1,721,096 research outputs found
Dielectric resonators for the measurement of superconductor thin films surface impedance in magnetic fields at high microwave frequencies
No full textWe present the design and realization of cylindrical dielectric resonators operating in the 40-60 GHz frequency range, designed for the measurement of the surface resistance and of the surface reactance shift in High-T (c) Superconductors (HTS) thin films in a dc magnetic field. The resonators are single tone, based on the TE (011) mode, and multiple tone, the latter allowing in principle to exploit the simultaneous determination of the surface impedance at different frequencies. As an application example, we report the temperature and field dependencies of the effective surface impedance of some cuprate superconductors thin films. The results are compared with those obtained through the use of a standard metal cavity with a similar Q-factor and operating in the same frequency range. The comparison highlights a superior stability and a higher sensitivity, resulting in an increase of about two orders of magnitude in the resolution of the surface impedance measurement. By contrast, the dissipative part of the superconducting transition (above T/T (c) = 0.97) is better studied with the metal cavity. We also present measurements in the vortex state in YBaCa2Cu3O7-delta and Tl2Ba2CaCu2O8+x that show significantly different physics
Microwave surface resistance peak at T-c in Bi2Sr2CaCu2O8+x film
No full textWe observe an anomalous peak in the microwave surface resistance of a Bi2Sr2CaCu2O8+x (Bi:2212) thin film, at a temperature close, but different, to the transition temperature. The peak is frequency-dependent. Measurements of the resistive transitions between 19.5 and 26.5 GHz show that the peak temperature decreases with increasing frequency. The peak is most clearly visible at high frequency. We show that the peak is not due to physical phenomena in the superconductor, but instead it is due to substrate effects. A recently developed simplified model for the microwave surface resistance of a thin superconducting film, together with a two-fluid expression for the conductivity, can fit both the resistive transition, including the peak, and the frequency dependence of the peak. So, we do not find it necessary to invoke a distribution of T-c values, or inhomogeneities of the sample
The effective surface resistance of superconductor/dielectric/metal structures
No full textWe investigate the effective microwave surface resistance of superconductor/dielectric/normal metal structures through numerical simulations, in particular, the setup with a thin superconducting film on a dielectric substrate, backed by a metallic endplate, is analysed, The results demonstrate that the effects of the substrate on the measured surface resistance of superconducting thin films must be taken into account. We show that in this setup several spurious effects are predicted. At characteristic frequencies, dependant on the substrate material, the commonly used thin-film approximation fails even for films thinner than the London penetration depth. In general, when the film is thicker than, but of the same order as, lambda, the measured surface resistance can exceed both the thin-film and the bulk values. We investigate how the effective surface resistance of a superconducting film can be approximated by the bulk or the thin-film value, as a function of frequency, film and substrate thickness, and temperature. We exploit a thin-film model to take into account the effects of the backing materials. Measurements of surface resistance are performed on a BSCCO/SrTiO3/metal structure to stress the nonintrinsic behaviours and to test the proposed model
FLUX-FLOW DISSIPATION IN GDBA2CU3O7 AT MICROWAVE-FREQUENCIES
No full textWe report measurements of the magnetic-field-dependent microwave dissipation in high-purity GdBa2Cu3O7-delta pellets with different grain sizes, by means of a cavity technique with no field modulation. The dissipation is due to the intragranular viscous vortex motion at fields greater than or similar to 0.02 T, and to weak-links at low fields (H 0.02 T) has to be ascribed to the free motion of flux lines
Microwave surface resistance peaks in BSCCO thin films.
No full textWe measure the microwave surface resistance R-S at 24 GHz in BSCCO films of different thickness on two different substrates: LaAlO3 and SrTiO3. One of the films deposited on LaAlO3 presents an anomalous peak in R, at a temperature close to the transition temperature. The film deposited on SrTiO3 presents multiple peaks as a function of the temperature. We show that both effects can be ascribed to substrate effects, instead of physical phenomena in the superconducting films. To reflect the actual experimental setup, we develop a simplified model for the microwave surface resistance of thin superconducting film deposited onto a dielectric substrate backed by a metal. We show that at characteristic frequencies, dependent on the substrate material, the commonly used thin film approximation fails even for films thinner than the London penetration depth. With this model, supplemented with a two-fluid expression for the conductivity, the single peak in the film deposited onto LaAlO3 and the multiple peaks of the SrTiO3-deposited film find a quantitative explanation. The results demonstrate that the effects of the substrate on the measured R-S of superconducting thin films must be taken into account
Substrate contribution to the surface impedance of HTS films on Si
No full textWe study the effects of semiconductor substrates on the surface impedance of high-T-c Superconductor (HTS) YBa2Cu3O7-delta (YBCO) films. The characteristic impedance of silicon (Si) ( for different doping levels and for different charge carrier scattering times) is evaluated. In particular, the most relevant features of Si electrodynamics are highlighted by the introduction of suitable normalized quantities. The effective surface impedance of the YBCO films on Si substrates is then calculated and discussed for different temperatures and frequencies in the microwave range, comparing the obtained results to their limiting expressions for bulk and thin-film HTS. Our analysis shows how the widely used thin-film approximation for the surface impedance can fail, critically highlighting the conditions it requires to be correctly used. We show that substrate contributions can heavily influence the overall response
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
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