1,721,002 research outputs found
Local structure and magnetotransport in Sr2FeMoO6 double perovskite compounds: an EXAFS study
No full textSr2FeMoO6 oxides with double perovskite structure1 are half metallic ferromagnetic with elevated Curie temperature (Tc > 400 K) and present large magnetoresistance at room temperature2. In the last few years these compounds stimulated large interest as potential electrodes in magnetic tunnel junction and innovative spintronic materials. On the other hand these peculiar magnetic and electronic properties attracted the fundamental research in the field of heavily correlated electron systems. Their crystallographic structure derives from the ideal cubic perovskites: it is made of weakly tetragonal distorted cubic units with the Sr ions located at the center and the Fe(Mo)O6 sharing the cube edges. The regular alternation of Fe and Mo along the lattice edges (chemical order) strengthens the magnetoresistance while increasing the miss-site defects (chemical disorder) weakens the magnetoresistance. A kind of double exchange interaction (DE) between Fe ions along the Fe-O-Mo-O-Fe chains mediated by Mo, has been suggested to explain the metallic state. Chemical disorder reduces the DE coupling and gives rise to tunnel type conductibility. The most striking features in this picture is the high Curie temperature, well above that observed in the well known Mn-perovskites (La1-xCaxMnO3, La1-xSrxMnO3 and related systems), which imply huge exchange interactions despite the very long distance among Fe ions (~8 Å)3. This work proposes a detailed EXAFS study on two Sr2FeMoO6, a fully chemically ordered and a fully chemically disordered sample, in order to get insights on the micro-structural origin of their magnetotransport properties. X-ray absorption experiments were performed at the Italian beamline (GILDA-BM8) at the ESRF. In order to have a complete and exhaustive description of local order Fe K-edge (~7112 eV), Mo-K edge (~20000 eV) and W-LIII (~10200 eV) edge EXAFS data were analysed. The complete EXAFS spectra were refined taking into account for single as well multiple scattering contributions till about 6 Å. This permitted to probe the relative arrangement of Fe and Mo ions. The coherence of the structural models, as resulting from the three edges analysis strengthens and gives confidence on the results. The main result is that either ordered and disordered compounds show very similar chemically ordered local structure, i.e. the presence of Fe-Fe and Mo-Mo coordination is weak (almost negligible) in both the samples. These finding contrasts with the information from X-ray diffraction analysis, i.e. a high ordered (order parameter ~ 90%) and a completely disordered sample. Our results suggest that the long-range chemically disordered sample is made of small (20-30 Å) chemically ordered clusters. Such a small size would produce weak and broad superlattice reflections easily confused with the background in standard diffraction patterns
Amorphous Aggregation of Amyloid Beta 1-40 Peptide in Confined Space
No full textThe amorphous aggregation of Aβ1-40 peptide is addressed by using micromolding in capillaries. Both the morphology and the size of the aggregates are modulated by changing the contact angle of the sub-micrometric channel walls. Upon decreasing the hydrophilicity of the channels, the aggregates change their morphology from small aligned drops to discontinuous lines, thereby keeping their amorphous structure. Aβ1-40 fibrils are observed at high contact angles
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
Long-Lived Photoinduced Polarons in Organohalide Perovskites
No full textThe long diffusion length of charge carriers in the CH3NH3PbI3 perovskite is one of the most relevant properties for explaining the high photovoltaic efficiency of perovskite solar cells. As a possible mechanism for the large diffusion length of electrons and holes, several authors suggested a reduced coulomb attraction of the carriers due to the formation of polarons. Here we performed continuous wave far-infrared photoinduced absorption (PIA) experiments on CH3NH3PbI3; spectral changes are associated with local deformation of the lattice around the photogenerated long-lived charges, a typical signature of photoinduced polarons. Ab initio calculations show confinement of charge carriers at the interface between structural domains characterized by a different tilting of the PbI6 octahedra. The differential IR spectrum between unperturbed and perturbed simulated structures shows a close pattern to the experimental PIA. Positive and negative charges are confined in different varieties of the perovskites coherent with the low recombination and long diffusion length of photogenerated carriers
Studio di assorbimento di acqua in membrane di ossido di grafene
Full text linkDa quando nel 2004 venne isolato per la prima volta il grafene, la ricerca sui composti del carbonio si è ampiamente intensificata: tra questi è particolarmente rilevante il caso dell'ossido di grafene (GO), interessante per la sua inusuale porosità selettiva alle molecole d'acqua e per la possibilità di ottenere proprietà simili al grafene se sottoposto a processi di riduzione.
Il presente studio ha avuto come argomento generale quello di comprendere i fenomeni di interazione di molecole di acqua con membrane di ossido di grafene con l’obiettivo a lungo termine di sviluppare membrane innovative basate su questo materiale per applicazioni di filtrazione e purificazione. Nello specifico si è studiato il comportamento strutturale del GO in presenza di acqua e la dipendenza del fenomeno di assorbimento dell'acqua dallo spessore della membrana.
L’elaborato di tesi segue l'attività sperimentale che ho svolto al seguito della Dottoressa Fabiola Liscio all'Istituto per la Microelettronica e i Microsistemi del Consiglio Nazionale delle Ricerche di Bologna: sono state eseguite misure di diffrazione a raggi X su campioni di ossido di grafene in condizioni di umidità controllata. Le misure sono state ripetute su diversi campioni sintetizzati con la medesima tecnica, studiando i dati in funzione dell'umidità.
Lo studio è riuscito a evidenziare alcune delle problematiche sperimentali relative a questa tipologia di misure, per le quali è stata modificata la metodologia ottenendo risultati migliori. Le misure condotte hanno evidenziato un comportamento inaspettato nei campioni, una isteresi nell'assorbimento dell'acqua da parte delle membrane, comportamento che andrebbe approfondito in ricerche future
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
Self-assembled magnetic nanostructured alloys prepared by molecular beam epitaxy on low energy surfaces
No full textThe objective of this work is to understand how the deposition temperature and the nature of substrate can tune the structural and magnetic properties and the morphology of MxPt1-x (M=Co and Fe) nanostructured alloys. The metallic adatoms have been co-deposited by molecular beam epitaxy at different deposition temperature on low energy surfaces as WSe2(0001) and NaCl(001). The characterization has been performed by the using of several techniques. Reflection high energy electron diffraction (RHEED) has been use to follow the epitaxy growth during the deposition; the X-ray diffraction measurements (XRD) has been performed to characterize the structure and extrapolate the long range chemical order parameter; the X-ray absorption spectroscopy (XAS) has been use to probe the local structure; the magnetic anisotropy has been investigated by the superconducting quantum interference devices (SQUID); the morphology was studied by grazing incidence small angle x-ray scattering (GISAXS) measurements, coupled to other characterization techniques (TEM, STM and Field Emission-SEM).
A first result is the structural anisotropy at short range observed in the CoPt3 grown at room temperature on WSe2(0001). It is characterized by preferential Co-Co correlations in the film plane extending up
to third shell that suggests the formation of Co rich thin disks in the film plane. The cobalt segregation is accompanied with a lattice distortion, where the in-plane bonds are shorter than the out-of-plane bonds. These effects disappear for the samples prepared at higher deposition temperature, Td, in which partial
L12 order and larger in-plane bonds are observed. To understand the reason of such Co segregation, CoPt3 nanostructures were formed on the low energy surface, NaCl(001). No L12 ordering was observed by RHEED in nanostructures grown at 670K, and no local anisotropic order in nanostructures grown at
370K was detected by XAS. It suggests that the short range anisotropic order observed in nanostructures grown at room temperature on WSe2 would be driven by Se surfactant effect. An XRD study has been performed on the equiatomic CoPt and FePt alloys. The grown of CoPt on WSe2(0001) leads to the formation of a single crystalline fcc CoPt nanoparticles with [111] orientation even for samples prepared at room temperature. The L10 chemical order starts at 470K. However, increasing the deposition temperature to 720K does not enhance the chemical order parameter, which is
only about 0.35. The segregation of Se at elevated deposition temperatures and surface disorder effects associated with a large surface-to-volume ratio might limit the long-range chemical order. Magnetic studies do not indicate the presence of an out-of-plane magnetic anisotropy as previously observed for CoPt3(111) alloys grown on WSe2(0001).
The growth of FePt on NaCl(001) leads to the formation of nanoparticles oriented along the [001] direction for Td=520K; the chemical order parameter has been found to be 0.34 and increases to 0.4 for Td=620K. However at this deposition temperature a [110] growth direction is also observed.
The size and morphology of the self-assembled CoPt and CoPt3 were studied. Deposits of alloys with various nominal thicknesses (t) and prepared at different Td were measured under incidence angles closed
to the critical values of the substrates. The STM observations in nanostructures grown on WSe2 show a decrease of the aspect ratio r=h/l (h and l being the height and the lateral size) with increasing the growth
temperature. A change from an hexagonal to triangular shape occurs above 700K in nanostructures higher than 3nm. Facetting is observed by GISAXS with a higher sensitivity to the less inclined 001 side-wall facets, even in minority. For highly dense assemblies as observed for t=1nm, information
extracted from GISAXS analysis would be restricted to the upper part of the particles due to a coalescence of their lower part. The nanostructures grown on NaCl(001) have a truncated pyramid shape with an
aspect ratio of 0.75 for Td=530K, i.e significantly larger than the ones grown on WSe2 (0.4). The 111 side-wall facets are well observed by GISAXS for nanostructures with lateral size of 10nm, while for
smaller nanostructures (l<5nm) simulated patterns cannot differentiate the cubooctahedral shape from the spherical shape
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
Nanostructures magnetiques auto-assemblees sur des surfaces a faible energie par epitaxie par jets moleculaires
No full textNanostructures of MxPt1x (M=Co and Fe) alloys have perpendicular magnetic anisotropy which makes them good candidates as high density magnetic recording media. In this thesis work, the structural and magnetic properties of these nanostructures were studied as a function of the substrate (structure, symmetry and surface energy) and the co-deposition temperature. The nanostructures were prepared by co-deposition of transition metal atoms on low energy surfaces WSe2(0001) and NaCl(001) by molecular beam epitaxy. This led to the formation of stress-free nanostructures whose properties were characterized by techniques including X-ray diffaction, XAFS, GISAXS, TEM, STM and SQUID magnetometry. The chemical order effects at short- and long-range and the in uence of morphology and growth direction on magnetic properties were clearly shown.Les nanostructures d'alliages MxPt1x (M=Co et Fe) developpent des anisotropies magnetiques perpendiculaires, très interessantes pour des applications dans le domaine de l'enregistrement a haute densite. Dans ce travail de these, la nature du substrat (structure, symetrie et energie de surface) et l'in uence de la temperature de co-deposition sur les proprietes structurales et magnetiques des nanostructures ont ete etudies. La methode a consiste a deposer des atomes de metaux de transition par epitaxie a jets moleculaires sur les surfaces WSe2(001) et NaCl(001) de faible energie. Elle a conduit a la formation de nanostructures modeles, non contraintes dont les proprietes ont ete determinees par differentes techniques (diffraction X, XAFS, GISAXS, MET, STM et magnetometrie SQUID). Les effets de mise en ordre chimique a courte et a longue distance et l'infl uence de la morphologie et de la direction de croissance sur les proprietes magnetiques ont ete clairement demontres
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