1,721,302 research outputs found
Dynamical mean-field theory calculation with the dynamical density-matrix renormalization group
No full textWe study the Hubbard model at half band-filling on a Bethe lattice with infinite coordination number at zero temperature. We use the dynamical mean-field theory (DMFT) mapping to a single-impurity Anderson model with a bath whose properties have to be determined self-consistently. For a controlled and systematic implementation of the self-consistency scheme we use the fixed-energy approach to the DMFT. Using the dynamical density matrix renormalization group method (DDMRG) we calculate the density of states (DOS) with a resolution ranging from 3% of the bare bandwidth W = 4t at high energies to 0.01 % for the quasi-particle peak. The DDMRG resolution and accuracy for the DOS is superior to those obtained with other numerical methods in previous DMFT investigations. We find that the critical couplings are Uc,1/t = 4.45 +/- 0.05 and Uc,2/t = 6.1 +/- 0. 1. Our calculation indicate the existence of two metallic solutions below U = Uc,1. (c) 2006 Elsevier B.V. All rights reserved
Die Beziehungen zwischen dem Mittelmeerraum und Europa. Die Rolle der Etrusker
No full textIl contributo mette in luce i rapporti culturali tra il Mediterraneo e l'Europa attraverso la mediazione degli Etruschi. In particolare si delinea un'evoluzione molto particolare da sud verso nord che investe soprattutto i caratteri fondamentali della statuaria monumentale
Phase diagram of the t-U-V-1-V-2 model at quarter filling
No full textWe examine the ground-state properties of the one-dimensional Hubbard model at quarter filling with Coulomb interactions between nearest-neighbors V-1 and next-nearest neighbors V-2. Using the density-matrix renormalization group and exact diagonalization methods, we obtain for U=10t three different phases in the V-1-V-2 plane: 2k(F)- and 4k(F)-charge-density-wave (CDW) and a broad metallic phase in between. Assuming that the metal is a Tomonaga-Luttinger liquid (TLL), we calculate the TLL parameter K-rho. It is largest when V-1 and V-2 are frustrated, and K-rho=0.25 at the boundaries between the metallic phase and each of the two CDW phases
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
Tomonaga-Luttinger parameters for doped Mott insulators
No full textThe Tomonaga-Luttinger parameter determines the
critical behavior in quasi–one-dimensional correlated electron
systems, e.g., the exponent α for the density of
states near the Fermi energy. We use the numerical density-matrix
renormalization group method to calculate from the
slope of the density-density correlation function in momentum
space at zero wave vector. We check the accuracy of our new
approach against exact results for the Hubbard and XXZ Heisenberg
models. We determine in the phase diagram of the
extended Hubbard model at quarter filling, n_{\ab{c}}=1/2, and
confirm the bosonization results K_{\rho}=n_{\ab{c}}^2=1/4 on
the critical line and K_{\rho}^{\ab{CDW}}=n_{\ab{c}}^2/2=1/8 at
infinitesimal doping of the charge-density-wave (CDW) insulator
for all interaction strengths. The doped CDW insulator exhibits
exponents only for small doping and strong
correlations
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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