1,721,008 research outputs found
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
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
Hysteresis of the Nearly-Commensurate to Commensurate Charge Density Wave Phase Transition in Tantalum Disulfide Using Raman Spectroscopy
Get PDFThe structure of tantalum disulfide at various temperatures was investigated through vibrational mode analysis using Raman scattering. Incident and scattered light was analyzed through polarization in order to decompose broadened features, allowing for the characterization of individual mode behavior. Temperature dependence of the vibrational modes in the region surrounding the nearly-commensurate to commensurate charge density wave phase transition shows strong hysteretic behavior. The widely-referenced macroscopic resistivity measurements, used in determining the charge density wave phase transition, have a different hysteresis. The resistivity displays sharp discontinuities at the phase transition whereas local vibrational mode measurements display a continuous evolution. The hysteresis of the vibrational modes extends over a significantly larger range than that of the resistivity. I conclude that tantalum disulfide has inhomogeneous, local structure on the microscopic scale
Hysteresis of the Nearly-Commensurate to Commensurate Charge Density Wave Phase Transition in Tantalum Disulfide Using Raman Spectroscopy
Get PDFThe structure of tantalum disulfide at various temperatures was investigated through vibrational mode analysis using Raman scattering. Incident and scattered light was analyzed through polarization in order to decompose broadened features, allowing for the characterization of individual mode behavior. Temperature dependence of the vibrational modes in the region surrounding the nearly-commensurate to commensurate charge density wave phase transition shows strong hysteretic behavior. The widely-referenced macroscopic resistivity measurements, used in determining the charge density wave phase transition, have a different hysteresis. The resistivity displays sharp discontinuities at the phase transition whereas local vibrational mode measurements display a continuous evolution. The hysteresis of the vibrational modes extends over a significantly larger range than that of the resistivity. I conclude that tantalum disulfide has inhomogeneous, local structure on the microscopic scale
Angle-Resolved Photoemission Spectroscopy Studies on Cuprate and Iron-Pnictide High-Tc Superconductors
Get PDFThe exotic physics in condensed matter systems, such as High-Tc superconductivity in cuprates and the newly discovered iron-pnictide superconductors, is due to the properties of the elementary excitations and their interactions. The "one-electron removal spectral function" measured by angle-resolved photoemission spectroscopy (ARPES) provides a chance to understand these excitations and reveal the mechanism of the high-Tc superconductivity. In most cases, ARPES studies focus on the excitations very close to the Fermi level (usually within tens to hundreds of meVs). In this region, by presuming that the correlation effect is not too strong, we usually can describe the correlated electron system in terms of well-defined "quasiparticles" , i.e. electrons dressed with a manifold of excited states. Then the spectral function measured by ARPES can be separated into two parts: a coherent pole part that contains the information about the dispersion relation E(k) and the lifetime "tau" of the quasiparticles, which is usually the main subject of ARPES studies; and an incoherent smooth part without poles which also contains important information about the many-body interactions in the system but is usually overlooked by physicists due to the lack of analysis techniques and theoretical understanding. In this thesis, we present ARPES measurement on the cuprate High-Tc superconductors PbxBi2-xSr2CaCu2O8 (Pb-Bi2212) and Bi2Sr2CaCu2O8+d (Bi2212) and the iron-pnictide High-Tc superconductor's parent compound CaFe2As2 (Ca122) and BaFe2As2 (Ba122). For Pb-Bi2212 and Bi2212 materials, whose quasiparticle dispersions have already been extensively studied, our work focuses on the incoherent part of the spectral function. By introducing a new ARPES lineshape analysis technique, we separate out the sharp coherent peaks from the higher energy incoherent "background" portions and uncover a new type of scaling behavior of the incoherent portions. In particular, the fraction of weight that is incoherent is found to be intimately linked to the energy of the dispersive coherent feature through a simple quadratic relationship with no special energy scales. This behavior in concert with strong momentum-dependent matrix element effects gives rise to the heavily studied "waterfall" behavior in cuprate superconductors. For the newly discovered Ca122and Ba122 materials, whose intrinsic electronic structure is still missing, our studies aim at understanding its quasiparticle dispersion relation E(k) and the Fermi surface geometry. We observed unequal dispersions and FS geometries along the orthogonal Fe-Fe bond directions. Comparing with the optimized LDA calculations, an orbital-dependent band shifting is introduced to get better agreement, which is consistent with the development of orbital ordering. More interestingly, unidirectional straight and at FS segments are observed near the zone center, which indicates the existence of a unidirectional charge density wave order. Therefore, our studies indicate that beyond the well-known spin density wave (SDW) order and superconducting state (SC), there are other competing orders in the iron-pnictide materials such as the orbital order (OO), the charge density wave (CDW) order and the possible nematic phase. The coexistence of all these competing orders puts strong constraints on theories for describing the iron-pnictide system
New Insights into High-Tc Superconductivity from Angle-Resolved Photoemission at Low Photon Energies
Get PDFAngle-resolved photoemission spectroscopy (ARPES) is one of the most direct and powerful probes for studying the physics of solids. ARPES takes a "snapshot" of electrons in momentum space (k-space) to reveal details of the dispersion relation E(k), as well as information about the lifetimes of interacting quasiparticles. From this we learn not only where the electrons live, but also, if we are crafty, what they are doing. Beginning with work by our group in 2006 using a 6-eV laser, ARPES experiments have begun to make use of a new, low photon energy regime (roughly hν = 6-9 eV). These low photon energies give drastic improvements in momentum resolution, photoelectron escape depths, and overall spectral sharpness. This has led to several important new findings in the intensively-studied problem of high-temperature superconductivity. This thesis will focus on two of the latest results from our group using low-energy ARPES (LE-ARPES) to study the cuprate high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi2212). The first of these is an investigation into the nature of many-body interactions at a well-known energy scale (~60-70 meV) where the dispersion shows a large bend, or "kink". Using LE-ARPES measurements, the k-dependence of this kink is investigated in unprecedented detail. An attempt is then made to map the feature's k evolution into the scattering q-space of boson dispersions. In our analysis, the q-dispersion of the kink bears more resemblance to dispersive spin excitations than phonons -- a surprising finding in light of previous evidence that the the kink originates from interactions with phonons. However, phonons cannot be ruled out, and the results may hint that both types of interactions contribute to the main nodal kink. A second result is the discovery of a new ultralow (< 10 meV) energy scale for electron interactions, corresponding to a distinct, smaller kink in the electron dispersion. The temperature and doping dependence of this feature show not only that it turns on near Tc -- signalling a possible relation to the mechanism of high-Tc superconductivity -- but also that it leads to a subtle breakdown of the so-called "universal" Fermi velocity vF along nodes of the anisotropic superconducting gap. Moreover, vF is found to depend quite strongly on temperature, which may be an important factor in the physics of cuprates
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