1,720,983 research outputs found
Temperature-induced neutral-to-ionic phase transition of the charge-transfer crystal tetrathiafulvalene-fluoranil
No full textThe temperature induced neutral to ionic phase transition (TI-NIT) is a rare phenomenon occurring in mixed stack charge transfer (CT) crystals made up of alternating π-electron donor (D) and acceptor (A) molecules. We were able to grow crystals of tetrathiafulvalene-fluoranil (TTF-FA), and to show that it undergoes TI-NIT like the prototype CT crystal TTF-chloranil. We characterized both room and low-T phases through IR and Raman spectroscopy and x-ray diffraction, demonstrating that while TTF-FA is quasineutral at room temperature, its ionicity jumps from 0.15 to 0.7 at low T, therefore crossing the neutral-ionic borderline. The transition, occurring around 150 K, is first order, with large thermal hysteresis and accompanied by crystal cracking. In the high-T phase D and A molecules lie on inversion center, i.e., the stacks are regular, whereas the low-T phase is characterized by the loss of the inversion symmetry along the stack as the stacks are strongly dimerized and by the doubling of the unit cell
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
Solvated and Ferroelectric Phases of the Charge Transfer Co-Crystal TMB-TCNQ
No full textWe have successfully determined the crystal structure of both the low temperature and solvated phases of 3,3′,5,5′-tetramethylbenzidine-tetracyanoquinodimethane (TMB-TCNQ). The triclinic solvate phase was erroneously believed for years to be a polymorph of TMB-TCNQ, while using X-rays diffraction, infrared spectroscopy, and differential scanning calorimetry, we have determined the presence of crystallization of acetonitrile molecules within the structure and observed the conversion to the room temperature monoclinic phase of TMB-TCNQ upon desolvation promoted both by heating and by grinding. The nonsolvated phase undergoes inversion symmetry breaking below 160 K, giving rise to stack dimerization. A combination of the X-ray crystal structure determination, Hirschfeld surface analysis, and density functional theory calculations suggest a rather complex scenario for the first order phase transition, which implies, besides stack dimerization, molecular inclination and small increase of the degree of charge transfer. The two donor-acceptor pairs within the unit cell are arranged in-phase, so that the low-temperature structure is potentially ferroelectric
Correlated electrons in soft lattices: Raman scattering evidence of the nonequilibrium dielectric divergence at the neutral-ionic phase transition
No full textA broad low-frequency Raman band is observed in the proximity of the temperature-induced neutral-ionic phase transition (NIT) of a mixed-stack charge-transfer crystal, 4,4'-dimethyltetrathiafulvalene-chloranil (DMTTF-CA). Adopting a modified Hubbard model with coupling to lattice phonons and molecular vibrations, we propose a time-correlation-function approach to the calculation of the Raman and infrared spectra of these quasi-one-dimensional systems. The approach provides a unifying picture for several puzzling features observed in vibrational spectra across the NIT, and ascribes the anomalous intensity of the Raman band to the divergent polarizability of the electronic system. © 2011 American Physical Society
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
Pressure-Induced Neutral to Ionic Phase Transition in TTF-Fluoranil, DimethylTTF-Fluoranil and DimethylTTF-Chloranil: A Comparative THz Raman Study
No full textThe Neutral to Ionic phase Transition (NIT) that occurs in few mixed stack charge transfer cocrystals at high pressure or low temperature is a charge instability combined with a structural instability. The lattice contraction, which increases the 3D Coulomb interactions, favors a higher degree of charge transfer. Due to Peierls instability, this leads to the dimerization of the stack, breaking its inversion symmetry. The 3D interactions also determine the arrangement of the adjacent dimerized polar stacks, making the ionic phase ferroelectric or antiferroelectric. The role of these parameters that modulate the NIT has been widely studied in Tetrathiafulvalene-haloquinone cocrystals. Here, we compare the high-pressure behavior of three of them: the newly synthesized TTF-FA and DMTTF-FA with the known DMTTF-CA and isostructural DMTTF-FA. We followed the evolution of the lattice phonons via THz Raman spectroscopy, assessing the pressure-dependent structural changes. While the FA-based crystals undergo strong first-order NIT, DMTTF-CA shows a continuous transition. The high-pressure behavior of each crystal is also compared with the low-temperature behavior
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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