1,721,212 research outputs found
Density functional theory studies of solid density plasmas
Full text linkIn warm dense matter (WDM) and dense plasma physics, Density Functional Theory (DFT) has become a standard approach over the past many years for simulating transport properties, equations of state, interpreting experimental measurements and many other applications. The main chapters, two to four, of this thesis cover original work by the author on three topics: excited state pseudopotentials, time-dependent DFT (TDDFT) and many-body theory.
For an excited state pseudopotential, a specific excited ion core configuration is generated by externally imposing a set of occupation numbers in the same way as can be rigorously done for a non-interacting electron system. In chapter 2 results and a physical argument are presented seeking to justify this process when generating excited configurations of bound electron systems.
Those electrons that might be considered as `free' within a plasma exhibit not only single-particle excitations, as one might label with a set of single-particle occupation numbers, but also significant collective behaviour i.e. plasmons. TDDFT linear-response theory is applied in chapter 3 as a rigorous means of modelling the general dynamic and wavelength-dependent response properties, and fluctuations, for quantum plasma systems. With help from the Langreth rules a fluctuation-dissipation relation for the electron dynamic structure factor is derived. Finally, the dynamic structure factor is computed for compressed Beryllium and CH plasma, with favourable comparison to experimental data and simulations by previous authors.
In chapter four the free-free opacity of solid density Al plasma is considered. Both the tensor nature of the dielectric function, in the form of local field corrections, and an accurate description of bound-state properties, in the form of correct binding energies, are required to reproduce experimental room temperature measurements. Commonly used exchange-correlation functionals are insufficient for predicting the energy gap between bound states and the continuum in a linear response theory context. To this end, the author has implemented and demonstrated finite-temperature many-body quasi-particle calculations in the Abinit code. These many-body calculations are expensive however they are a potential future source of accurate theoretical predictions, covering a wide range of plasma conditions to which other, perhaps simpler models might be benchmarked
Phonon instabilities in uniaxially compressed fcc metals as seen in molecular dynamics simulations
Full text linkWe show that the generation of stacking faults in perfect face-centered-cubic (fcc) crystals, uniaxially compressed along [001], is due to transverse-acoustic phonon instabilities. The position in reciprocal space where the instability first manifests itself is not a point of high symmetry in the Brillouin zone. This model provides a useful explanation for the magnitude of the elastic limit, in addition to the affects of box size, temperature, and compression on the time scale for the generation of stacking faults. We observe this phenomenon in both simulations that use the Lennard-Jones potential and embedded atom potentials. Not only does this work provide fundamental insight into the microscopic response of the material but it also describes certain behavior seen in previous molecular dynamics simulations of single-crystal fcc metals shock compressed along the principal axis.Fil: Kimminau, Giles. University of Oxford. Department of Physics; Reino UnidoFil: Erhart, Paul. Lawrence Livermore National Laboratory; Estados UnidosFil: Bringa, Eduardo Marcial. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Remington, Bruce. Lawrence Livermore National Laboratory; Estados UnidosFil: Wark, Justin S.. University of Oxford. Department of Physics; Reino Unid
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Creation and study of matter in extreme conditions by high-intensity free-electron laser radiation
Full text linkThe recent development of free-electron lasers operating at XUV and X-ray wavelengths are proving vital for the exploration of matter in extreme conditions. The ultra-short pulse length and high peak brightness these light sources provide, combined with a tunable X-ray wavelength range, makes them ideally suited both for creating high energy density samples and for their subsequent study. In this thesis I describe the work done on the XUV free-electron laser FLASH in Hamburg, aimed at creating homogeneous samples of warm dense matter through the process of volumetric XUV photo-absorption, and the theoretical work undertaken to understand the process of high-intensity laser-matter interactions. As a first step, we have successfully demonstrated intensities above 1017 Wcmâ2 at a wavelength of 13.5 nm, by focusing the FEL beam to micron and sub-micron spot sizes by means of a multilayer-coated off-axis parabolic mirror. Using these record high intensities, we have demonstrated for the first time saturable absorption in the XUV. The effect was observed in aluminium and magnesium samples and is due to the bleaching of a core-state absorption channel by the intense radiation field. This result has major implications for the creation of homogeneous high energy density systems, as a saturable absorption channel allows for a more homogeneous heating mechanism than previously thought possible. Further, we have conducted soft X-ray emission spectroscopy measurements which have delivered a wealth of information on the highly photo-excited system under irradiation, immediately after the excitation pulse, yet before the system evolves into the warm dense matter state. Such strongly photo-excited samples have also been studied theoretically, by means of density functional theory coupled to molecular dynamics calculations, yielding detailed electronic structure information. The use of emission spectroscopy as a probe for solid-density and finite-temperature systems is discussed in light of these results. Theoretical efforts have further been made in the study of the free-free absorption of aluminium as the system evolves from the solid state to warm dense matter. We predict an absorption peak in temperature as the system heats and forms a dense plasma. The physical significance of this effect is discussed in terms of intense light-matter interactions on both femtosecond and picosecond time-scales.</p
Experimental studies and simulations of shock-compressed silicon
Full text linkThe response of silicon to shock-compression has been an area of active research for decades. However, despite considerable improvements in both experimental techniques and computer simulations, the behaviour has not been unanimously constrained. This work introduces a new Lagrangian Elastic code which aims to approach the modelling of solids from a different direction to traditional hydrodynamics codes. We present data from a white-light Laue diffraction experiment, which along with molecular dynamics simulations, suggest that a kinetically inhibited phase-change may be indirectly responsible for the previously reported anomalous elastic response of silicon. We also present data from a monochromatic x-ray diffraction experiment which show the first observed diffraction from a shock-induced high-pressure phase in silicon.</p
X-ray diffraction studies of laser-shocked crystals
Full text linkWhen materials are shock compressed, they undergo changes in microstructure that act to relieve the large shear stresses associated with the compression. The plasticity mechanisms that mediate this transition such as slip and twinning remain poorly understood, especially in the case of polycrystals, which make up the majority of real world materials. This work presents a theoretical outline for analysing Debye-Scherrer diffraction experiments under large strains. A method is demonstrated to measure both the components of strain in the normal and transverse directions, as well as crystal orientation using highly textured samples. These theoretical predictions are compared with simulated diffraction patterns from molecular dynamics simulations. This technique is applied to two different experiments on tantalum. The first provides a measurement of the timescale for plastic deformation, which we find similar to comparable experiments in copper, while the second provides the first in situ of observation of twinning in shock compressed metal
Shock and release of tantalum studied via molecular dynamics and femtosecond x-ray diffraction
Full text linkThe response of solid matter to shock compression is complexified considerably by its strength, or its ability to withstand shear stress. Strength is challenging to measure experimentally under shock conditions and even harder to model, due to its being an extremely complicated function of the loading conditions. Our understanding of
material strength and the way it manifests under dynamic loading thus remains, to a great extent, incomplete. This work presents studies of two phenomena arising from strength under the conditions of shock compression and release by means of multimillion-atom molecular dynamics simulations and femtosecond x-ray diffraction. The role of shock-induced grain interactions is first explored via simulations of elementary polycrystals. Such interactions are found to control the plasticity mechanisms activated under shock compression and the limiting shear stress state to which the polycrystal settles in the wake of the shock. A combined experimental-computational study of plastic-work heating under the conditions of shock release is then presented. An algorithm for extracting the temperature of released samples from their diffraction image is derived and verified on synthetic data. When applied to experimental data, the algorithm shows that the temperatures of shock-released tantalum foils vastly exceed those expected from a conventional isentropic release. The underlying microphysical processes responsible for the heating are then interrogated via large-scale simulations of crystals under shock and release. A heat equation is used to identify plastic-work heating owed to the sample’s exceptional strength during its rapid release as the culprit, thus challenging the conventional assumption that shock release is a universally isentropic process
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