1,720,964 research outputs found
In situ TEM analysis of resistive switching in manganite based thin-film heterostructures
No full textThe mechanism of the electric-pulse induced resistance change effect in Au/Pr0.65Ca0.35MnO3/SrTi0.99Nb0.01O3 thin-film samples is studied by means of in situ electrical stimulation inside a transmission electron microscope. A detailed equivalent-circuit model analysis of the measured current voltage characteristics provides crucial information for the proper interpretation of the microscopy results. The electrical transport data of the electron-transparent samples used for the in situ investigations is verified by comparison to measurements of unpatterned thin-film samples. We find comprehensive evidence for electrochemical oxygen vacancy migration affecting the potential barrier of the pn junction between Pr0.65Ca0.35MnO3 and SrTi0.99Nb0.01O3 as well as the resistance of the manganite bulk. The high-resistance state formation in the Pr0.65Ca0.35MnO3 bulk is frequently accompanied by structural transformations, namely detwinning and superstructure formation, most likely as the result of the joint impact of dynamic charge inhomogenities by oxygen vacancy migration and injection of high carrier densities at the electrodes.Deutsche Forschungsgemeinschaft (DFG) [JO 348/10-1, SFB1073
Strain Driven Phase Decomposition in Ion-Beam Sputtered Pr1-XCaXMnO3 Films
Full text linkThe deposition of heteroepitaxial thin films on single crystalline substrates by means of physical deposition methods is commonly accompanied by mechanical strain due to lattice mismatch and defect generation. Here we present a detailed analysis of the influence of strain on the Mn solubility of Pr1-xCaXMnO3 thin films prepared by ion-beam sputtering. Combining results from X-ray diffraction, transmission electron microscopy and in situ hot-stage stress measurements, we give strong evidence that large tensile strain during deposition limits the Mn solubility range of the Perovskite phase to near-stoichiometric composition. Mn excess gives rise to MnO2. precipitates and the precipitation seems to represent a stress relaxation path. With respect to size and density of the precipitates, the relaxation process can be affected by the choice of substrate and the deposition parameters, that is, the deposition temperature and the used sputter gas.DFG [SFB 1073
Effects of interaction and disorder on polarons in colossal resistance manganite Pr0.68Ca0.32MnO3 thin films
No full textThe colossal magnetoresistance effect (CMR), the drop of the electric resistance by orders of magnitude in a strong magnetic field, is a fascinating property of strongly correlated electrons in doped manganites. Here, we present a detailed analysis of the magnetotransport properties of small polarons in thin films of the low bandwidth manganite Pr0.68Ca0.32MnO3 with different degrees of preparation- induced octahedral disorder. The crystal and defect structure is investigated by means of high-resolution transmission electron microscopy. We apply the small polaron theory developed by Firsov and Lang in order to study the hopping mobility in the paramagnetic phase and its changes due to the formation of the antiferromagnetic charge ordered (CO) and the ferromagnetic metallic phases. Although it represents a single particle theory, reasonable estimates of small polaron properties such as formation energy, activation energy and transfer integral are possible, if the effects of interactions and disorder are taken into account. Beyond the well-known effect of the magnetic double exchange on the transfer integral, we show that the emergence of band transport of small polarons in the CMR transition sensibly depends on the degree of octahedral disorder, the polaron-polaron interactions and the resulting long range order leading to a structural phase transition in the CO 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
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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
Temperature- and doping-dependent optical absorption in the small-polaron system
No full textSmall polaron optical properties are studied comprehensively in thin film samples of the narrow bandwidth manganite PrCaMnO by optical absorption spectroscopy as a function of doping and temperature. A broad near infrared double-peak absorption band in the optical conductivity spectras is observed and interpreted in the framework of photon-assisted small polaron intersite hopping and on-site Jahn-Teller excitation. Application of quasiclassical small polaron theory to both transitions allows an approximate determination of polaron specific parameters like the polaron binding energy, the characteristic phonon energy, as well as the Jahn-Teller splitting energy as a function of temperature and doping. Based on electronic structure calculations, we consider the impact of the hybridization of O2p and Mn 3d electronic states on the Jahn-Teller splitting and the polaron properties. The interplay between hopping and Jahn-Teller excitations is discussed in the alternative pictures of mixed valence Mn/Mn sites (Jahn-Teller polaron) and equivalent Mn sites (Zener polaron). We give a careful evaluation of the estimated polaron parameters and discuss the limitations of small polaron quasiclassical theory for application to narrow bandwidth manganites
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