Institute of Crystallography

Aachen, Germany

Institute of Crystallography

Aachen, Germany
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Caliandro R.,Institute of Crystallography | Guccione P.,Polytechnic of Bari | Nico G.,CNR Institute of Neuroscience | Tutuncu G.,Brookhaven National Laboratory | Hanson J.C.,Brookhaven National Laboratory
Journal of Applied Crystallography | Year: 2015

Modulated enhanced diffraction (MED) is a technique allowing the dynamic structural characterization of crystalline materials subjected to an external stimulus, which is particularly suited for in situ and operando structural investigations at synchrotron sources. Contributions from the (active) part of the crystal system that varies synchronously with the stimulus can be extracted by an offline analysis, which can only be applied in the case of periodic stimuli and linear system responses. In this paper a new decomposition approach based on multivariate analysis is proposed. The standard principal component analysis (PCA) is adapted to treat MED data: specific figures of merit based on their scores and loadings are found, and the directions of the principal components obtained by PCA are modified to maximize such figures of merit. As a result, a general method to decompose MED data, called optimum constrained components rotation (OCCR), is developed, which produces very precise results on simulated data, even in the case of nonperiodic stimuli and/or nonlinear responses. The multivariate analysis approach is able to supply in one shot both the diffraction pattern related to the active atoms (through the OCCR loadings) and the time dependence of the system response (through the OCCR scores). When applied to real data, OCCR was able to supply only the latter information, as the former was hindered by changes in abundances of different crystal phases, which occurred besides structural variations in the specific case considered. To develop a decomposition procedure able to cope with this combined effect represents the next challenge in MED analysis. © 2015 International Union of Crystallography.

Samoylova L.,German Electron Synchrotron | Buzmakov A.,Institute of Crystallography | Chubar O.,Brookhaven National Laboratory | Sinn H.,German Electron Synchrotron
Journal of Applied Crystallography | Year: 2016

This article describes the WavePropaGator (WPG) package, a new interactive software framework for coherent and partially coherent X-ray wavefront propagation simulations. The package has been developed at European XFEL for users at the existing and emerging free-electron laser (FEL) facilities, as well as at the third-generation synchrotron sources and future diffraction-limited storage rings. The WPG addresses the needs of beamline scientists and user groups to facilitate the design, optimization and improvement of X-ray optics to meet their experimental requirements. The package uses the Synchrotron Radiation Workshop (SRW) C/C++ library and its Python binding for numerical wavefront propagation simulations. The framework runs reliably under Linux, Microsoft Windows 7 and Apple Mac OS X and is distributed under an open-source license. The available tools allow for varying source parameters and optics layouts and visualizing the results interactively. The wavefront history structure can be used for tracking changes in every particular wavefront during propagation. The batch propagation mode enables processing of multiple wavefronts in workflow mode. The paper presents a general description of the package and gives some recent application examples, including modeling of full X-ray FEL beamlines and start-to-end simulation of experiments.The WavePropaGator (WPG) package is a new interactive cross-platform open-source software framework for modeling of coherent and partially coherent X-ray wavefront propagation. The WPG addresses the needs of beamline scientists and user groups to facilitate the design, optimization and improvement of X-ray optics to meet their experimental requirements. The paper presents a general description of the package and gives some recent application examples. © Liubov Samoylova et al. 2016.

Ziegler E.,European Synchrotron Radiation Facility | Peverini L.,European Synchrotron Radiation Facility | Vaxelaire N.,European Synchrotron Radiation Facility | Cordon-Rodriguez A.,European Synchrotron Radiation Facility | And 3 more authors.
Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment | Year: 2010

The possibility of smoothening aspherical X-ray mirrors by irradiation of the surface with a low-energy ion beam is investigated. Nanofocusing being the primary application of these mirrors the ion beam conditions must be optimized to achieve a surface roughness of the order of 0.1-0.2 nm. To address this issue a first study was performed on silicon flat substrates etched using ion energies ranging from 400 to 1200 eV. A second study consisted of eroding the silicon surface while varying the ion grazing incidence angle between 10° and 90° for a fixed value of the ion energy. The surface topography of the samples was characterized at various scales using atomic force microscopy (probed area: 1-10 μm2), interferential optical microscopy (probed area: 1 mm2) and X-ray scattering (probed area: 100 mm2). Finally, a study by AFM of the evolution of the surface finish level of a silicon mirror after ion erosion at various depth values up to 10 μm allowed a trade off to be found between total etch time and the finish quality level in view of profiling a highly aspherically shaped mirror starting from a flat surface. © 2010 Elsevier B.V. All rights reserved.

Kozhevnikov I.V.,Institute of Crystallography | Peverini L.,European Synchrotron Radiation Facility | Peverini L.,Societe Europeenne de Systemes Optiques | Ziegler E.,European Synchrotron Radiation Facility
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

A method capable of extracting the depth distribution of the dielectric constant of a thin film deposited on a substrate and the three power spectral density (PSD) functions characterizing its roughness is presented. It is based on the concurrent analysis of x-ray reflectivity and scattering measurements obtained at different glancing angle values of the probe beam so that the effect of roughness is taken into account during reconstruction of the dielectric constant profile. Likewise, the latter is taken into account when determining the PSD functions describing the film roughness. This approach is using a numerical computation iterative procedure that demonstrated a rapid convergence for the overall set of data leading to a precise description of the three-dimensional morphology of a film. In the case of a tungsten thin film deposited by dc-magnetron sputtering onto a silicon substrate and characterized under vacuum, the analysis of the x-ray data showed the tungsten density to vary with depth from 95% of the bulk density at the top of the film to about 80% near the substrate, where the presence of an interlayer, estimated to be 0.7 nm thick, was evidenced. The latter may be due to diffusion and/or implantation of tungsten atoms into the silicon substrate. In the reconstruction of the depth profile, the resolution (minimum feature size correctly reconstructed) was estimated to be of the order of 0.4-0.5 nm. The depth distribution of the dielectric constant was shown to affect the roughness conformity coefficient extracted from the measured x-ray scattering distributions, while the deposition process increased the film roughness at high spatial frequency as compared to the virgin substrate. On the contrary, the roughness showed a weak influence on the dielectric constant depth profile extracted, as the sample used in our particular experiment was extremely smooth. © 2012 American Physical Society.

PubMed | Helmholtz Center Berlin, Saint Petersburg State University, Fraunhofer Institute Angewandte Optik und Feinmechanik and Institute of Crystallography
Type: Journal Article | Journal: Science and technology of advanced materials | Year: 2016

We developed a mathematical analysis method of reflectometry data and used it to characterize the internal structure of TiO

Bianconi A.,Rome International Center for Materials Science Superstripes | Bianconi A.,Institute of Crystallography | Bianconi A.,National Institute of Nuclear Physics, Italy
Journal of Superconductivity and Novel Magnetism | Year: 2016

The presence of two components in the electron fluid of high-temperature superconductors and the complex charge and lattice inhomogeneity have been the hot topics of the international conference of the superstripes series, SUPERSTRIPES 2015, held in Ischia in 2015. The debate on the mechanisms for reaching room-temperature superconductors has been boosted by the discovery of superconductivity with the highest critical temperature in pressurized sulfur hydride. Different complex electronic and structural landscapes showing up in superconductors which resist to the decoherence effects of high temperature have been discussed. While low-temperature superconductors described by the BCS approximation are made of a single condensate in the weak coupling, the high-temperature superconductors are made of coexisting multiple condensates (in different spots of the k-space and the real space) some in the BCS-BEC crossover regime and others in the BCS regime. The role of “shape resonance” in the exchange interaction between these different condensates, like “the Fano-Feshbach resonance” in ultracold gasses, is emerging as a key term for high-temperature superconductivity. © 2016 Springer Science+Business Media New York

Kozhevnikov I.V.,Institute of Crystallography | Montcalm C.,XENOCS | Montcalm C.,Iridian Technologies
Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment | Year: 2010

We describe several techniques to design X-ray multilayer mirrors with enhanced bandwidth and, more especially, bandwidth with a shape that optimizes its matching with a given source angular distribution. A Gaussian angular distribution source emitting at the characteristic Mo-Kα radiation (λ=0.071 nm), a radiation regularly used in X-ray analytical applications, is used for the demonstration. Three approaches are developed and compared to design multilayer structures with maximum efficiency, where efficiency is defined here as the convolution of the source and reflectance angular profiles. The first (most general) approach, which can be applied for arbitrary source profiles, is based on the exact EulerLagrange equation and a general expression for the multilayer mirror reflectivity of arbitrary wideband. With this method, the depth-distribution of the multilayer bi-layer thickness was found to be smooth. Numerical refinements of the obtained analytical solution were not useful, giving insignificant efficiency increases despite significant complication of the thickness depth-distribution of the multilayer stack. The efficiency of some of these depth-graded structures exceeds that of a periodic mirror by a factor of two in our case. The main factor limiting the efficiency was absorption of the radiation by the materials. The second approach is based on designing a simpler depth-graded multilayer mirror with a linear variation of bi-layer thickness and a constant thickness ratio. Such a mirror proved to have an efficiency close to the maximum possible one. The same conclusion is also true for the simplest design approach, where several periodic substructures are stacked on top of each other, our third described approach. We found that about only four substructures are necessary. In this case the optimization procedure is based on simply three fitting parameters, independently of the number of periodic substructures. We show that the efficiency decreases quickly with increasing interface width. Therefore, multilayer structures with abrupt interfaces, e.g., Mo2C/Si multilayers instead of the more conventional Mo/Si multilayers, are preferable in practice. The efficiency also depends strongly on the mean grazing angle incident on the mirror and the angular width of the source (for a fixed number of bi-layers). This must be taken into consideration when designing wide-band multilayer coated X-ray collimators or concentrators. © 2010 Elsevier B.V. All rights reserved.

Shekhar Pandey C.,Ruhr University Bochum | Schreuer J.,Ruhr University Bochum | Burianek M.,Institute of Crystallography | Muhlberg M.,Institute of Crystallography
Applied Physics Letters | Year: 2013

The relaxor behavior of tetragonal tungsten bronze uniaxial relaxor ferroelectric calcium strontium barium niobate (Ca0.22Sr 0.12Ba0.66Nb2O6 or CSBN-22) single crystal was studied by measuring elastic constants and thermal expansion with the aid of resonant ultrasound spectroscopy and dilatometry, respectively, in the temperature range 300 K-1503 K. Thermal expansion yields evidence of the Burns temperature TB and the intermediate characteristic temperature T*, which was also supported by the temperature evolutions of the elastic constants cij. CSBN-22 was found to be ∼2%-3% elastically stiffer than CBN-28. The presented results open the perspective to understand the relaxor behavior of CSBN. © 2013 American Institute of Physics.

Yakshin A.E.,FOM Institute for Atomic and Molecular Physics | Kozhevnikov I.V.,Institute of Crystallography | Zoethout E.,FOM Institute for Atomic and Molecular Physics | Louis E.,FOM Institute for Atomic and Molecular Physics | And 2 more authors.
Optics Express | Year: 2010

The optical properties of a-periodic, depth-graded multilayer mirrors operating at 13.5 nm wavelength are investigated using different compositions and designs to provide a constant reflectivity over an essentially wider angular range than periodic multilayers. A reflectivity of up to about 60% is achieved in these calculation in the [0, 18°] range of the angle of incidence for the structures without roughness. The effects of different physical and technological factors (interfacial roughness, natural interlayers, number of bi-layers, minimum layer thickness, inaccuracy of optical constants, and thickness errors) are discussed. The results from an experiment on the fabrication of a depth-graded Mo/Si multilayer mirror with a wide angular bandpass in the [0, 16°] range are presented and analyzed. © 2010 Optical Society of America.

Altomare A.,Institute of Crystallography | Cuocci C.,Institute of Crystallography | Moliterni A.,Institute of Crystallography | Rizzi R.,Institute of Crystallography
Journal of Applied Crystallography | Year: 2013

The new method RAMM (random-model-based method) has been developed and implemented in the EXPO computing program for improving the ab initio crystal structure solution process. When the available information consists of only the experimental powder diffraction pattern and the chemical formula of the compound under study, the classical structure solution approach follows two main steps: (1) phasing by direct methods (or by Patterson methods) in order to obtain a structure model (this last is usually incomplete and/or approximate); (2) improving the model by structure optimization techniques. This article proposes the alternative procedure RAMM, which skips step (1) and supplies a fully random model to step (2). This model is then submitted to effective structure optimization tools present in EXPO - wLSQ (weighted least squares), RBM (resolution bias minimization) and COVMAP (a procedure of electron density modification based on the concept of covariance between points of the map) - which are able to lead to the correct structure. RAMM is based on a cyclic process, generating several random models which are then optimized. The process stops automatically when it recognizes the correct structure. Copyright © International Union of Crystallography 2013.

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