore, using a virtual structure notion, we are able to identify the power travelling within the impedance structure associated with a given external surface wave mode. Thus, a definition of the total surface wave power is applied in a manner that modal power flow separability is maintained. The far field pattern is observed to vanish in certain directions, in particular along the impedance plane. Furthermore, the far field power is calculated in terms of a non-elementary definite integral which is then estimated at high and low frequencies so that a partial verification of our calculations can be made by observing that power is conserved to these orders.     

Design of two-dimensional photonic crystal structure based in all-angle negative refractive effect for application in focusing systems

A design of a polarisation independent focusing system of electromagnetic waves in a two-dimensional photonic crystal (PC) is proposed and numerically demonstrated by using the finite-difference time-domain method. We have shown that by the careful selection of both the refractive index of the high index material and the air cylinder radius, a complete all-angle negative refraction can be obtained for both the transverse electric (TE) and the transverse magnetic (TM) polarisations. We have demonstrated that the proposed PC structures can focus either the TE or the TM or both TE and TM polarised waves with sub-wavelength resolution. In addition, a significant improvement of the image resolution has been presented     

Scattering of a plane harmonic SH wave by a completely embedded corrugated scatterer

Anti‐plane‐strain model for steady‐state scattering of elastic waves by a rough inclusion or a cavity embedded in a half space is considered by using a direct boundary integral equation method. The roughness of the scatterer is assumed to be periodic with arbitrary amplitude and period. Detailed testing of the numerical results is presented. The motion along the half‐space surface is evaluated for different corrugations, frequencies and impedance contrast of the materials. The importance of the scatterer roughness upon the displacement field is clearly demonstrated. It was shown that larger corrugation amplitudes, shorter corrugation periods and higher frequencies may produce significant change in the displacement field when compared with the corresponding smooth scatterer result. This effect strongly depends upon the impedance contrast of the materials. Copyright © 2008 John Wiley & Sons, Ltd.     

Angular distribution of coherent bremsstrahlung

The angular distribution of the linearly polarised photon beam produced by coherent bremsstrahlung from an aligned diamond radiator has been measured at the MAMI A2 tagged photon facility. The measurements were made with a prototype position sensitive photon detector which utilises the pair production process and a double sided silicon strip detector. This polarised photon beam is used for nuclear and hadronic experiments and in their analysis the polarisation is obtained from a calculation, which matches the experimental intensity spectrum. As the polarisation is related to the photon beam angular distribution, the present measurements can be used to test this calculation. The overall agreement is found to be good although there are some regions where significant discrepancies exist.     

ON THE INADMISSIBILITY OF SEPARATION OF VARIABLES SOLUTIONS IN LINEAR ANISOTROPIC VISCOELASTICITY

SUMMARY

The field and constitutive equations as well as the boundary conditions of linear anisotropic viscoelasticity are examined for possible solutions which are separable into products of time and spatial functions. It is shown that under no circumstances other than for constant Poisson's ratios are such separation of variables solutions admissible, even though under these or some additional restrictive conditions they can exist in isotropic viscoelasticity.     

Quasi-optical theory of radiation amplification by electron flow above resistive metal surface

Linear and nonlinear stages of a dissipative instability that arises in an electron flow above a resistive metal surface, have been studied in the framework of a quasi-optical approach with the Leontovich impedance boundary conditions. It is shown that this instability can be used for the amplification of short-wavelength radiation including that in the terahertz frequency range.     

The calibration of slow neutron polarised beam instruments

A new procedure for calibrating the efficiencies of the polarisation sensitive components in slow neutron polarised beam instruments is described. The measurement of one “flipping ratio” and two “shim ratios” allows the effective polarisation, the spin flipping efficiency, and a depolarising shim efficiency to be determined unambiguously. Results of its application to the calibration of a neutron time-of-flight polarised beam reflectometer are presented.     

Improvement of a Differential Method for Metallic Diffraction Gratings by Means of an Integral Method

Abstract

It is shown how a result from the integral method for calculating the diffraction from a metallic grating can be used to improve the differential method. The result from the integral formalism can be regarded as a vigorous theoretical basis for those methods that employ surface impedance boundary conditions.     

Crystal field effects using polarised neutron spectroscopy

Crystal field transitions and quasielastic magnetic scattering were observed in PrAl₃ using a polarised neutron diffractometer/spectrometer. These were positively identified using neutron polarisation analysis. Transitions were observed at 3.5 and 4.5 meV, and both magnetic and nuclear elastic scattering were successfully separated.     

Elastic analysis of a half-plane containing an inclusion and a void using a mixed volume and boundary integral equation method

A mixed volume and boundary integral equation method is used to calculate the plane elastostatic field in an isotropic elastic half-plane containing an isotropic or anisotropic inclusion and a void subject to remote loading parallel to a traction-free boundary. A detailed analysis of the stress field is carried out for three different geometries of the problem. It is demonstrated that the method is very accurate and effective for investigating local stresses in an isotropic elastic half-plane containing multiple isotropic or anisotropic inclusions and multiple voids.     

薄膜高频磁阻抗效应及其应用

本文介绍了Ｆｅ－Ｎｉ软磁薄膜高频磁阻抗效应及在磁传感技术中的应用，分析了讨论了获得高灵敏度磁传感的途径，磁性薄膜厚度的一般控制在４００ｎｍ以上抑制Ｎｅｅｌ壁的出现，为了提高电压输出变化灵敏度，需要沿被测场方向加向Ｏｅ的偏磁场并使高频磁场形成闭合回路，感生各向异性通过在溅射薄膜过程中加几百Ｏｅ平行于膜的直流磁场获得，与各向异性磁阻效应和巨磁阻效应相比，高频磁阻抗效应传感技术灵敏度高，无巴克豪森噪声，     

Corrosion mechanism and hydrogen evolution on Mg

Magnesium (Mg) dissolution is distinct from other engineering metals, as Mg can support cathodic hydrogen evolution on its surface during anodic polarisation. The phenomenon of cathodic hydrogen evolution upon anodically polarised Mg is characterised by the rate of the hydrogen evolution reaction (HER) increasing with anodic polarisation, a phenomenon called the negative different effect (NDE). Mg has a tendency to aggressively corrode in aqueous solutions, impairing its application as a durable engineering material or a predictable electrode material, which is also influenced by the NDE. Over the last century a number of different theories have sought to explain the NDE. However, recent progress in research upon Mg utilising contemporary methods including advanced electrochemical techniques, on-line elemental analysis and cross-sectional electron microscopy, have not only refined the understanding of Mg dissolution, but discredited almost a century of alternate theories. During anodic polarisation, a bilayered MgO/Mg(OH)₂ film forms on Mg, appearing as a dark region on visual inspection. This film gradually occupies the bulk of the previously pristine Mg surface, and importantly sustains (and enhances) the HER. This phenomenon of cathodic activation may also be catalysed by an enrichment of noble elements or impurities on the Mg surface, which could play an important role in promoting the HER. A phenomenological model for the dissolution of Mg encompassing the current opinion of many researchers is presented herein.