1,720,973 research outputs found
Dielectric properties of crystalline organic molecular films in the limit of zero overlap
No full textWe present the calculation of the static dielectric susceptibility tensor and dipole field sums in thin molecular films in the well-defined limit of zero intermolecular overlap. Microelectrostatic and charge redistribution approaches are applied to study the evolution of dielectric properties from one to a few molecular layers in films of different conjugated molecules with organic electronics applications. Because of the conditional convergence of dipolar interactions, dipole fields depend on the shape of the sample and different values are found in the middle layer of a thick film and in the bulk. The shape dependence is eliminated when depolarization is taken into account, and the dielectric tensor of molecular films converges to the bulk limit within a few molecular layers. We quantify the magnitude of surface effects and interpret general trends among different systems in terms of molecular properties, such as shape, polarizability anisotropy, and supramolecular organization. A connection between atomistic models for molecular dielectrics and simpler theories for polarizable atomic lattices is also provided
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
How much hydropeaking? Development of a first screening approach to assess the level of hydropeaking pressures
No full textThe general aim of this work is to develop and test a preliminaryscreening approach to assess the level of hydropeaking pressure on a river reach, without explicitly focusing on its impacts. A combination of two indicators is proposed, in order to provide an easy-to-use methodology which can be applied on commonly available stream-flow data in alpine regions. The two indicators, HP1 and HP2, quantify the daily maximum intensity and the velocity of discharge variations, respectively. We analyzed data from two different Alpine areas: 28 flow gauging stations located in the Trentino area (NE Italy, 16 subject to hydropeaking and 12 not impacted reaches) and 37 flow gauging stations located in Switzerland (20 impacted and 17 not impacted reaches).The approach is based on the analysis of statistical properties of time series that can be built with two quantities related with the indicators. In the end every reach is represented by an unique value of HP1 and HP2, which reveals a statistically significant difference between impacted and non-impacted reaches, the latter considered as reference group. In particular, higher values of HP1and HP2 are associated with reaches which experience hydropeaking releases.In the space of the indicators we provide a quantitative definition of the “hydropeaking pressure” as a relative distance from not impacted conditions, based on limits for HP1 and HP2 calculated on not impacted group. Overall the proposed approach can easily inform on (i) relative distance of impacted reaches from not impacted conditions, (ii) provide thresholds to identify hydropeaking impacted reache
Variations on the Author
Get PDF“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
Get PDFWe 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
Get PDFWe 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
Mathematical study of linear morphodynamic acceleration and derivation of the MASSPEED approach
No full textMorphological accelerators, such as the MORFAC (MORphological acceleration FACtor) approach (Roelvink, 2006), are widely adopted techniques for the acceleration of the bed evolution, which reduce the computational cost of morphodynamic numerical simulations. In this work we apply an acceleration to the one-dimensional morphodynamic problem described by the de Saint Venant–Exner model by multiplying all the spatial derivatives related to the mass or momentum flux by an acceleration factor ≥ 1 which may be different for each equation. The goal is to identify the best combination of the accelerating factors for which (i) the bed responds linearly to hydrodynamic changes; (ii) a decrease of the computational cost is obtained. The sought combination is obtained by studying the behavior of an approximate solution of the three eigenvalues associated with the flux matrix of the accelerated system. This approach allows to derive a new linear morphodynamic acceleration technique, the MASSPEED (MASs equations SPEEDup) approach, and the a priori determination of the highest possible acceleration for a given simulation. In this new approach both mass conservation equations (water and sediment) are accelerated by the same factor, differently from the MORFAC approach where only the sediment mass equation is modified. The analysis shows that the MASSPEED gives a larger validity range for linear acceleration and requires smaller computational costs than that of the MORFAC approach. The MASSPEED approach is then implemented using an adaptive approach that applies the maximum linear acceleration similarly to the implementation of the Courant–-Friedrichs–-Lewy stability condition. Finally, numerical simulations have been performed in order to assess accuracy and efficiency of the new approach. Results obtained in the long-term propagation of a sediment hump demonstrate the advantages of the new approach. The validation of the method is performed under steady or quasi-steady flow conditions, whereas further investigation is needed to extend morphological accelerators to fully unsteady flows
An algorithm for constructing certain differential operators in positive characteristic
Get PDFGiven a non-zero polynomial f in a polynomial ring R with coefficients in a finite field of prime characteristic p, we present an algorithm to compute a differential operator δ which raises 1/ f to its pth power. For some specific families of polynomials, we also study the level of such a differential operator δ , i.e., the least integer e such that δ is R^{p^e} -linear. In particular, we obtain a characterization of supersingular elliptic curves in terms of the level of the associated differential operator.<br /
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