177,722 research outputs found
Cosmological evolution of thermal relic particles in f (R) gravity
No full textBy considering f(R) gravity models, the cosmic evolution is modified with respect to the standard ΛCDM scenario. In particular, the thermal history of particles results is modified. In this paper, we derive the evolution of relics particles (weakly interacting massive particles) assuming a reliable f(R) cosmological solution and taking into account observational constraints. The connection to the PAMELA experiment is also discussed. Results are consistent with constraints coming from BICEP2 and PLANCK experiments
Measurements and removal of substrate effects on the microwave surface impedance of YBCO films on SrTiO3
No full textWe reconsider the problem of the measurements of the microwave complex surface impedance in thin superconducting films deposited on SrTiO3 substrates. We perform measurements of the complex surface impedance Z(s)(') = R-s(') + i Delta X-s(') of thin YBa2Cu3O7-delta films deposited by laser ablation on SrTiO3 substrates. The typical oscillations due to the strong temperature variation of the SrTiO3 permittivity are confirmed in R-s(') and observed in Delta X-s('). The effects of the SrTiO3 substrate are evident even well below the superconducting transition temperature of YBa2Cu3O7-delta. Similarly to previous works, we describe the overall response in terms of impedance transformations. We extend the known results by (i) considering the measurements of the imaginary part, (ii) comparing the measurements to the absolute dc resistivity measured on the same sample, and (iii) suggesting a method for measuring the intrinsic thin film surface impedance by adjusting the substrate impedance. To demonstrate the feasibility of microwave measurements of intrinsic properties of films grown onto SrTiO3 substrates, we check the proposed method by measuring the field dependent surface impedance before and after removal of the substrate resonance
Change of strength of vortex pinning in YBCO due to BaZrO3 inclusions
No full textWe probe the short-range pinning properties with the application of microwave currents at very high driving frequencies (47.7 GHz) on YBa2Cu3O7-delta films with and without sub-micrometer BaZrO3 inclusions. We explore the temperature and field ranges 60 K, < T < T-c and 0 <mu H-0 < 0.8 T, with the field applied along the c-axis. The magnetic field induces a much smaller increase of the microwave resistivity, Delta rho(1)(H)+i Delta rho(2)(H), in YBa2Cu3O7-delta/BaZrO3 with respect to pure YBa2Cu3O7-delta.Delta rho(1)(H) is slightly superlinear in pure YBa2Cu3O7-delta (suggesting a possible contribution of thermal activation), but linear or sublinear in YBa2Cu3O7-delta/BaZrO3 (suggesting a possible suppression of thermal activation as a consequence of BaZrO3 inclusions). These features persist up to close to T-c. We discuss our data in terms of the ratio r = Delta rho(2)(H)/Delta rho(1)(H) in the framework of the models for the microwave surface impedance in the mixed state. Large r are found in YBa2Cu3O7-delta/BaZrO3, with little field dependence. By contrast, smaller values and stronger field dependences are found in pure YBa2Cu3O7-delta. We discuss the different field dependence of the pinning constant. (C) 2008 Elsevier B.V. All rights reserved
The Painter and the Moon: Cosmology issues in the Codex Leicester
No full textAll’interno del catalogo scientifico con comitato internazionale della mostra sul Codice Leicester (Foundation Bill & Melinda Gates) di Leonardo da Vinci alla Galleria degli Uffizi (ottobre 2018 - gennaio 2019, editore Giunti), il saggio indaga la struttura acquea della luna così come immaginata e disegnata da Leonardo. Si contestualizza la funzione del pittore/astronomo in epoca rinascimentale e si delucidano le note e i diagrammi relativi alle acque nella luna del Codice Leicester, con opportuni riferimenti ad analoghi passi di altri manoscritti leonardiani. In particolare i disegni del “lumen cinereum” e della superficie ondosa della luna sono spiegati in relazione agli studi del vinciano sull’ottica e gli specchi sferici
Le anomalie neurologiche transitorie del primo anno di vita: considerazioni sulle caratteristiche dello sviluppo neuromotorio in un campione di bambini a “basso rischio”
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n+-SnO2/a-SiC:H/Metal Thin Film Photodiodes with Voltage-Controlled Spectral Sensitivity
No full textAppropriate Similarity Measures for Author Cocitation Analysis
No full textWe 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
"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
No full textBoth spending and tax policies have been implemented in the United States with the goal of stimulating private sector research and development (R&D). Karier questions whether current R&D policy, especially the research and experimentation tax credit, can contribute to closing the gap between nondefense expenditures on R&D in the United States and such expenditures in other countries, such as Japan and Germany. He also explores possible changes to our current R&D policy to make it more effective.
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
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