1,720,987 research outputs found
Physics of iron and earth's inner core.
No full textThe Earth's core is primarily composed of iron and I studied the physics of this element. This dissertation addresses two important questions concerning the physical properties of hexagonally close packed (hcp) iron, the high pressure polymorph. In the first part (chapter II and III) the focus is on the thermoelastic properties of hcp iron at the thermodynamic conditions of the deep interior of the Earth, advancing our understanding of inner core processes and structure. Combining accurate band structure total energy methods for electronic contributions to the Helmholtz free energy with a mean field approach to the lattice vibrations, I have predicted the structure and elastic constants of dense hcp iron at high temperature from first principles. The axial ratio c/ a in the hcp cell increases substantially with temperature, reaching a value of nearly 1.7 at 5700 K, where aggregate elastic properties match those of the inner core. As a consequence of increasing c/ a the single crystal longitudinal anisotropy at high temperature has the opposite sense from that at low temperature. Combining the single crystal elasticity with a simple model of inner core texture, in which basal planes are preferentially aligned with Earth's rotation axis, one can account for seismological observations of inner core anisotropy. The second part (chapters IV and V) investigates the effect of a theoretically predicted anti-ferromagnetic ground state at pressure on spectroscopic and elastic properties, the results adding to a growing body of theoretical evidence for magnetism in hcp iron at pressure. The orthorhombic anti-ferromagnetic ordering leads to two Raman active transverse optical modes, as found experimentally, while only one such mode is predicted in the hcp structure. Moreover, we find good agreement with experimental observations of the frequencies of the two modes. The influence of anti-ferromagnetic ordering on the elasticity of hcp iron reduces the discrepancy between theory and experiment in equation of state and aggregate elastic properties.PhDEarth SciencesGeophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/128722/2/3029439.pd
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Thermal effects of metamorphic reactions in a three-component slab
No full textThermal evolution of a subducting crust is of primary importance for understanding physical properties, phase transformations, fluid migration and melting regimes at convergent plate boundaries. Various factors influencing the thermal structure of a subduction zone have been considered previously: age, geometry and rate of subducting lithosphere, shear stress across the subduction interface, radioactive heating, etc. Recently, emphasis has been placed on significant heating of the slab due to rheologically favourable convection in the mantle wedge. However, substantial heat production or consumption can occur due to metamorphic reactions, including endothermic devolatilization. We investigate enthalpy budget in a subducting slab using a self-consistent thermodynamic model. Petrological model of a subducting slab consists of three layers: oceanic subducting sediment (GLOSS), oceanic basalt (OB), and moderately serpentinized harzburgite (SHB). These layers are examined over the range of pressure-temperature conditions of interest by computing metamorphic phase diagrams and retrieving whole-rock thermodynamic properties. Our results suggest that metamorphic reactions consume a significant amount of slab heat budget and induce substantial cooling. In contrast to previous thermal models of subduction zones, actual slab temperatures may be lower by as much as 250 °C
- …
