1,720,968 research outputs found
Superfluorescence from Erbium-doped crystals
Get PDFIn Superfluorescence process, an initially incoherent ensemble of excited atoms (molecules, etc) gives rise to a macroscopic polarization due to spontaneously synchronization of the atomic dipoles. In this coupled system, every single dipole is correlated with all the others and the atomic transition rate is enhanced by a factor μN, with N the number of correlated atoms and μ a geometrical factor that accounts for the interference effects. The cooperative radiative decay results in the emission of coherent photon bunches whose peak intensity is proportional to μN^2 and the duration scales with (μN)^−1. In this thesis, we present the clear signatures of the Superfluorescence from Erbium-doped Y2SiO5 and YLiF4 crystals at liquid helium temperature. The process is observed for two transitions and involves the remarkable number of 10^12 Erbium ions, accelerating the radiative decay by millions of times. Several aspects of Superfluorescence are deeply investigated and a remarkable agreement with the theoretical expectation is reported
A new technique for infrared scintillation measurements
Get PDFWe propose a new technique to measure the infrared scintillation light yield of rare earth doped crystals by comparing it to near UV–visible scintillation of a calibrated Pr:(Lu0.75Y0.25)3Al5O12 sample. As an example, we apply this technique to provide the light yield in visible and infrared range up to 1700 nm of this crystal
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
Optical coherence and spin population dynamics in <sup>171</sup>Yb<sup>3+</sup>:Y<sub>2</sub>SiO<sub>5</sub> single crystals
No full textYb3+171-doped Y2SiO5 (YSO) crystals are a promising platform for optical quantum memories in long-distance quantum communications. The relevance of this material lies in Yb171 long optical and spin coherence times, along with a large hyperfine splitting, enabling long quantum storage over large bandwidths. Mechanisms affecting the optical decoherence are, however, not precisely known, especially since low-temperature measurements have so far focused on the 2 to 4 K range. In this work, we performed two- and three-pulse photon echoes and spectral hole burning to determine optical homogeneous linewidths in two Yb171:YSO crystals doped at 2 and 10 ppm. Experiments were performed in the 40 mK to 18 K temperature range, leading to linewidths between 320 Hz, among the narrowest reported for rare-earth ions, and several MHz. Our results show that above ∼6 K, the homogeneous linewidth Γh is mainly due to an elastic two-phonon process which results in a slow broadening with temperature, with Γh reaching only 25 kHz at 10 K. At lower temperatures, interactions with Y89 nuclear spin flips, paramagnetic defects or impurities, and also Yb-Yb interactions for the higher concentrated crystal are likely the main limiting factor to Γh. In particular, we conclude that the direct effect of a spin and optical excited state lifetime is a minor contribution to optical decoherence in the whole temperature range that is studied. Our results indicate possible paths and regimes for further decreasing homogeneous linewidths or maintaining narrow lines at higher Yb171 concentration
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
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