1,720,969 research outputs found
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
1H NMR Relaxometry Study of a Rod-Like Chiral Liquid Crystal in Its Isotropic, Cholesteric, TGBA*, and TGBC* Phases
No full textThe molecular dynamics of a chiral liquid crystal showing a rich variety of frustrated mesophases has been investigated by means of (1)H NMR relaxometry. The interest in this lactate derivative, HZL 7/*, is related to a large range of thermal stabilities of the twist grain boundary (TGB) phases. Dispersions of the (1)H spin-lattice relaxation times, T(1), in the frequency range from 300 MHz to 5 kHz were measured and consistently analyzed in the isotropic, chiral nematic, TGBA*, and two TGBC* phases. In the isotropic and N* phases, a three-exponential magnetization decay was observed and assigned to three specific molecular groups of the HZL 7/* (molecular core, methyl, and methylene groups). In the TGB phases, all T(1) components merge into a single one. The analysis of the T(1) dispersion in the TGBA* phase shows that the translational self-diffusion relaxation mechanism dominates over a broad frequency range and that layer undulations are less relevant than the relaxation contribution associated with the diffusion process across the TGB structure. In the TGBC(1)* phase, the T(1) dispersion presents a strong contribution of in-layer tilt direction fluctuations (T(1)(-1) proportional to v(-1/2)), while, in the TGBC(2)* phase, the linear frequency dependence of T(1) could be associated with a much stronger contribution of layer undulations than for the other TGB phases. This is at present the first molecular dynamics investigation on several TGB phases by means of (1)H NMR relaxometry
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
Transverse nuclear spin relaxation studies of viscoelastic properties of membrane vescicles. II. Experimental results
No full textTransverse nuclear spin relaxation measurements employing Carr-Purcell (CP) pulse sequences have been used to determine the viscoelastic properties of quasi-spherical membrane vesicles with controlled radii R0. CP The observed relaxation rates, R_2infinity^CP (omega), exhibit a linear dependence on the inverse pulse frequency over a wide frequency range in the kHz regime znd then level off to a constant "plateau" value independent of omega. Within the linear dispersion regime, the same relaxation rates are detected for unilamellar and oligolamellar vesicles, indicating that the interbilayer coupling is weak and has no effect on the measured relaxation curves. Analysis of the experimental dispersion profiles is performed using a slow-motional model in which two different relaxation processes are considered (i.e., vesicle shape fluctuations and molecular translational diffusion). It is shown that for vesicle radii R0 greater than or equal to 200 nm lateral diffusion across the vesicle shell is too slow to contribute significantly to transverse spin relaxation in the kHz range. Rather, vesicle shape fluctuations constitute the dominant transverse relaxation process. Model calculations reveal that R_2infinity^CP(omega), induced by vesicle fluctuations, depends linearly on omega^-1 over a wide frequency range in the kHz regime. Notably, within this linear dispersion regime, the bending elastic modulus is the only relevant parameter because the magnitude of
R_2infinity^CP(w) does not depend on R0, the effective lateral tension, and the viscosity of the surrounding fluid. On the other hand, all these parameters determine the frequency at which R_2 (w) levels off to a constant plateau value. Thus, analysis of the linear dispersion regime is a direct way to determine the bending rigidity. For the studied DMPC and DMPC/cholesterol vesicles, the K values vary from (1.5 +/-0.1) X 10^-20 J to (8.3 +/- 0.1) x 10^-20 J. From the plateau in the experimental dispersion profiles, values for the effective lateral tension of 3+/-1 and 4+/-1 have been extracted. It appears that transverse NMR relaxation involving CP sequences represents a powerful tool for the study of the viscoelastic properties of membrane vesicles
Transverse nuclear spin relaxation studies of viscoelastic properties in membrane vesicles. I. Theory
No full textTransverse nuclear spin relaxation measurements employing Carr-Purcell (CP) pulse sequences can provide detailed information on the slow-motional dynamics in biomembranes. In this paper, a comprehensive relaxation model is developed for the analysis of such experiments performed on unilamellar quasi-spherical vesicles. The basis of the model is the stochastic Liouville equation in which two different relaxation processes are considered (i.e., vesicle shape fluctuations and molecular translational diffusion ). It is shown that for vesicle radii R0 greater than or equal to 200 nm, translational diffusion of the lipid molecules along the vesicle shell is too slow to contribute significantly to transverse spin relaxation in the kHz range, whereas vesicle shape fluctuations constitute the dominant transverse relaxation process. The theory is employed in model calculations for pulse frequency-dependent transverse nuclear spin relaxation rates,
R_2infinity^CP(omega), from CP sequences. The analysis reveals that R_2infinity^CP(omega), induced by vesicle fluctuations, depends linearly on omega^-1 over a wide frequency range in the kHz regime. Notably, within this linear dispersion regime, the bending elastic modulus is the only relevant parameter because the magnitude of
R_2infinity^CP(omega) does not depend on the size of the vesicle R0, the effective Lateral tension, or the viscosity of the surrounding fluid. On the other hand, all these parameters determine the frequency at which
R_2infinity^CP(omega) levels off to a constant "plateau" value independent of omega. Thus, analysis of the R_2infinity^CP(omega) dispersion profiles is a direct way to determine the bending elastic modulus and other viscoelastic parameters of membrane vesicles
Transverse nuclear spin relaxation induced by shape fluctuations in membrane vesicles. Theory and experiments
No full textTransverse nuclear spin relaxation measurements, employing the Carr-Purcell-Meiboom-Gill (CPMG) sequence, represent a powerful tool to stud), the dynamics of director fluctuations in liquid crystalline mesophases. Generally, however, the analysis of these experiments requires a slow-motional theory based on the stochastic Liouville equation. In this paper, such an approach is applied to stud), the shape fluctuations of dimyristoyl-phosphatidylcholine membrane vesicles with controlled size. Analysis of the transverse nuclear spin relaxation rates, measured as a function of the pulse frequency in the CPMG sequence, provides values for the bending rigidity, kappa, and the lateral tension, sigma, of the membrane vesicles. The results are of major importance in the understanding of the biological membrane function
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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