1,720,962 research outputs found
Low energy excitations of vector spin glasses
Full text linkIl lavoro contenuto in questa tesi riguarda il problema dell'eccitazioni di bassa energia dei modelli di vetri di spin vettoriali. Viene proposto uno studio analitico e numerico di tre modelli: il primo consiste in un vetro di spin di Heisenberg con campo magnetico esterno random con grafo di interazioni denso, il secondo in un modello p spin di Heisenberg con grafo di interazioni denso, il terzo infine in un modello di Heisenberg con campo magnetico esterno random e grafo di interazioni diluito.
Questi modelli sono valutati rispetto al comportamento dell'eccitazioni dei sistemi vetrosi a basse temperature: in particolare, nella tesi si mostra che questi modelli posseggono delle fasi in cui la densità degli stati è senza gap e i modi sono quasi localizzati. Nel modello sparso la densità degli stati ha una dipendenza quartica dalla frequenza, in accordo con molteplici misure di questa quantità reperibili dalla letteratura sui modelli vetrosi computazionali. In tutti e tre i casi di studio, la transizione nella fase del vetro di spin è caratterizzata rispetto al comportamento dei modi soffici. Troviamo che nei modelli densi la transizione del vetro di spin è una transizione di delocalizzazione dei modi soffici. Nel caso sparso, la delocalizzazione alla transizione si manifesta in forma più debole. Questi risultati ampliano la nostra comprensione del punto critico di temperatura nulla, mostrando come l'emergenza di un ordinamento da vetro di spin modifichi la risposta del sistema a piccole perturbazioni magnetiche.The work of this thesis concerns the problem of linear low energy excitations of vector spin glass models. An analytical and numerical study is carried out, considering a fully connected random-field Heisenberg model at zero temperature, a fully-connected vector p-spin glass model and a sparse random-field Heisenberg model. We test these models against the low temperature behavior of finite dimensional glassy systems, in particular we show that they posses phases where the density of states is gapless with quasi-localised modes. In the case of the sparse model, we show that the density of states follows a quartic law at low frequency, consistently with several recent measures of this quantity that can be found in the literature of computer glasses. In all the three models, the spin glass transition is characterised in terms of the behavior of the softest excitations. We found that in the fully connected models the zero temperature spin glass transition in a field is a delocalisation transition of the softest modes. In the sparse case, a weaker form of delocalisation appears at the transition. These results broaden our understanding of the zero temperature critical point, by showing how spin glass ordering affects the way the system responds to small magnetic perturbations.Le travail de cette thèse concerne le problème des excitations linéaires à basse énergie des modèles de verre de spin vectoriel. Une étude analytique et numérique est menée, considérant un modèle de Heisenberg à champ aléatoire entièrement connecté à température nulle, un modèle de verre de spin p vectoriel entièrement connecté et un modèle de Heisenberg dilué à champ aléatoire. Nous testons ces modèles par rapport au comportement à basse température des systèmes vitreux de dimension finie, en particulier nous montrons qu'ils possèdent des phases où la densité d'états est sans lacunes avec des modes quasi localisés. Dans le cas du modèle dilué, nous montrons que la densité d'états suit une loi quartique à basse fréquence, en accord avec plusieurs mesures récentes de cette quantité que l'on peut trouver dans la littérature des modèles computationnels de verres. Dans les trois modèles, la transition de verre de spin est caractérisée en termes de comportement des excitations les plus douces. Nous avons constaté que dans les modèles entièrement connectés, la transition de verre de spin à température zéro dans un champ est une transition de délocalisation des modes les plus douces. Dans le cas dilué, une forme plus faible de délocalisation apparaît à la transition. Ces résultats élargissent notre compréhension du point critique à température zéro, en montrant comment l'ordre du verre de spin affecte la façon dont le système répond à de petites perturbations magnétiques
Analytical solution to Heisenberg spin glass models on sparse random graphs and their de Almeida–Thouless line
No full textResults regarding spin glass models are, to this day, mainly confined to models with discrete (usually Ising) spins. Spin glass models with continuous spins exhibit interesting new physical behaviors related to the additional degrees of freedom, but have been primarily studied on fully connected topologies. Only recently some advancements have been made in the study of continuous models on sparse graphs. In this work we partially fill this void by introducing a method to solve numerically the belief propagation equations for systems of Heisenberg spins on sparse random graphs via a discretization of the sphere. We introduce techniques to study the finite-temperature, finite-connectivity case as well as algorithms to deal with the zero-temperature and large-connectivity limits. As an application, we locate the de Almeida–Thouless line for this class of models and the critical field at zero temperature, showing the full consistency of the methods presented. Beyond the specific results reported for Heisenberg models, the approaches presented in this paper have a much broader scope of application and pave the way to the solution of strongly disordered models with continuous variables
The most uniform distribution of points on the sphere
No full textHow to distribute a set of points uniformly on a spherical surface is a longstanding problem that still lacks a definite answer. In this work, we introduce a physical measure of uniformity based on the distribution of distances between points, as an alternative to commonly adopted measures based on interaction potentials. We then use this new measure of uniformity to characterize several algorithms available in the literature. We also study the effect of optimizing the position of the points through the minimization of different interaction potentials via a gradient descent procedure. In this way, we can classify different algorithms and interaction potentials to find the one that generates the most uniform distribution of points on the sphere
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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