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Effects of a magnetic field on the microwave surface impedance of Bi2Sr2CaCu2O8+x single crystals
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Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Effect of BaZrO(3) Inclusions on the Microwave Surface Impedance of YBCO Films in a Magnetic Field
No full textWe perform measurements of high-frequency (similar to 48 GHz) microwave surface impedance with an applied magnetic field in YBa(2)Cu(3)O(7-delta) ( YBCO) laser-ablated films with various amounts of BaZrO(3) ( BZO) sub-micrometric inclusions, up to 7 mol % concentration. BZO inclusions are very effective in the reduction of the field-induced surface resistance in our experimentally accessible field range [ 0, 0.8] T. At temperatures low enough, the application of a moderate (similar to 0.2 T) field makes the YBCO/BZO films markedly less dissipative than pure YBCO. This result, examined in the light of the very high measuring frequency ( very small vortex oscillation amplitude) shows that BZO inclusions are even more effective pinning centers than columnar defects. We study the dependence of the vortex parameters ( vortex viscosity, pinning constant) on the BZO concentration. We examine the correlation between the reduction of the microwave dissipation and the areal density of BZO-induced defects. We argue that the very improved performances in a magnetic field are due to individual pinning of vortices on BZO inclusions
Angular dependence of the high-frequency vortex response in YBa2Cu3O7-x thin film with self-assembled BaZrO3 nanorods
No full textNanosize particles are known to increase dramatically the vortex pinning in YBa2Cu3O7-x (YBCO).
Among the various inclusions that give rise to an increased pinning, BaZrO3 (BZO) is one of the most
studied. Interestingly, under certain circumstances BZO spontaneously self-assemble along columnar-like
structures, with a diameter of a few nm and directed approximately along the c-axis of the YBCO film.
This feature has relevant consequences for the practical use of YBCO superconductors. In this work we
present a microwave study of the enhancement of pinning in YBCO thin films due to the addition of BZO
particles, with particular emphasis on the angular dependence. The samples under study have been grown
by pulsed laser ablation using a composite YBCO/BZO target containing BZO at 5 mol %. X-ray
diffraction and TEM analysis were performed, and confirmed the excellent orientation of the YBCO film
and the presence of columnar-like structures across the film. Microwave measurements were performed
by means of a dielectric-loaded resonator operating at 48 GHz, in the temperature range between 60 K
and Tc and in a static magnetic field up to 0.8 T, applied with a variable angle with the c-axis.
Comparison with a pure YBCO sample revealed a very strong increase in the pinning strength,
particularly relevant in the small-vortex-displacement regime here probed. The angular dependence of the
complex microwave response directly showed that the balance between vortex dissipation and reactance,
which is a measure of the pinning strength, changes from nearly unity (dissipation = reactance) when the
field is directed along the c-axis (thus, along the BZO nanorods), to more dissipative when the field is
directed in-plane. Thus, BZO nanorods are found to be more efficient in pinning the vortices in YBCO
than the anisotropic (layered) structure itself. In order to describe more quantitatively the experimental
results, we extract from our data the vortex parameters (vortex drag coefficient, vortex pinning
frequency). We developed a model for the anisotropic motion of vortices in uniaxially anisotropic
superconductor, at an arbitrary angle with the crystallographic axes, from which an expression of the
effective anisotropic vortex drag coefficient is derived. The known limiting cases of vortices and motion
along the crystallographic axes are recovered. We discuss our results in the light of the mentioned model.
This work has been partially supported by the Italian FIRB project "SURE:ARTYST" and by
EURATOM. N.P. acknowledges support from Regione Lazi
Variations on the Author
Get PDF“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
Effects of a magnetic field on the microwave surface impedance of Bi2Sr2CaCu2O8+x single crystals.
No full textWe present measurements of the field-induced change of the microwave surface impedance in optimally doped Bi2Sr2CaCu2O8- crystals. Measurements are taken with microwave fields in the (a,b) planes. A moderate (<0.7 T) dc magnetic field is applied normal to the (a,b) planes. Measurements are taken for temperatures above 70 K. At fixed temperature, with increasing field the surface resistance presents a sharp onset at low fields, a steep rise then a plateau or a peak, followed by a second, slower rise with the field. The surface reactance presents similarly a steep raise at low fields above the onset, then a sudden decrease. We discuss the applicability of commonly established vortex motion models to the observed response, and we find that substantial inconsistencies emerge. Possible influences of the layered nature on the vortex response are investigated
Angular dependence of the high-frequency vortex response in YBa2Cu3O7−x thin film with self-assembled BaZrO3 nanorods
No full text"We present a microwave study of the angular dependence of the flux-flow resistivity qff and of the pin-. ning constant kp in YBCO thin films containing BZO nanorods. We find that BZO nanorods are very effi-. cient pinning centers, even in tilted fields. We find that qff is a scaling function of a reduced field. H\/f(h). We extend a model for the anisotropic motion of vortices in uniaxially anisotropic superconductor,. able to describe the experimental f(h) on the basis of only the intrinsic anisotropy of YBCO. The pinning. constant kp, by contrast, exhibits different field dependences in different angular ranges, consistent with. pinning by BZO at angles as large as 60°, and with pinning along the a,b planes as originating from the. same mechanism as in pure YBCO with the field along the c axis..
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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
Get PDFWe 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
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