1,723,296 research outputs found
Positron- electron autocorrelation function study of e-center in phosphorus-doped silicon
No full textTwo dimensional Fourier transformed angular correlation of annihilation radiation (2D-FT-ACAR) spectra have been taken for 1019 cm -3 Phosphorus-doped Si in the as grown state and after being subjected to 1.8MeV e- fluences of 2x1018cm -2. In the spectra of the irradiated samples, the zero-crossing points are observed to displace outwards from the Bravais lattice positions. It is suggested that this results from positrons annihilating with electrons in localized orbitals at the defect site. An attempt is made to extract just the component of the defect's positron-electron autocorrelation function that relates to the localized defect orbitals. It is argued that such an extracted real-space function may provide a suitable means for obtaining a mapping of localized defect orbitals
Positron-electron autocorrelation function study of E-center in silicon
No full textA study was conducted on the positron-electron autocorrelation function of defect center in silicon. The positron annihilation lifetime spectroscopy (PALS) spectra taken on the as-grown and defected samples of silicon were studied. It was found that the positron binding energy to the defect could be estimated from the ratio of the positron-electron autocorrelates for bulk crystal and defect trapped positron states
F-electron behavior in rare earth based systems: Localization or itinerancy?
No full textMeasurement of the two-dimensional angular correlation of the electron-positron annihilation radiation complemented with ab initio calculations can provide decisive information about the character of the /-electrons in rare earth compounds. We provide examples of f-electron localized and f-electron itinerant systems, respectively. (i) In the case of the antiferromagnetic heavy fermion and superconductor CeIn3 the multisheet Fermi surface, reconstructed from our measurements in the paramagnetic phase, agrees closely with the predictions of band structure calculations regarding the Ce 4f electrons as fully localized. (ii) On the other hand, our studies of the antiferromagnet actinide based UGa3 in the paramagnetic phase, compared with calculations which include the effects due to the non-uniform positron density and the electron-positron correlations, produce a substantial evidence that an unconstrained 5 f-electron itinerant description applies
Positron spectroscopy analysis of vacancy-solute nanoaggregates in Al-Cu
No full textThis work addresses the process of ageing in Al-4wt.% Cu at two temperatures (150°C, 180°C) and in conditions of secondary ageing at temperatures up to 60°C, after artificial ageing treatment at 150°C. Positron lifetime and coincidence Doppler broadening measurements are reported. The analysis shows an initial association of positron trapping sites with Cu (about 9 at. % Cu at the annihilation site) that decreases to 7+6 % in the initial stages of ageing, but then increases sharply (up to 25 at. % Cu after 300 hours at 180°C). The positron lifetime also displays an initial decrease followed by a final increase. The results confirm the hypothesis that positrons are not trapped in GP[1] zones, but show that trapping occurs in GP[2] zones formed at 150°C. During secondary ageing, the vacancy-Cu association as well as the positron lifetime decrease with the ageing time. This is attributed to the formation of GP[1] zones due to residual supersaturation
Vacancy-solute aggregates in Al-Cu-Mg alloys studied by positron annihilation techniques
No full textFermi-surface topology of the heavy-fermion antiferromagnetic superconductor CeIn3
No full textA three-dimensional mapping of the Fermi surface (FS) of the heavy-fermion antiferromagnetic system CeIn3 in the paramagnetic phase was determined via measurements of the angular correlation of the electron-positron annihilation radiation for five crystal orientations. The two equivolume FS sheets observed agree with the predictions of our band-structure calculations regarding the Ce 4f electrons as fully localized. Moreover, a comparison of our results with the de Haas-van Alphen signals observed in the antiferromagnetic phase suggests that the f electrons retain the local atomic character in the magnetically ordered phase
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
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