Probing the dynamics of quasicrystal growth using synchrotron live imaging

The dynamics of quasicrystal growth remains an unsolved problem in condensed matter. By means of synchrotron live imaging, facetted growth proceeding by the tangential motion of ledges at the solid-melt interface is clearly evidenced all along the solidification of icosahedral AlPdMn quasicrystals. The effect of interface kinetics is significant so that nucleation and free growth of new facetted grains occur in the melt when the solidification rate is increased. The evolution of these grains is explained in details, which reveals the crucial role of aluminum rejection, both in the poisoning of grain growth and driving fluid flow

Electronic and physico-chemical properties of nanmetric boron delta-doped diamond structures

Heavily boron doped diamond epilayers with thicknesses ranging from 40 to less than 2 nm and buried between nominally undoped thicker layers have been grown in two different reactors. Two types of [100]-oriented single crystal diamond substrates were used after being characterized by X-ray white beam topography. The chemical composition and thickness of these so-called deltadoped structures have been studied by secondary ion mass spectrometry, transmission electron microscopy, and spectroscopic ellipsometry. Temperature-dependent Hall effect and four probe resistivity measurements have been performed on mesa-patterned Hall bars. The temperature dependence of the hole sheet carrier density and mobility has been investigated over a broad temperature range (6K<T<450 K). Depending on the sample, metallic or non-metallic behavior was observed. A hopping conduction mechanism with an anomalous hopping exponent was detected in the non-metallic samples. All metallic delta-doped layers exhibited the same mobility value, around 3.660.8 cm2/Vs, independently of the layer thickness and the substrate type. Comparison with previously published data and theoretical calculations showed that scattering by ionized impurities explained only partially this low common value. None of the delta-layers showed any sign of confinement-induced mobility enhancement, even for thicknesses lower than 2 nm.14 page

One-dimensional Model of a Gamma Klystron

A new scheme for amplification of coherent gamma rays is proposed. The key elements are crystalline undulators - single crystals with periodically bent crystallographic planes exposed to a high energy beam of charged particles undergoing channeling inside the crystals. The scheme consists of two such crystals separated by a vacuum gap. The beam passes the crystals successively. The particles perform undulator motion inside the crystals following the periodic shape of the crystallographic planes. Gamma rays passing the crystals parallel to the beam get amplified due to interaction with the particles inside the crystals. The term `gamma klystron' is proposed for the scheme because its operational principles are similar to those of the optical klystron. A more simple one-crystal scheme is considered as well for the sake of comparison. It is shown that the gamma ray amplification in the klystron scheme can be reached at considerably lower particle densities than in the one-crystal scheme, provided that the gap between the crystals is sufficiently large.Comment: RevTeX4, 22 pages, 4 figure

New possibilities of diffraction topography at third generation synchrotron radiation facilities

Diffraction topography is an imaging technique for single crystals which allows, through the mapping of the local Bragg reflecting power, the visualization of defects, domains, phases,..., present within the crystal volume. We describe some of the new possibilities of this technique associated with the third generation synchrotron radiation machines : real time observations in the 10-2 second range, investigation of heavy or bulky materials, and high resolution experiments. These new possibilities are illustrated by the first diffraction topographic observations performed at the ESRF, which were carried out, in white beam, either on the wiggler of the ID11 "Materials Science" beamline or on the D5 Optics "Open Bending Magnet' beamline. The main features of these topographs are the short exposure times (≈10-2s), the high energy corresponding to the recorded Bragg spots (≈ 50-100 keV), and the good spatial resolution retained even when setting the film far (≈ 60 cm) from the sample. We finally briefly describe the ESRF ID19 beamline devoted to high resolution diffraction and topography

Characterization of dislocations in protein crystals by means of synchrotron double-crystal topography

5 pages, 8 figures.Hen egg-white lysozyme (HEWL) crystals have been studied by means of double-crystal synchrotron topography. The crystals reveal a number of features that are quite well known in hydrothermally grown inorganic crystals: dislocations, growth bands and growth sector boundaries. Dislocations in the sectors have been characterized as edge dislocations with Burgers vector parallel to the c axis. They are distinguishable only under weak beam conditions. The presence of edge dislocations shown in this paper is consistent with the spiral growth steps previously reported. This spiral growth on protein crystals has been observed many times by surface techniques.We are very indebted to Dr M. C. Robert who kindly helped us to examine the topographs and to analyse the various features of their contrast.Peer reviewe

Hybrid and effective satellites for studying superlattices

Abstract Observation of new synchrotron X-ray scattering processes in semiconductor superlattice structures are reported. They are analogous to the three-beam diffraction in single crystal; however, the basic difference is that in these new processes superlattice-satellite reflections came to play. They give rise to effective-satellite reflections (superlattice -superlattice coupling) and hybrid-satellite reflections (substratesuperlattice coupling). These sort of reflections are features that depend on the rotation of the sample around the surface-normal direction, i.e. an azimuthal or f rotation. Their positions in f are very sensitive to the in-plane projection of the reciprocal space, but while the effectivesatellite reflections are sensitive to the superlattice parameters, the positions of the hybrid-satellite reflections depend mostly on the substrate ones. The selective sensitivity of these two sort of reflections is the physical fact that can be used as a new tool for studying superlattices.

Hydrostatic Pressure Effect on Oxygen Precipitates in Silicon Single Crystal

The effect of hydrostatic pressure on some properties of Cz-Si with oxygen precipitates is investigated. The observed phenomena are discussed in terms of misfit between the precipitates and Si matrix

Transformation of AlGaAs/GaAs Interface under Hydrostatic Pressure

AlGaAs layers grown by molecular beam epitaxy on GaAs substrates were investigated before and after high hydrostatic pressure (1.2 GPa) at high temperature (770 K) treatment (HP-HT treatment). An influence of HP-HT treatment on the properties of the AlGaAs/GaAs system was studied by lattice parameter measurements using the high resolution diffractometer and by X-ray topography. Observed changes in the lattice parameter of the AlGaAs layers after HP-HT treatment are related to the strain relaxation and explained by the creation of misfit dislocations and other extended defects which are visible on the topographs