1,721,008 research outputs found
Incoherent Bi off-centering in Bi_{2}Ti_{2}O_{6}O^{?} and Bi_{2}Ru_{2}O_{6}O^{?}: Insulator versus metal
No full textIn the cubic, stoichiometric oxide compounds Bi2Ti2O6O? (also written as Bi2Ti2O7) and Bi2Ru2O6O? (also written as Bi2Ru2O7) Bi3+ ions on the pyrochlore A site display a propensity to off-center. Unlike Bi2Ti2O6O?, Bi2Ru2O6O? is a metal, so it is of interest to ask whether conduction electrons and/or involvement of Bi 6s states at the Fermi energy influence Bi3+ displacements. The Bi3+ off-centering in Bi2Ti2O6O? has previously been revealed to be incoherent from detailed reverse Monte Carlo analysis of total neutron scattering. Similar analysis of Bi2Ru2O6O? reveals incoherent off-centering as well, but of smaller magnitude and with distinctly different orientational preference. Analysis of the distributions of metal to oxygen distances presented suggests that Bi in both compounds is entirely Bi3+. Disorder in Bi2Ti2O6O? has the effect of stabilizing valence while simultaneously satisfying the steric constraint imposed by the presence of the lone pair of electrons. In Bi2Ru2O6O?, off-centering is not required to satisfy valence and seems to be driven by the lone pair. Decreased volume of the lone pair may be a result of partial screening by conduction electrons.<br/
Atomic displacements in the charge ice pyrochlore Bi_{2}Ti_{2}O_{6}O^{?} studied by neutron total scattering
No full textThe oxide pyrochlore Bi2Ti2O6O? is known to be associated with large displacements of Bi and O? atoms from their ideal crystallographic positions. Neutron total scattering, analyzed in both reciprocal and real space, is employed here to understand the nature of these displacements.Rietveld analysis and maximum entropy methods are used to produce an average picture of the structural nonideality. Local structure is modeled via large-box reverse Monte Carlo simulations constrained simultaneously by the Bragg profile and real-space pair distribution function. Direct visualization and statistical analyses of these models show the precise nature of the static Bi and O? displacements. Correlations between neighboring Bi displacements are analyzed using coordinates from the large-box simulations. The framework of continuous symmetry measures has been applied to distributions of O?Bi4 tetrahedra to examine deviations from ideality. Bi displacements from ideal positions appear correlated over local length scales. The results are consistent with the idea that these nonmagnetic lone-pair containing pyrochlore compounds can be regarded as highly structurally frustrated systems
Exploration of the CU-MN-AS ternary phase diagram and MN3B4 antiferromagnets
Full text linkCuMnAs was first studied more than fifty years ago, both in a tetragonal phase (t-CuMnAs) and orthorhombic phase (o-CuMnAs). Today, the prospect of spintronics has created a new focus on antiferromagnetic materials with Néel temperatures above room temperature, with the goal of reading and writing spin orientations by electrical current. Tetragonal CuMnAs is a candidate whose experimental reading and writing with electrical field has been realized as a proof of concept. Our fundamental knowledge of other phases in the Cu-Mn-As ternary system remains to be improved. In this study, we study a broader part of the phase diagram of this system. We examine the structure of different phases, their thermal and magnetic properties, and the relationship between stoichiometry and structure. We also present our synthetic and magnetic characterization of antiferromagnetic Mn3B4. Single crystals larger than 200 microns were obtained and examined by single-crystal diffraction and magnetometry.Submission published under a 24 month embargo labeled 'Closed Access', the embargo will last until 2020-08-01The student, Jingwen Yu, accepted the attached license on 2018-05-02 at 10:19.The student, Jingwen Yu, submitted this Thesis for approval on 2018-05-02 at 10:27.This Thesis was approved for publication on 2018-05-03 at 15:39.DSpace SAF Submission Ingestion Package generated from Vireo submission #12499 on 2018-09-27 at 11:32:47Made available in DSpace on 2018-09-27T16:45:27Z (GMT). No. of bitstreams: 2
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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
Heavy-metal-free colloidal nanocrystal heterostructures – synthetic chemistry and growth mechanisms
Full text linkBuilding heterostructures of colloidal nanocrystals by bringing different components into contact can improve existing or impart novel properties, which enables applications as light emitters in already-commercialized and near-future products. However, the majority of the research has been based on compositions with heavy metals (e.g., cadmium or lead) posing toxicity concerns to the consumers. Therefore, a better understanding of the synthetic chemistry and the growth mechanisms for heavy-metal-free nanocrystal heterostructures is critical before their commercialization potential becomes reality. Here, we have developed novel synthetic strategies to achieve the anisotropic growth of colloidal nanocrystals and the introduction of multiple components with tunability of their positions in the heterostructures, as well as to improve the photoluminescence quantum yield (PL QY) of nanocrystals.
We first examine how different Cu precursors in the synthesis of colloidal copper sulfide (Cu2-xS) nanocrystals affect the resulting shape and phase. Decreasing aspect ratio in 1D nanorods (eventually transitioning to 2D nanodisks) observed is consistent with the expected effects of decreasing Cu precursor reactivity. Nanorods are predominantly chalcocite at the early stages of growth but a phase transition to djurleite occurs accompanied by a change in tip faceting upon further growth. In contrast, nanodisks appear in the djurleite phase early on and remain so upon continued growth. Secondly, the high copper vacancy density and the high cation mobility in the Cu2-xS nanorods favor cation exchange, and the remaining Cu2-xS after partial cation exchange can serve as the catalyst for the subsequent solution-liquid-solid (SLS)-like growth. The interplay between cation exchange and SLS-like growth leads to tapered rod-rod, body/tail, or barbell-like Cu2-xS/ZnS heterostructures, which can be controlled by the Zn precursor and ligand choice. Finally, using a similar procedure, spontaneous multiple segment formation has been achieved in Cu2-xS/CuGaS2 heterostructured nanorods for the first time. Large strain due to lattice mismatch (-7.4% in the axial direction and -10.6% in the other direction) is responsible, because such segmentation is absent in Cu2-xS/CuInS2 heterostructured nanorods with a smaller mismatch (-4.3% in the axial direction and -6.5% in the other direction). More importantly, ligands with a large steric hindrance are critical to the multiple segment formation, which pack less densely on the surface and enables nucleation in the middle of the nanorods. In contrast, linear ligands provide better surface protection and the new components tend to form at the tips of the nanorods where the ligand packing density is relatively low.
In addition to Cu2-xS-based anisotropic heterostructures, we have also explored how to enhance the PL QY for heavy-metal-free color-center (manganese)-doped nanocrystals and ZnTe-based core/shell nanocrystals with spherical shapes. Annealing or growth at high temperatures for an extended period of time is considered detrimental for the synthesis of high-quality Mn-doped II-VI semiconductor nanocrystals, which can lead to the broadening of size distribution and, more importantly, to the loss of dopants. However, ripening can be beneficial to doping in a simple heat-up approach, where high dopant concentrations can be achieved. Smaller nanocrystals in a reaction batch, on average, exhibit higher undesirable band edge PL and lower desirable dopant PL. The optimization of dopant loss and the removal of such smaller undesirable nanocrystals through Ostwald ripening along with surface exchange/passivation to remove Mn clustering lead to high Mn PL QYs (45 to 55 %) for ZnSxSe1-x, ZnS, CdS, and CdSxSe1-x host nanocrystals. Similar with the synthetic optimization for doped nanocrystals, surface passivation by a high-quality ZnSe shell and a cation-rich surface by Zn-cation treatment enable a high PL QY (~40%) for ZnTe-based nanocrystals. The highly reactive selenium powder provides better synthetic control of the ZnSe shell. In addition, highly reductive ligand (diphenylphosphine) can further increase the PL QY presumably by filling surface electron trap states.Submission published under a 24 month embargo labeled 'Closed Access', the embargo will last until 2019-05-01The student, You Zhai, accepted the attached license on 2017-04-15 at 11:53.The student, You Zhai, submitted this Dissertation for approval on 2017-04-17 at 11:03.This Dissertation was approved for publication on 2017-04-17 at 13:52.DSpace SAF Submission Ingestion Package generated from Vireo submission #10778 on 2017-08-10 at 14:30:49Made available in DSpace on 2017-08-10T19:51:58Z (GMT). No. of bitstreams: 3
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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
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