Showing papers on "Diffraction published in 1984"
TL;DR: In this article, a martensitic phase transition to a complex tetragonal structure occurs on cooling below 202 K. Reasons for the structural transformation are discussed, and details of the low temperature crystallographic superlattice and magnetic structure are determined.
Abstract: Magnetization, magnetic susceptibility, optical microscopy, X-ray diffraction and neutron diffraction measurements have been made on a quenched sample of Ni2MnGa. Room-temperature neutron diffraction patterns indicate a highly ordered Heusler alloy, L21 type, structure. The alloy is ferromagnetic with a Curie temperature of 376 K, and a magnetic moment of 4·17μB largely confined to the Mn sites, but probably with a small moment <07·3μB associated with the Ni sites. A martensitic phase transition to a complex tetragonal structure occurs on cooling below 202 K. Neutron diffraction oscillation photographs taken using the D12 facility at Institut Laue Langevin, Grenoble, and low-temperature powder neutron diffraction data enabled details of the low temperature crystallographic superlattice and magnetic structure to be determined. Reasons for the structural transformation are discussed.
1,048 citations
TL;DR: In this article, Planar as well as spherical targets were irradiated for the first time by the random-phased wave and the targets were uniformly accelerated without being affected by small-scale intensity nonuniformities.
Abstract: By converting a coherent wave to a random-phased wave, the intensity profile on a target becomes easily controllable. Planar as well as spherical targets were irradiated for the first time by the random-phased wave. The targets were uniformly accelerated without being affected by the small-scale intensity nonuniformities. The $\frac{3}{2}$-harmonic emission shows that the plasma waves at $\frac{{n}_{c}}{4}$ are only weakly excited in a spherical plasma. Irradiation with short-wavelength, random-phased beams will be suitable for compression.
603 citations
TL;DR: In this paper, the foundations of diffraction tomography for offset vertical seismic profiling and well-to-well tomography are presented for weakly inhomogeneous formations for which the Born or Rytov approximations can be employed.
Abstract: Diffraction tomography is the generalization of X-ray tomography to applications such as seismic exploration where diffraction effects must be taken into account. In this paper, the foundations of diffraction tomography for offset vertical seismic profiling and well-to-well tomography are presented for weakly inhomogeneous formations for which the Born or Rytov approximations can be employed. Reconstruction algorithms are derived for approximately determining the acoustic or electromagnetic velocity profile of such formations from borehole measurements of acoustic or electromagnetic fields generated by sources located on the surface or in an adjacent borehole. Computer simulations are presented for the case of offset vertical seismic profiling.
594 citations
TL;DR: In this article, the results of computer simulations used to determine the domains of applicability of the first-order Born and Rytov approximations in diffraction tomography for cross-sectional (or three-dimensional) imaging of biosystems are shown.
Abstract: In this paper, the results of computer simulations used to determine the domains of applicability of the first-order Born and Rytov approximations in diffraction tomography for cross-sectional (or three-dimensional) imaging of biosystems are shown. These computer simulations were conducted on single cylinders, since in this case analytical expressions are available for the exact scattered fields. The simulations establish the first-order Born approximation to be valid for objects where the product of the relative refractive index and the diameter of the cylinder is less than 0.35 lambda. The first-order Rytov approximation is valid with essentially no constraint on the size of the cylinders; however, the relative refractive index must be less than a few percent. We have also reviewed the assumptions made in the first-order Born and Rytov approximations for diffraction tomography. Further, we have reviewed the derivation of the Fourier Diffraction projection Theorem, which forms the basis of the first-order reconstruction algorithms. We then show how this derivation points to new FFT-based implementations for the higher order diffraction tomography algorithms that are currently being developed.
590 citations
TL;DR: In this article, a parabolic model for calculating the combined refraction/diffraction of monochromatic linear waves is developed, including a term which allows for the dissipation of wave energy.
Abstract: A parabolic model for calculating the combined refraction/diffraction of monochromatic linear waves is developed, including a term which allows for the dissipation of wave energy. The coefficient of the dissipation term is related to a number of dissipative models. Wave calculations are performed for a localized area of dissipation, based on a friction model for a spatial distribution of rigid vertical cylinders. The region of localized dissipation creates a shadow region of low wave energy, which may have important implications for the response of neighboring shore lines.
484 citations
TL;DR: In this paper, the authors proposed to apply wedge diffraction in the format of the geometrical theory of diffraction (GTD), modified to include finite conductivity and local surface roughness effects.
Abstract: Diffraction propagation over hills and ridges at VHF and UHF is commonly estimated using Fresnel knife edge diffraction. This approach has the advantage of simplicity, and for many geometries yields accurate results. However, since it neglects the shape and composition of the diffracting surface, it can in some cases yield results which are in serious disagreement with measurements. To remedy this, attempts have been made to approximate the diffracting hill or ridge by other shapes, most notably cylinders. These approaches have not been widely adopted, due in large part to their greater numerical complexity. In this paper it is proposed to apply wedge diffraction in the format of the geometrical theory of diffraction (GTD), modified to include finite conductivity and local surface roughness effects. It is shown that, for geometries with grazing incidence and/or diffraction angles, significant improvement in accuracy is obtained. Further, the GTD wedge diffraction form used is based on the Fresnel integral, so that it is only slightly more complex numerically than knife edge diffraction. Finally, the GTD includes reflections from the sides of the ridge (wedge faces), and can be extended to multiple ridge diffraction and three-dimensional terrain variations.
431 citations
TL;DR: In this article, the most commonly used modes (Fresnel, Foucault, holographic, differential phase contrast and low angle diffraction) are compared and intercompared.
Abstract: Magnetic domain structures in thin foils may be revealed in a number of ways in the transmission electron microscope. The most commonly used modes (Fresnel, Foucault, holographic, differential phase contrast and low angle diffraction) are described and intercompared. The Fresnel and Foucault modes are simple to implement but provide a mainly qualitative description of the spatial variation of induction throughout the foil. Quantitative information may be obtained from the other two imaging modes but they require more sophisticated equipment. Electron diffraction is particularly useful for studying periodic magnetic structures. Examples of the use of these, and further specialised techniques, are presented by reference to some applications of current interest within magnetism.
428 citations
TL;DR: In this article, high-energy elastic and inelastic scattering at small t are discussed in terms of pomeron and double-pomeron exchange and the similarity between them allows an almost parameter-free description of various elastic process, a and diffraction dissociation, in good agreement with experiment.
333 citations
TL;DR: In this article, it was shown that hydrogen-bonding schemes cannot be assigned with confidence if only oxygen atoms are located from x-ray diffraction data, which is not the case in this paper.
Abstract: ..beta..-cyclodextrin (CD) crystallizes from water in monoclinic space group P2/sub 1/. The crystal structure was initially determined from x-ray data. Neutron diffraction studies were carried out to determine the hydrogen-bonding schemes of ..beta..-CD, which are broken down into a few clearly defined motifs. This study again demonstrates that hydrogen-bonding schemes cannot be assigned with confidence if only oxygen atoms are located from x-ray diffraction data.
315 citations
TL;DR: In this paper, the authors analyzed diffracted beam profiles from stepped surfaces that are reversible and obtained functional expressions for these angular profiles by a Markov method of treating a one-dimensional geometric distribution of steps.
298 citations
TL;DR: The crystal structures of Si and Ge were studied by energy dispersive X-ray diffraction at room temperature and pressures up to 50 GPa as mentioned in this paper, showing that Si transforms to a primitive hexagonal (Si-V) structure around 16 GPa, to an intermediate phase Si-VI between 35 and 40 GPa.
TL;DR: In this paper, the mechanisms responsible for the growth of periodic surface structure on Ge irradiated by 1.06-m laser pulses from a single beam are investigated using time-resolved diffraction of a weak cw probe beam from the developing structures, coupled with electron-microscope pictures of the resulting morphology.
Abstract: The mechanisms responsible for the growth of periodic surface structure on Ge irradiated by 1.06-\ensuremath{\mu}m laser pulses from a single beam are investigated. Time-resolved diffraction of a weak cw probe beam from the developing structures, coupled with electron-microscope pictures of the resulting morphology, is used to identify four distinct regimes of ripple formation at different incident laser fluences. At low fluences, the structure develops when thin (1 \ensuremath{\mu}M wide) isolated, molten strips resolidify on the solid substrate, while at high fluences, the structure results from freezing of capillary waves which are generated on the surface that the laser pulse has melted uniformly. Models are presented which clearly demonstrate how the incident electromagnetic field interacts with the evolving structures (in different fluence regimes) to provide feedback mechanisms which sustain their growth.
TL;DR: In this article, a coupledmode analysis has been developed to describe the output of phase-locked injection laser arrays, showing that an array of emitters with weak coupling can only operate in a set of discrete modes determined by the number and the spacing of the emitters.
Abstract: A coupled‐mode analysis has been developed to describe the output of phase‐locked injection laser arrays. We show that an array of emitters with weak coupling can only operate in a set of discrete modes determined by the number and the spacing of the emitters. The interaction between emitters leads to a splitting of the common frequency of operation that can be estimated from the coupling strength. The coupled‐mode analysis is compared to calculations based on simple diffraction theory. A consequence of the analysis is an explanation for the commonly observed discrepancy between experimentally observed far‐field lobe(s) widths and those predicted by simple diffraction theory.
TL;DR: In this article, the authors derived explicit expressions for equivalent edge currents for an arbitrary local wedge angle and arbitrary directions of illumination and observation, based on an asymptotic relationship between the surface radiation integral of the physical theory of diffraction and the line radiation integral.
Abstract: Explicit expressions for equivalent edge currents are derived for an arbitrary local wedge angle and arbitrary directions of illumination and observation. Thereby the method of equivalent currents (MEC) is completed as a practically applicable theory of the electromagnetic high-frequency diffraction by edges. The derivation is based on an asymptotic relationship between the surface radiation integral of the physical theory of diffraction (PTD) and the line radiation integral of MEC, and the resulting expressions are deduced from the exact solutions of the canonical wedge problem.
TL;DR: In this article, a polycrystalline SnO films prepared by evaporation were studied by Raman scattering, IR reflection and X-ray diffraction, and all the allowed Raman-active and IR-active phonon modes were observed.
TL;DR: Transmission electron microscopy and x-rays diffraction demonstrate that the x-ray diffraction characteristics of a wide range of interstratification can be modeled experimentally by utilizing materials containing only three types of particles.
Abstract: Materials representing common interstratified clay minerals are shown to be composed of aggregates of fundamental particles. Transmission electron microscopy and x-ray diffraction demonstrate that the x-ray diffraction characteristics of a wide range of interstratification can be modeled experimentally by utilizing materials containing only three types of particles. The data have been incorporated into a new model that regards interstratified clay minerals as populations of fundamental particles whose x-ray diffraction patterns result from interparticle diffraction.
01 Jan 1984
TL;DR: In this article, the authors proposed a method for computing the power transfer of multiple diffraction using a reference vector and a Kossel pattern, and showed that it is possible to compute the number of diffraction points in a single image.
Abstract: 1. Introduction.- 2. Geometry, Peak Indexing, and Experimental Techniques.- 2.1 Geometry of Multiple Diffraction.- 2.1.1 Real-Space and Reciprocal-Space Representations.- 2.1.2 Persistent and Coincidental Multiple Diffractions.- 2.1.3 Lattice-Symmetry Dependence-Intrinsic Multiple Diffraction.- 2.1.4 Multiple-Diffraction Possibilities.- 2.1.5 Decomposition of Multiple Diffraction.- 2.2 Experimental Techniques for Obtaining Multiple Diffraction.- 2.2.1 Collimated-Beam Technique.- 2.2.2 Divergent-Beam Techniques.- 2.3 Indexing Multiple Diffraction Patterns.- 2.3.1 Reference Vector Method.- 2.3.2 Orientation Matrix Method.- 2.4 Indexing Kossel Patterns.- 2.4.1 Stereographic Projection Method.- 2.4.2 Combined Gnomonic-Stereographic Projection Method.- 3. Kinematical Theory of Diffraction.- 3.1 Equation of Power Transfer for Multi-Beam Cases.- 3.2 Approximate Solutions to the Equation of Power Transfer.- 3.3 Integrated Intensity and the Lorentz-Polarization Factors.- 3.4 Path Lengths of X-Ray Beams in Crystals.- 3.5 Exact Solution to the Power-Transfer Equation.- 3.6 Iterative Calculation for Reflection Power.- 3.7 Dynamical Treatment for Kinematical Reflections.- 3.8 Diffraction in Multi-Layered Crystals.- 3.9 Peak Width, Beam Divergence, and Mosaic Spread.- 4. Dynamical Theory of X-Ray Diffraction.- 4.1 Fundamental Equation of Wavefields.- 4.2 Polarization of Wavefields.- 4.3 Dispersion Surface.- 4.3.1 Geometry of the Dispersion Surface.- 4.3.2 Excitation of the Dispersion Surface.- 4.4 Energy Flow.- 4.4.1 Poynting Vectors and the Dispersion Surface.- 4.4.2 Group Velocity and Energy Flow.- 4.5 Modes of Wave Propagation.- 4.5.1 Wavefields of Modes.- 4.5.2 Number of Permitted Modes.- 4.6 Absorption.- 4.7 Boundary Conditions.- 4.8 Excitation of Mode and Excitation of Beam.- 4.9 Intensity of Wavefield (Standing-Wave) in Crystal.- 4.10 Consideration of the Spherical-Wave Nature of the Incident X-Rays.- 5. Approximations, Numerical Computing, and Other Approaches.- 5.1 Two-Beam Approximation for Three-Beam Diffraction.- 5.1.1 General Considerations.- 5.1.2 Three-Beam Transmission Case (Borrmann Diffraction).- 5.1.3 Diffracted Intensity of Three-Beam Bragg Reflection.- 5.1.4 Integrated Reflection for Non-Absorbing Crystals.- 5.1.5 Diffraction in Absorbing Crystals.- 5.2 Procedures for Numerical Computing.- 5.3 Quantum Mechanical Approach.- 5.4 N-Beam Diffraction in Other Types of Interaction.- 5.4.1 Two-Beam Diffraction with Specular Reflection.- 5.4.2 Interaction of X-Rays with Phonons.- 6. Case Studies.- 6.1 Bragg-Type Multiple Diffraction from Gallium Arsenide, Indium Arsenide and Indium Phosphide-Kinematical Interpretation.- 6.2 Three-Beam Borrmann Diffraction-Dynamical Calculation.- 6.3 Simultaneous Four-Beam Borrmann Diffraction.- 6.4 Three-Beam Bragg-Laue and Bragg-Bragg Diffraction.- 6.5 Four-Beam Bragg-Laue Diffraction.- 7. Applications.- 7.1 Experimental Determination of X-Ray Reflection Phases Application to Crystal Structure Determination.- 7.1.1 General Consideration of the X-Ray Phase Problem.- 7.1.2 Reflection Phases and Multiple Diffraction.- 7.1.3 Experimental Methods for Phase Determination Using Multiple Diffraction.- 7.1.4 Determination of Centrosymmetric Crystal Structures in Practice.- 7.1.5 Phase Determination for Non-Centrosymmetric Crystals.- 7.2 Determination of Lattice Constants of Single Crystals.- 7.2.1 Divergent-Beam Photographic Methods.- 7.2.2 Collimated-Beam Method.- 7.3 Determination of Lattice Mismatch in Thin Layered Materials.- 7.4 Multi-Beam X-Ray Topography.- 7.5 Multi-Beam X-Ray Interferometer.- 7.6 Monochromatization of X-Ray Beams.- 7.7 Plasma Diagnosis.- 7.8 Determination of Mosaic Spread of Crystals.- 7.9 Multi-Beam X-Ray Standing-Wave Excited Fluorescence Technique for Surface Studies-A Proposed Method.- 7.10 Possible Future Trend of Development.- References.
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15 Nov 1984
TL;DR: In this article, the authors present an overview of electron beam instruments, including TEM, STEM, HREM, and SEM, as well as their operational modes and analysis of micrographs.
Abstract: Preface. Introduction to electron beam instruments. Electron-specimen interactions. Layout and operational modes of electron beam instuments. Interpretation of diffraction information. Analysis of micrographs in TEM, STEM, HREM, and SEM. Interpretation of analytical data. Appendices. Index.
TL;DR: In this paper, the difference in lattice parameter between the silicon substrate and the porous layer and the strains of the porous lattice have been determined accurately on a double crystal diffractometer for a series of porous structures.
TL;DR: In this article, qualitative and quantitative arguments are presented that elucidate the modifications that the whole three-dimensional structure of the diffracted field undergoes as the Fresnel number is gradually decreased, and contours of equal intensity in the focal region are presented for systems of selected Fresnel numbers, which focus uniform waves.
Abstract: It was recently shown that, when a converging spherical wave is focused in a diffraction-limited system of sufficiently low Fresnel numbers, the point of maximum intensity does not coincide with the geometrical focus but is located closer to the exit pupil. In the present paper both qualitative and quantitative arguments are presented that elucidate the modifications that the whole three-dimensional structure of the diffracted field undergoes as the Fresnel number is gradually decreased. Contours of equal intensity in the focal region are presented for systems of selected Fresnel numbers, which focus uniform waves.
TL;DR: In this paper, a new technique using Kossel rings is described which simultaneously measures structure, lattice spacings, and crystallite orientation, and the experimental determination of the bulk modulus is further illustrated.
Abstract: Bragg diffraction of laser light from crystalline aqueous colloids of polystyrene spheres is examined to determine crystal structure, orientation, and elasticity. A new technique using Kossel rings is described which simultaneously measures structure, lattice spacings, and crystallite orientation. The monodisperse polystyrene sphere latex dispersions crystallize into large single crystals, which, depending on sphere concentration, are either face-centered or body-centered cubic. The interparticle spacings in the crystals are many times larger than the sphere diameter (0.109 μm). The use of tunable lasers to easily determine crystal structure is described, and the technique is further illustrated by the experimental determination of the bulk modulus. The bulk modulus is a macroscopic physical constant which can be used to monitor intersphere potentials and the screening of the particle charges by electrolytes in the solution. Data are presented which suggest that crystallite orientation occurs with the closest packed sphere layers parallel to the sample cell quartz walls.
TL;DR: An experimental and theoretical analysis is presented involving the effect of variation in fiber and beam diameter upon the determination of average sarcomere length in isolated single muscle fibers using laser light diffraction.
TL;DR: In this article, the diffraction of axisymmetric borehole waves by bed boundary discontinuities is calculated by using axioms of the bed boundary and the inhomogeneities.
Abstract: This paper presents the calculation of the diffraction of axisymmetric borehole waves by bed boundary discontinuities. The bed boundary is assumed to be horizontal and the inhomogeneities to be axi...
TL;DR: In this article, electron diffraction patterns from two different crystal forms of purple membrane have been obtained, and the diffraction spots extend to beyond 3.0 A resolution, using specimens tilted at angles up to 60°.
TL;DR: In this article, a terrain sensitive propagation model based on the geometrical theory of diffraction (GTD) modified for finite conductivity and local surface roughness is applied to hilly terrain including multiple diffraction, with the results compared with measurements and the Longley-Rice model.
Abstract: A terrain sensitive propagation model based on the geometrical theory of diffraction (GTD) modified for finite conductivity and local surface roughness is applied to hilly terrain including multiple diffraction, with the results compared with measurements and the Longley-Rice model. Even though the approach used to calculate multiple-wedge diffraction is known to be invalid in certain situations, the comparison is favorable to the GTD model results.
TL;DR: In this article, an interparticle diffraction effect from the sedimented aggregates of oriented particles was proposed to reconcile the discrepancy between TEM and X-ray diffraction.
Abstract: Transmission electron microscopic (TEM) examination of the <0·1-µm fraction of montmorillonite and regularly interstratified illite-smectite (I-S) shows that these clays, when dried from suspension, consist primarily of particles 10 A and 20 A thick respectively. However, X-ray diffraction (XRD) examination of sedimented aggregates of montmorillonite indicate that the effective number of unit cells that are diffracting coherently is ∼9. This discrepancy can be reconciled by postulating an interparticle diffraction effect from the sedimented aggregates of oriented particles. The interfaces of these particles are capable of adsorbing water, ethylene glycol etc. so that on this basis smectite is composed of elementary silicate particles 10 A thick, and regularly interstratified I-S is primarily composed of elementary ‘illite’ particles 20 A thick, values which are in agreement with the TEM observations. This concept is confirmed experimentally by XRD examination of sedimented aggregates from mixed suspensions of both materials; the resulting patterns are identical to those of randomly interstratified illite and smectite layers, which indicates that the layer sequence examined by XRD has been entirely rearranged. It is concluded that the use of XRD peak breadth to determine mean crystal thickness cannot be reliably applied to these systems. Standard XRD data from sedimented aggregates may not be able to distinguish between true interstratification and interparticle diffraction effects of intimate physical mixtures.
TL;DR: The Laue technique offers advantages for the recording of diffraction patterns from short-lived structural intermediates; that is, for time-resolved crystallography.
Abstract: In conventional x-ray diffraction experiments on single crystals, essentially monochromatic x-rays are used. If polychromatic x-rays derived from a synchrotron radiation spectrum are used, they generate a Laue diffraction pattern. Laue patterns from single crystals of macromolecules can be obtained in less-than 1 second, and significant radiation damage does not occur over the course of an exposure. Integrated intensities are obtained without rotation of the crystal, and individual structure factors may be extracted for most reflections. The Laue technique thus offers advantages for the recording of diffraction patterns from short-lived structural intermediates; that is, for time-resolved crystallography.
TL;DR: In this paper, a finite-difference algorithm is used to generate synthetic seismograms for waves propagating through two-dimensional random media, which are meant to approximate the heterogeneity of the crust and upper mantle.
Abstract: A finite-difference algorithm is used to generate synthetic seismograms for waves propagating through two-dimensional random media. The media have a significant component of their material properties varying randomly over length scales smaller than the seismic wavelength and are meant to approximate the heterogeneity of the crust and upper mantle. The finite-difference technique retains all multiply scattered and diffracted waves, and also accounts for transmission losses.
The synthetic seismograms clearly exhibit coda and apparent attenuation caused by scattering. For a medium with a white wavenumber spectrum of velocity fluctuations, the coda is higher frequency than the initial pulse. The apparent attenuation is greatest when the scatterer size is comparable to the seismic wavelength. The spectra of the coda generally increase in frequency as the scatterers decrease in size. Examples demonstrate how scattering can produce spectra with broad peaks and sharp fall-offs that can make the determination of the source spectra and corner frequencies of small earthquakes extremely difficult.
TL;DR: In this article, it is shown that in the case that the geometrical optics approximation holds, it is possible to calculate or measure range error for wave propagation in the atmosphere of the earth (troposphere and ionosphere) from knowledge of the refractive index as a function of space (location) and time.
Abstract: Range and range rate measurements play an important role in geodetic applications of electromagnetic waves (terrestrial as well as from space, including Very Long Baseline Interferometry (VLBI)) Recent developments of measuring, techniques have led to the situation that the measuring accuracy is no longer the limiting factor in the final accuracies of range and range rate measurements. The main error stems now from the influence of the atmosphere and from assumptions and approximations in context with the derivation of refractive index used to describe the wave propagation properties of the atmosphere. Only in the case that the geometrical optics approximation holds is it possible to calculate or to measure range error for wave propagation in the atmosphere of the earth (troposphereand ionosphere) from knowledge of the refractive index as a function of space (location) and time. Otherwise we have to consides additionally scattering and/or diffraction effects.
TL;DR: In this paper, a set of superlattices of the dilute magnetic semiconductor Cd1−x Mnx Te (x∼0.2) alternating with CdTe have been successfully grown for the first time using the molecular beam epitaxy technique.
Abstract: Single‐crystal multilayers of the dilute magnetic semiconductor Cd1−x Mnx Te (x∼0.2) alternating with CdTe have been successfully grown for the first time using the molecular beam epitaxy technique. Four sets of superlattices have been prepared consisting of 14, 60, 90, and 240 double layers of average thickness 460, 140, 75, and 37 A, respectively. Each set consists of two samples grown simultaneously using 7×15×1‐mm thick (0001) sapphire substrates onto which 5.0‐μm‐thick CdTe buffer layers were first deposited. X‐ray diffraction techniques were employed to verify that epitaxy had been achieved and to obtain the average lattice constant of each of the multilayer structures. X‐ray diffraction satellites were observed on both sides of the (111) diffraction peak of the superlattices composed of 14 and 60 alternating layers, respectively, which allowed an accurate estimate of the superlattice period, or double‐layer thickness, for these samples. Results of UV reflectance studies and photoluminescence experi...