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This thesis addresses one of the major outstanding problems in the study of self-assembled InAs quantum dots (QDs): their physical profile after deposition of a capping layer and post-growth processing. The optical properties of QDs depend critically on the shape, composition and strain profile, yet these parameters are inaccessible to most experimental techniques once the dots are buried. Data from various x-ray scattering experiments are presented here, along with a novel approach to simulating diffuse scattering using an atomistic model based on Keating energy minimisation. The size and position of the diffuse scattering on the low-Q side of the Bragg peak, which are strongly influenced by the shape and composition of the QDs, has been used to determine that the QDs are truncated pyramids with a diagonal base length of 28 nm, with their edges aligned along the [100] and [010] directions. The composition profile varies from pure InAs at the top to 40-60% InAs at the base. These properties all agree with recent cross-sectional scanning tunnelling microscopy (X-STM) measurements by Bruls et al. It was shown that post-growth annealing causes a reduction in the In content of the QDs, primarily by diffusion from the base of the dot into the wetting layer. Grazing incidence small angle x-ray scattering (GISAXS) measurements have been made from samples of QDs produced with varying growth interruptions (GI) before deposition of the capping layer. The QDs were found to be highly diffuse. After a GI, the dots have been shown to change shape anisotropically, with two facets becoming sharper. An investigation of the use of resonant scattering to study buried QDs has shown that the method of contrast variation is of limited use for enhancing the measurement of diffuse features away from the Bragg peak. It is unsuitable for the study of buried nanostructures.

Thesis (Ph. D.)--University of Hong Kong, 1990.

The second report from the Project-Group ESRF-Beamline of the Forschungszentrum Rossendorf covers the period from July 1999 until December 2000. The ROssendorf BeamLine (ROBL) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France performed quite well during this time. In the beamtime used by the FZR and collaborating institutes 44 scheduled experiments were performed, while in the ESRF scheduled beamtime 12 experiments. Additionally, a distinct amount of beamtime was devoted to in-house research of the FZR and methodical experiments. Since February 2000 ROBL is part of the European Commission's programme "Access to Research Infrastructure" which supports user groups from member and associated states of the EU during experiments at ROBL. In the year 2000 ROBL hosted 6 groups for experiments. The report is organised in three main parts. The first part contains extended contributions on results obtained at ROBL. The second part gives an overview about the scheduled experiments, publications, guests having visited ROBL with support of the EC, and some other information. Finally, the third part collects the experimental reports of the user groups received.

The Rossendorf Beamline (ROBL) - located at BM20 of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France - is in operation since 1998. This 6th report covers the period from January 2007 to December 2008. In these two years, 50 peerreviewed papers have been published based on experiments done at the beamline. The average citation index, which increased constantly over the years, has now reached 3.5 (RCH) and 3.0 (MRH), indicating that papers are predominately published in journals with high impact factors. Six exemplary highlight reports on the following pages should demonstrate the scientific strength and diversity of the experiments performed on the two end-stations of the beamline, dedicated to Radiochemistry (RCH) and Materials Research (MRH). Demand for beamtime remains very high as in the previous years, with an average oversubscription rate of 1.8 for ESRF experiments. The attractiveness of our beamline is based upon the high specialization of its two end-stations. RCH is one of only two stations in Europe dedicated to x-ray absorption spectroscopy of actinides and other radionuclides. The INE beamline at ANKA provides superior experimental flexibility and extends to lower energies, including important elements like P and S. In contrast, ROBL-RCH provides a much higher photon flux, hence lower detection limits crucial for environmental samples, and a higher energy range extending to elements like Sb and I. Therefore, both beamlines are highly complementary, covering different aspects of radiochemistry research. Once the MARS beamline at SOLEIL is ready to run radionuclides (>2010), it will cover a third niche (Materials Science of actinides, including irradiated fuel) not accessible for the two other beamlines. The Materials Research Hutch MRH has realized an increasing number of in-situ investigations in the last years. On the one hand thin film systems were characterized during magnetron sputtering. On the other hand diffraction experiments under controlled atmosphere were performed. A high variety of experimental parameters was covered by varying pressure, temperature and atmospheric compositions including highly reactive gases. Furthermore structural investigations were combined with electrical conductivity measurements. These kind of in-situ experiments are the key to monitor and understand reaction mechanism or the influence of process parameters, which are again the basis to tailor materials properties on demand. The core competences of MRH are these experimental possibilities, which make it unique among other diffraction beamlines. In fall 2007, ROBL was reviewed by an international panel on behalf of the ESRF. The very positive panel report recommended a renewal of the contract between ESRF and FZD for the next five years, and a major upgrade of critical optical components of the beamline to keep ROBL competitive for the next decade. The FZD will provide 2 Mio € from 2009 to 2011 for this upgrade, which will be performed in parallel to the major upgrade of the ESRF to minimize the downtime. According to the current plans of the ESRF, our users have to expect that ROBL will have only limited or no operation for several months from August 2011 on. Since July 2004 the beamline is a member of the pooled facilities of ACTINET – European Network of Excellence. In the reported period, RCH has provided 27 % of its inhouse beamtime to perform 11 ACTINET experiments. The success of ACTINET within FP-6 has now led to a renewal of ACTINET within FP-7, running until end of 2011.

No description available.

Particle accelerators are machines to accelerate and store charged particle beams, such as electrons or protons, to the energy levels for various scientific applications. There are three basic types of particle accelerators: linear accelerators (linac), storage-ring (or circular) accelerators, and recirculating accelerators. The third type, also the most recent one, is designed to accelerate a particle beam in a short section of linac, circulate and then continue to accelerate it for energy boost or decelerate it for energy recovery. The modern recirculating machines possess the advantages to both accelerate and preserve the beam with high beam quality, as well as efficiently reuse the accelerating components. As modern accelerators push toward the high-brightness or high-intensity frontier by demanding particles in a highly charged bunch to concentrate in an ever-decreasing beam phase space, the interaction amongst particles via their self-generated electromagnetic fields can potentially lead to coherent instabilities of the beam and thus pose significant challenges to the machine design and operation. Microbunching instability (MBI) has been one of the most challenging issues for such high-brightness or high-intensity beam transport, as it would degrade lasing performance in the fourth-generation light sources, reduce cooling efficiency in electron cooling facilities, and eventually compromise the luminosity of colliding beams in lepton or lepton-hadron colliders. The dissertation work will focus on the MBI in modern recirculating electron accelerators. The research attempts to develop a comprehensive theoretical formulation of MBI with aspects including among various degrees of freedoms the beam itself, the beamline lattice optics, and incorporation of all relevant collective effects that the beam encounters, for example the coherent synchrotron radiation (CSR) and the longitudinal space charge (LSC) effects. This dissertation includes the following seven themes: 1) Development and generalization of MBI theory to arbitrary linear lattices and coupled beams with constant and varying energies; 2) Construction of CSR impedance models from steady state to transient state and from high to low energy regime; 3) Numerical implementation of the developed theory as a fast and numerical-noise-free Vlasov solver and benchmarking with massive particle tracking simulation; 4) Exploration of multistage cascaded amplification mechanism of CSR microbunching development; 5) Control of CSR-induced MBI in multi-bend transport or recirculation arcs; 6) Study of more aspects of microbunched structures in beam phase spaces; and 7) Study of MBI for magnetized beams and confirming the suppression of MBI for a recent cooler design for Jefferson Lab Electron Ion Collider project. / Ph. D.

<p>This thesis is concerned with <i>in-situ</i> time-, temperature- and pressure-resolved synchrotron X-ray powder diffraction investigations of a variety of inorganic compounds with twodimensional layer structures and three-dimensional framework structures. In particular, phase stability, reaction kinetics, thermal expansion and compressibility at non-ambient conditions has been studied for 1) Phosphates with composition <i>M</i><i>IV</i>(HPO₄)₂·<i>n</i>H₂O (<i>M</i><i>IV</i> = Ti, Zr); 2) Pyrophosphates and pyrovanadates with composition<i> M</i><i>IV</i>X₂O₇(<i>M</i><i>IV</i> = Ti, Zr and X = P, V); 3) Molybdates with composition ZrMo₂O₈. The results are compiled in seven published papers and two manuscripts.</p><p>Reaction kinetics for the hydrothermal synthesis of α-Ti(HPO₄)₂·H₂O and intercalation of alkane diamines in α-Zr(HPO₄)₂·H₂O was studied using time-resolved experiments. In the high-temperature transformation of γ-Ti(PO₄)(H₂PO₄)·2H₂O to TiP₂O₇ three intermediate phases, γ'-Ti(PO₄)(H₂PO₄)·(2-x)H₂O, β-Ti(PO₄)(H₂PO₄) and Ti(PO₄)(H₂P₂O₇)_0.5 were found to crystallise at 323, 373 and 748 K, respectively. A new tetragonal three-dimensional phosphate phase called τ-Zr(HPO₄)₂ was prepared, and subsequently its structure was determined and refined using the Rietveld method. In the high-temperature transformation from τ-Zr(HPO₄)₂ to cubic α-ZrP₂O₇two new orthorhombic intermediate phases were found. The first intermediate phase, ρ-Zr(HPO₄)₂, forms at 598 K, and the second phase, β-ZrP₂O₇, at 688 K. Their respective structures were solved using direct methods and refined using the Rietveld method. <i>In-situ</i> high-pressure studies of τ-Zr(HPO₄)₂revealed two new phases, tetragonal ν-Zr(HPO₄)₂and orthorhombic ω-Zr(HPO₄)₂ that crystallise at 1.1 and 8.2 GPa. The structure of ν-Zr(HPO₄)₂ was solved and refined using the Rietveld method.</p><p>The high-pressure properties of the pyrophosphates ZrP₂O₇ and TiP₂O₇, and the pyrovanadate ZrV₂O₇were studied up to 40 GPa. Both pyrophosphates display smooth compression up to the highest pressures, while ZrV₂O₇ has a phase transformation at 1.38 GPa from cubic to pseudo-tetragonal β-ZrV₂O₇ and becomes X-ray amorphous at pressures above 4 GPa.</p><p>In-situ high-pressure studies of trigonal α-ZrMo₂O₈ revealed the existence of two new phases, monoclinic δ-ZrMo₂O₈and triclinic ε-ZrMo₂O₈ that crystallises at 1.1 and 2.5 GPa, respectively. The structure of δ-ZrMo₂O₈was solved by direct methods and refined using the Rietveld method.</p>

No description available.

No description available.