1,720,961 research outputs found
Thermal hysteresis in the luminescence of Yb3+ ions in Sr0.6Ba0.4Nb2O6
No full textThe optical spectra of Yb(3+) ions in congruent, Sr(0.6)Ba(0.4)Nb(2)O(6) (SBN) crystals have been systematically investigated in a broad temperature range (10-420 K). Laser site selective low temperature fluorescence spectra have shown that Yb(3+) ions enter in four different cationic lattice sites. A thermal hysteresis in both bandwidth and luminescence intensity of Yb(3+) ions has been detected in the temperature range (300-400 K) around the ferro to paraelectric phase transition. This behavior has been explained as a result of the local structural modifications that take place around the Yb(3+) ions when the crystal becomes nonpolar, which affect in a different way the luminescence of Yb(3+) ions located in the different sites. Off-congruent SBN crystals activated with Yb(3+) ions have been grown and investigated in order to examine the possibilities of extending the detected hysteretic behavior to other temperature ranges
Bistable chromatic switching in Yb3+ doped NdPO4 crystals
No full textPump-induced chromatic switching is reported for an 808 nm pumped Yb3+ -doped NdP O4 crystal. This effect is based on the interplay between the Nd3+ ↔ Yb3+ energy transfer processes and on the large pump-induced thermal loading characteristic of this neodymium compound. For temperatures below 300 K, a noticeable hysteresis is observed in the relative Nd3+ and Yb3+ contributions to the emission spectra; this leads to a bistable chromatic switching. This optical bistability is related to a nonlinear pump-induced heating rate that produces abrupt changes in the local temperatur
Energy transfer processes in the ytterbium doped NdPO4 stoichiometric crystal
No full textWe report on crystal growth and fluorescence characterization of Yb3+ doped NdPO4 single crystals. From the analysis of fluorescence decay curves, the temperature dependence of Nd3+ → Yb3+ and Nd3+ ← Yb3+ energy transfer processes has been obtained. We have found that the dominant process is strongly temperature dependent, this leading to a temperature induced luminescence switching from Yb3+ to Nd3+ ion
Lanthanide-doped strontium barium niobate (SBN) materials: A vibrational investigation
No full textVibrational Raman spectra have been measured for strontium barium niobate (Sr0.5Ba0.5Nb2O6, SBN50) single crystals and nanopowders doped with 1 mol% of the luminescent ions Eu3+ and Er3+. The nanocrystalline materials show slightly broader spectra with respect to the single crystals with the same composition. The presence of the Eu3+ and Er3+ ions at the 1 mol% doping level, and the present particle size (200 nm) do not appear to affect the ferro-to-paraelectric phase transition temperature typical of undoped SBN50 single crystals, as detected from anomalies in the dependence of the position of the Raman peak around 635 cm(-1) as a function of the temperature
Optical properties of active ions around the ferro-paraelectric phase transition in SBN crystals
No full textNew results on the evolution of the emission of Nd3+ doped SrxBa1-xNb2O5 crystal during the ferroelectric phase transition are presented and compared to those previously obtained for Cr3+ and Yb3+ doped SrxBa1-xNb2O5. We have observed that Nd3+ emission is insensitive to the structural changes suffered by the crystal during the phase transition, contrary to the observations obtained for the cases of the Cr3+ and Yb3+ doped crystals. The results are interpreted in terms of the different lattice sites occupied by the optically active ions in the SrxBa1-xNb2O5 crystal
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
Phase transition in SrxBa1-xNb2O6 ferroelectric crystals probed by raman spectroscopy
No full textIn this paper the temperature evolution of the main Raman peak at about 630 cm(-1) has been studied in order to detect the ferroelectric to paraelectric phase transition in strontium barium niobate (SrxBa1-xNb2O6) crystals with different stoichiometries (x = 0.33, 0.5 and 0.61). Both its peak wavenumber and width have been shown to be strongly dependent on temperature and display clear anomalies around the Curie temperature. The obtained results manifest the usefulness of simple unpolarized Raman spectra to probe the changes in T-c as a result of modifications in the crystal composition
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
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