3D DuMond diagrams of multi-crystal Bragg-case synchrotron topography. I. Flat sample

The 3D representation of the DuMond diagram is used to explain the dimensional features of X-ray topographs obtained by multi-crystal configuration with a synchrotron beam. Symmetric Bragg-case reflections are considered for a flat double-crystal monochromator and a flat sample. Two ways of sample alignment are taken into account. They are referred to as σ–σ and σ–π geometries, where the diffraction plane of the sample is parallel and perpendicular, respectively, to the vertical diffraction plane of the monochromator (σ polarization). It is shown that the shape of the sample image is closely connected to the shape the diffraction domain common to monochromator and sample assumes in the 3D DuMond diagram. An experiment is reported for the less commonly used σ–π topography, showing how the lattice mismatch and its lateral homogeneity are determined in samples made by epilayer and substrate. Received: 31 July 2000 / Accepted: 25 January 2001 / Published online: 3 May 2001     

Structure and dynamics of clean and adsorbate-covered crystal surfaces studied by surface X-ray diffraction

Received: 24 March 1998/Accepted: 21 August 1998     

Chemical-state specificity in surface structure determination

In the pursuit of an understanding of complex surfaces, the problem of obtaining quantitative structural information about local adsorbate geometry is especially difficult. Conventional diffraction methods rely on long-range order of the adsorbed species, rarely present in complex coadsorption systems. Elementally specific local structural probes can help, but ultimately one also requires chemical-state specificity. This can be achieved in structural methods that involve detection of photoelectrons through the well-known ‘chemical shifts’ in core-level photoelectron binding energies; specific methods of this type are scanned-energy mode photoelectron diffraction (PhD) and normal-incidence X-ray standing waves (NISXW). Recent examples of chemical-shift PhD and NIXSW applications to complex coadsorption systems and to larger molecular adsorbates demonstrate this potential. Received: 28 April 2000 / Accepted: 6 June 2000 / Published online: 7 March 2001     

Influence of Zn doping on electrooptical properties and structure parameters of lithium niobate crystals

We report the dependence of the unit cell parameters and the EO coefficients on Zn doping in optical-damage-resistant LiNbO₃:Zn crystals. Both properties depend in a non-monotonic manner on the Zn content. This is accounted for by different types of Zn ion incorporation into the lattice depending on the Zn concentration in the melt. Extrema observed in the concentration dependence of the EO coefficients at about 2–3 and 6.4 mol.% Zn correlate with an unusual concentration dependence of the unit cell parameters a and c. The low-concentration anomalies may be accounted for by a decrease of the Li vacancy concentration due to the Zn incorporation into Li sites. Anomalies at high concentrations are obviously due to a partial incorporation of Zn ions on Nb sites, which is reflected in the structure data. Anomalies in the concentration dependence of other optical properties at about 6–7 mol.% Zn reported recently are obviously related to a change in the localization of the Zn ions. The combination of high EO coefficients with a reduced optical damage for these concentrations make these crystals attractive for applications as Q-switching or electrooptical modulation. Received: 16 Nowember 1998 / Revised version: 8 December 1998 / Published online: 24 March 1999     

Structural and optical properties of nanophase zinc oxide

Nanophase zinc oxide samples were synthesized by a two-step solid-state reaction method. The phase structure and microstructure were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The vibrational Raman spectra were compared with those from the bulk and their grain size dependence was also examined. Their photoelectric behavior was studied by X-ray photoelectron spectroscopy (XPS). The peaks at 1044.5 and 1021.4 eV were recorded as corresponding to the respective binding energies of Zn 2p_1/2 and Zn 2p_3/2, and the photoelectron spectrum of O 1s in the as-prepared powder was located at 531.2 eV. A strong visible emission centered at 580 nm was clearly observed in the nanosized zinc oxide at room temperature. Photoluminescence (PL) spectra were investigated as a function of grain size after different heat treatments. The origin of the luminescence is attributed to the recombination of electrons in singly occupied oxygen vacancies with photoexcited holes in the valence band. Received: 30 June 2001 / Accepted: 20 February 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: mszhang@nju.edu.cn     

Inelastic X-ray scattering: new possibilities for Raman spectroscopy

Inelastic X-ray scattering with meV resolution has recently become available for studies of dynamical properties and elementary excitations in solids. Contrary to Raman spectroscopy at visible wavelengths, which in first order is limited to the Brillouin-zone center, the wave vectors in hard X-ray Raman scattering are very large, and the crystal-momentum transfer to elementary excitations, whose energies may range from a few meV up to several eV, can be tuned continuously across the whole Brillouin zone. This paper reviews new and unique possibilities offered by X-ray Raman spectroscopy for crystalline solids, such as phonon-dispersion measurements (GaN), the determination of phonon self-energies (isotopically mixed diamond), and resonance effects and studies of electronic excitations (copper oxides). Received: 19 October 2001 / Accepted: 12 December 2001 / Published online: 27 March 2002 / Published online: 27 March 2002 RID="*" ID="*"Present address: Agilent Technologies Deutschland GmbH, Herrenberger Str. 130, 71034 B?blingen, Germany     

Parallel excitation pathways in ultrafast interferometric pump-probe correlation measurements of hot-electron lifetimes in metals

Hot electron (E-E_Fermi=0.75 to 1.55 eV) lifetimes for cesiated Cu(100) and Cu(111) surfaces are measured via interferometric time-resolved two-photon photoemission with a 19-fs intensity FWHM mode locked Ti:sapphire laser at 1.55 eV. The data are analyzed using the optical Bloch equations and a laser pulse characterized in situ via surface second-harmonic generation interferometric autocorrelation. It is found that the retrieved hot-electron lifetimes are unphysically fast, and have a strong dependence on the temperature of the sample and the polarization of the laser. A simple explanation for the data is that the measured signal consists of contributions from transitions through both virtual and real intermediate states. Received: 26 July 2000 / Accepted: 8 September 2000 / Published online: 12 October 2000     

Heteroepitaxial growth of oxides on sapphire induced by laser radiation in the solid–liquid interface

2 O₃, Fe₂O₃ and MnO₂ on sapphire from an aqueous solutions of either CrO₃, FeCl₃, or KMnO₄, respectively, under laser irradiation of the interface sapphire/liquid. The interface is exposed through the sapphire substrate to the radiation of a copper vapor laser (wavelength of 510 nm). The etching of sapphire is accompanied by the deposition of oxide films, which are shown to grow epitaxially on the sapphire substrate, while the deposition of the polycrystalline oxide film occurs on a glass substrate under the same experimental conditions. Similarly, the epitaxial growth of cubic Fe₂O₃ and orthorhombic MnO₂ is observed, though their crystallographic structure is different from the hexagonal structure of sapphire. Received: 26 June 1997/Accepted: 7 July 1997     

Curved crystal analysis using a triple-crystal X-ray spectrometer

A Bragg crystal spectrometer using cylindrically curved crystals is designed to focus a divergent X-ray beam. The spectral resolution and precision of focus is determined by the quality of the concavely curved crystal. The analysis of the quality of the crystal was done using a triplecrystal X-ray spectrometer, comparing parallel and antiparallel X-ray scans of the third curved crystal, determining flaws resulting from the bend. Topics discussed include the derivation of the crystal's quality, conditions of alignment for the Bragg reflection, and X-ray beam width contributions to the FWHM. Organic and inorganic crystals with varying values of Bragg angles are used.     

Stereoscopic flow-tagging velocimetry

A laser-based method for measuring the three components of the velocity in a plane simultaneously and instantaneously without seed particles is presented. This is achieved by combining a laser flow-tagging technique with stereoscopic detection, in which the tagged flow is viewed from two different directions. A single CCD camera is employed for this purpose by using a new optical detection system. The flow tagging is performed by two consecutive laser pulses, i.e., “write” and “read” laser pulses. The write laser creates a grid of tracer molecules (NO) by inducing a photodissociation process. The three-dimensional motion of the tracer molecules is measured by a thick read laser sheet. Received: 22 July 1999 / Revised version: 5 August 1999 / Published online: 30 September 1999     

Electronic structure of light emitting centers in Er doped Si

Ab-initio calculations are carried out for the Er-related electrically active centers in Si. Our proposed microscopic model is consistent with photoluminescence measurements on Si:Er and Si:Er:O samples. For isolated Er, the tetrahedral interstitial site is the stable configuration, being related to the photoluminescence lines in Si:Er. Several configurations containing oxygen and fluorine atoms, surrounding the Er impurities, are proposed to simulate the effects of co-implantation. The results suggest that six oxygen atoms around substitutional Er can stabilize the center, which can be related to the strong photoluminescence lines in Si:Er:O samples. On the other hand, no configuration containing fluorine atoms could explain the stronger photoluminescence lines resulting from fluorine co-implantation. Received: 9 September 2002 / Accepted: 12 September 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +55-11/3091-5585, E-mail: jjusto@lme.usp.br     

Flame growth and wrinkling in a turbulent flow

High-speed planar laser-induced fluorescence (PLIF) and 3-D large eddy simulations (LES) are used to study turbulent flame kernel growth, wrinkling and the formation of separated flame pockets in methane/air mixtures. Turbulence was effected by a set of rotary fans situated in a cylindrical enclosure. Flame wrinkling was followed on sequential 2-D OH images captured at kHz repetition rates. Under stoichiometric conditions and low turbulence levels the flame kernel remains singly connected and close to spherical in shape. By increasing turbulence or reducing the stoichiometry of the mixture the formation of separated pockets could be observed and studied. The mechanisms behind these phenomena are investigated qualitatively by LES of a level-set G-equation describing the flame surface propagation in turbulent flows. Received: 12 April 2000 / Revised version: 26 June 2000 / Published online: 5 October 2000     

Layer-resolved microscopy of magnetic domains in multi-layered systems

Photoelectron emission microscopy in connection with magnetic circular dichroism in soft X-ray absorption can be used for the microscopic imaging of magnetic domains in layered thin film structures consisting of several magnetic layers. Due to the element-selectivity of the method, the different magnetic layers in such a structure can be imaged separately, provided that they contain different elements. This has been applied for the investigation of Co/Cu/Ni trilayers, epitaxially grown on Cu (001). The magnetic coupling between the Co and Ni layers can be directly visualized from comparing layer-resolved magnetic domain images of both layers. As a consequence of the competition between the anisotropy energies of the two magnetic layers and the magnetic coupling energy, spin-reorientation transitions between collinear and non-collinear magnetic configurations are observed. Apart from this globally observable magnetic interlayer coupling a micromagnetic coupling mechanism is also evident from the layer-resolved domain images. It is caused by magnetostatic interaction of local stray fields from domain walls. Received: 22 August 2002 / Accepted: 2 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-345/5511-223, E-mail: kuch@mpi-halle.de     

Surface electronic structure of Gd(0001) films on W(110)

Received: 1 September 1996/Accepted: 6 January 1997     

Simultaneous multiple-line Raman/Rayleigh/LIF measurements in combustion

It is demonstrated that multiple 1D Raman scattering, Rayleigh scattering, and laser-induced fluorescence (LIF) measurements can be performed simultaneously. This can be used for quasi-2D (or quasi-3D) single-shot measurements of multiple species and the temperature in turbulent reacting and non-reacting flows. The technique has the potential to yield more precise information than most competitive planar imaging approaches in combustion. For example, it can be used to overcome Raman/LIF interference problems in technical flames. This is achieved by a new optical set-up that makes use of an imaging spectrograph combined with fiber optics. Received: 13 October 1999 / Revised version: 26 November 1999 / Published online: 27 January 2000     

Molar volume and compactness versus mean coordination number in GeSeFe glasses: observation of a threshold behavior

Molar volumes, V, and compactness values, α, of 34 glassy compositions of the GeSeFe system have been obtained using their measured densities. The property–composition dependences are examined in light of the models proposed for the structure of these network glasses. Received: 28 August 2000 / Accepted: 9 January 2001 / Published online: 23 March 2001     

Structural and Magnetic Properties of Codoped ZnO based Diluted Magnetic Semiconductors

Zn1-xCoxO （x = 0.01, 0.02, 0.05, 0.10 and 0.20） diluted magnetic semiconductors are prepared by the sol-gel method. The structural and magnetic properties of the samples are studied using x-ray diffraction （XRD）, extended x-ray absorption fine structure （EXAFS） and superconducting quantum interference device （SQUID）. The XRD patterns does not show any signal of precipitates that are different from wurtzite type ZnO when Co content is lower than x = 0.10. An EXAFS technique for the Co K-edge has been employed to probe the local structures around Co atoms doped in ZnO powders by fluorescence mode. The simulation results for the first shell EXAFS signals indicate that Zn sites can be substituted by Co atoms when Co content is lower than x = 0.05. The SQUID results show that the samples （x 〈 0.05） exhibit clear hysteresis loops at 300K, and magnetization versus temperature from 5 K to 350K at H = 100 Oe for the sample x = 0.02 shows that the samples have ferromagnetism above room temperature. A double-exchange mechanism is proposed to explain the ferromagnetic properties of the samples.     

Growth of Semi-Insulating GaN by Using Two-Step AIN Buffer Layer

Semi-insulating GaN is grown by using a two-step A1N buffer layer by metalorganic chemical vapour deposition. The sheet resistance of as-grown semi-insulating GaN is dramatically increased to 10^13 Ω/sq by using two-step A1N buffer instead of the traditional low-temperature GaN buffer. The high sheet resistance of as-grown GaN over 10^13 Ω/sq is due to inserting an insulating buffer layer （two-step A1N buffer） between the high-temperature GaN layer and a sapphire substrate which blocks diffusion of oxygen and overcomes the weakness of generating high density carrier near interface of GaN and sapphire when a low-temperature GaN buffer is used. The result suggests that the high conductive feature of unintentionally doped GaN is mainly contributed from the highly conductive channel near interface between GaN and the sapphire substrate, which is indirectly manifested by room-temperature photoluminescence excited by an incident laser beam radiating on growth surface and on the substrate. The functions of the two-step A1N buffer layer in reducing screw dislocation and improving crystal quality of GaN are also discussed.     

High repetition rate planar laser induced fluorescence of OH in a turbulent non-premixed flame

Received: 1 February 1999 / Revised version: 17 February 1999     

Reconstruction of parasitic holograms to characterize photorefractive materials

Photoinduced light scattering is a serious drawback that limits the applicability of thick holographic recording media but provides valuable information on the recording medium. As long as there is no correlation between the scattering centers in the crystal, photoinduced light scattering may be explained to result from the interference pattern of the incident beam and the field scattered from a single point-like scattering center. The hologram of this ellipsoidally scattered wave field will have practically the same structure in the reciprocal space modified by a response function which reflects the anisotropic properties of the recording medium. We studied photoinduced light scattering in LiNbO₃:Fe, a model system for photorefractive materials. The transmitted intensity in the stationary state of the scattering process is investigated as a function of the reconstruction angle at different wavelengths and polarizations of the reconstructing beam. The experimental results are analyzed by a simple phenomenological model based on the Ewald construction and can be used to choose suitable conditions at which holographic scattering can be minimized as well as to extract some physical parameters of the crystal. Received: 27 September 2000 / Revised version: 1 December 2000 / Published online: 21 February 2001