3,951 research outputs found

    Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials

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    Quantum ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). Quantum ESPRESSO stands for "opEn Source Package for Research in Electronic Structure, Simulation, and Optimization". It is freely available to researchers around the world under the terms of the GNU General Public License. Quantum ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively-parallel architectures, and a great effort being devoted to user friendliness. Quantum ESPRESSO is evolving towards a distribution of independent and inter-operable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.Comment: 36 pages, 5 figures, resubmitted to J.Phys.: Condens. Matte

    GEANT4 : a simulation toolkit

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    Abstract Geant4 is a toolkit for simulating the passage of particles through matter. It includes a complete range of functionality including tracking, geometry, physics models and hits. The physics processes offered cover a comprehensive range, including electromagnetic, hadronic and optical processes, a large set of long-lived particles, materials and elements, over a wide energy range starting, in some cases, from 250 eV and extending in others to the TeV energy range. It has been designed and constructed to expose the physics models utilised, to handle complex geometries, and to enable its easy adaptation for optimal use in different sets of applications. The toolkit is the result of a worldwide collaboration of physicists and software engineers. It has been created exploiting software engineering and object-oriented technology and implemented in the C++ programming language. It has been used in applications in particle physics, nuclear physics, accelerator design, space engineering and medical physics. PACS: 07.05.Tp; 13; 2

    Roadmap of ultrafast x-ray atomic and molecular physics

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    X-ray free-electron lasers (XFELs) and table-top sources of x-rays based upon high harmonic generation (HHG) have revolutionized the field of ultrafast x-ray atomic and molecular physics, largely due to an explosive growth in capabilities in the past decade. XFELs now provide unprecedented intensity (1020 W cm−2) of x-rays at wavelengths down to ~1 Ångstrom, and HHG provides unprecedented time resolution (~50 attoseconds) and a correspondingly large coherent bandwidth at longer wavelengths. For context, timescales can be referenced to the Bohr orbital period in hydrogen atom of 150 attoseconds and the hydrogen-molecule vibrational period of 8 femtoseconds; wavelength scales can be referenced to the chemically significant carbon K-edge at a photon energy of ~280 eV (44 Ångstroms) and the bond length in methane of ~1 Ångstrom. With these modern x-ray sources one now has the ability to focus on individual atoms, even when embedded in a complex molecule, and view electronic and nuclear motion on their intrinsic scales (attoseconds and Ångstroms). These sources have enabled coherent diffractive imaging, where one can image non-crystalline objects in three dimensions on ultrafast timescales, potentially with atomic resolution. The unprecedented intensity available with XFELs has opened new fields of multiphoton and nonlinear x-ray physics where behavior of matter under extreme conditions can be explored. The unprecedented time resolution and pulse synchronization provided by HHG sources has kindled fundamental investigations of time delays in photoionization, charge migration in molecules, and dynamics near conical intersections that are foundational to AMO physics and chemistry. This roadmap coincides with the year when three new XFEL facilities, operating at Ångstrom wavelengths, opened for users (European XFEL, Swiss-FEL and PAL-FEL in Korea) almost doubling the present worldwide number of XFELs, and documents the remarkable progress in HHG capabilities since its discovery roughly 30 years ago, showcasing experiments in AMO physics and other applications. Here we capture the perspectives of 17 leading groups and organize the contributions into four categories: ultrafast molecular dynamics, multidimensional x-ray spectroscopies; high-intensity x-ray phenomena; attosecond x-ray science

    Transition Metal Impurities in Semiconductors: Induced Magnetism and Band Gap Engineering

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    The main subject of this thesis is the study of electronic and magnetic properties of materials containing 3d transition metal atoms. Our motivation stems mainly from the modern fields of spintronic computing and solar energy conversion. The two primary goals of this work are to determine (i) why certain transition metal impurities in certain semiconductors can induce magnetic properties suitable for spintronic computing applications, and (ii) how transition metal impurities can be used to modify the electronic band gaps of semiconductors and insulators in ways useful for harnessing solar energy and for other applications. To accomplish these goals, we have applied both experimental and theoretical tools. We studied high quality materials prepared by advanced synthesis techniques using x-ray spectroscopy methods at synchrotron light sources. The results of these experiments were interpreted using a variety of theoretical techniques, primarily using computational software developed as part of this thesis and discussed herein. Regarding the study of introducing transition metal impurities into semiconductors to induce magnetic properties, we first developed and demonstrated a method to determine the location of impurity atoms within the host semiconductor lattice. This allowed to us explain the presence and absence of ferromagnetism in samples prepared under only slightly different synthesis conditions, which helped to address some long--standing issues in the spintronics field. We then studied an advanced and promising material -- indium (III) oxide with iron impurities -- to determine how magnetic ordering was maintained up to room temperatures. Our techniques unveiled that a portion of the iron atoms were coupled to oxygen vacancies in the material to create conditions which propelled the observed magnetism. This finding confirmed some earlier theoretical predictions by others in the field. For the study of electronic band gap modifications in semiconductors and insulators via the incorporation of transition metal atoms, we investigated a wide range of materials synthesized using different techniques. Again, we used experimental techniques to determine the location of impurity atoms within the materials, and used this to understand how band gaps were modified upon the introduction of the impurities. For Ti implantation into SiO2, Ni substitution into ZnO, and a new material, MnNCN, we have determined the electronic band gaps and used our techniques to explain how the values for the gaps arise. Finally, an additional outcome of this thesis work is a software program capable of simulating x-ray spectra using various advanced quantum models. We rewrote and built upon powerful existing programs and applied the result to the above studies. Our software was further applied in a collaborative effort with other researchers at the Canadian Light Source to study the differences in two experimental techniques for measuring x-ray absorption: partial and inverse partial fluorescence yields. By using the proper absorption and scattering formalisms to simulate each technique, we were able to explain the differences between the experimental spectra obtained from each. We explain fluorescence yield deviations using an analysis based on the spin configuration of different states, suggesting that the technique can be further extended as a quantitative spin state probe. These results could have significant implications for the field of soft x-ray absorption spectroscopy

    Publications of the Jet Propulsion Laboratory July 1965 through July 1966

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    Bibliography on Jet Propulsion Laboratory technical reports and memorandums, space programs summary, astronautics information, and literature searche

    Resonant Inelastic X-ray Scattering Studies of Elementary Excitations

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    In the past decade, Resonant Inelastic X-ray Scattering (RIXS) has made remarkable progress as a spectroscopic technique. This is a direct result of the availability of high-brilliance synchrotron X-ray radiation sources and of advanced photon detection instrumentation. The technique's unique capability to probe elementary excitations in complex materials by measuring their energy-, momentum-, and polarization-dependence has brought RIXS to the forefront of experimental photon science. We review both the experimental and theoretical RIXS investigations of the past decade, focusing on those determining the low-energy charge, spin, orbital and lattice excitations of solids. We present the fundamentals of RIXS as an experimental method and then review the theoretical state of affairs, its recent developments and discuss the different (approximate) methods to compute the dynamical RIXS response. The last decade's body of experimental RIXS data and its interpretation is surveyed, with an emphasis on RIXS studies of correlated electron systems, especially transition metal compounds. Finally, we discuss the promise that RIXS holds for the near future, particularly in view of the advent of x-ray laser photon sources.Comment: Review, 67 pages, 44 figure

    Soft X-ray Spectroscopy of Metal Nitrides and Oxides: Uncovering Structure-property Relationships in Phosphors for pc-LEDs

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    This thesis engages a problem that is seemingly a little incongruous: to develop an under- standing of, and link together, the structure-property relationships of LED-phosphors and frontier polymorphs of vanadium oxides using the same experimental and theoretical frame- work. Soft x-ray spectroscopy and density functional theory calculations are used as the probe of, and interpretive tool for, the electronic structures of these materials. By comparing and contrasting the observed electronic structures to those of other, similar systems, changes in electronic structure are linked to changes in crystal structure and atomic composition. From this vantage point the LED-phosphors, which are essentially metal nitrides, and vana- dium oxides are not as disparate as they would seem at first glance. Their properties lie on a continuum, allowing them to be readily understood within a single framework. At the same time, there are significant insights to be gained about each group, by studying the other. In both groups the interplay between localization and delocalization of metal d-states is seen to be key to their functional properties, both as a result of and in conjunction with, the influence of their ligands. The effects of the ligands are seen to stem directly from their local charge densities, as well as their separation from and arrangement around the metal sites. In addition the interaction of non-metal sites is seen to be critical in many cases. Structure-property relationships are outlined for this suite of materials. Critical material properties, such as the band gaps, and the location of metal states in the band gaps, are determined experimentally. Several analysis techniques are developed that prove critical for the analyses of these compounds. Included in this is a thorough analysis of x-ray self- absorption in vanadium oxides, which is too often ignored. Ultimately a coherent framework for understanding the properties of these compounds is developed and it is hoped that this will serve as the basis of further refinement of their useful properties

    Research and Technology Operating Plan. Summary: Fiscal year 1976 research and technology program

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    A compilation of the summary portions of each of the Research and Technology Operating Plans (RTOP) used for management review and control of research currently in progress throughout NASA was presented. The document is arranged in five sections. The first one contains citations and abstracts of the RTOP. This is followed by four indexes: subject, technical monitor, responsible NASA organization, and RTOP number

    GEANT4--a simulation toolkikt

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    Geant4 is a toolkit for simulating the passage of particles through matter. It includes a complete range of functionality including tracking, geometry, physics models and hits. The physics processes offered cover a comprehensive range, including electromagnetic, hadronic and optical processes, a large set of long-lived particles, materials and elements, over a wide energy range starting, in some cases, from 250 eV and extending in others to the TeV energy range. It has been designed and constructed to expose the physics models utilised, to handle complex geometries, and to enable its easy adaptation for optimal use in different sets of applications. The toolkit is the result of a worldwide collaboration of physicists and software engineers. It has been created exploiting software engineering and object-oriented technology and implemented in the C++ programming language. It has been used in applications in particle physics, nuclear physics, accelerator design, space engineering and medical physics
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