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Start Over Author "Fastampa R" Journal physica c: superconductivity and its applications
32 results on '"Fastampa R"'

18. Microwave complex resistivity in YBCO films above Tc: effects of fluctuations

Author

Neri, D., Marcon, R., Rogai, R., Silva, E., Fastampa, R., Giura, M., Sarti, S., Cucolo, A.M., Beneduce, C., Bobba, F., Boffa, M., and Cucolo, M.C.

Abstract

We present measurements of the (a,b) plane complex resistivity at 24 GHz in nearly-optimally doped YBCO films grown by dc sputtering with Tc ≅ 90 K. The attention is focused above Tc, where we discuss the data in terms of 3D, anisotropic fluctuations. We find that at microwave frequencies simple gaussian fluctuations are unable to describe our data both well above and in the vicinity of the critical temperature. The discrepancy well above Tc is recovered by including a short-wavelength cutoff in the frequency-dependent fluctuational conductivity. In order to follow the slowing down of the fluctuations at Tc, we treat the region close to Tc including the quartic term of the order parameter in the Ginzburg-Landau functional within the Hartree approximation. We then find excellent agreement with our data from high temperature (T = 1.3 Tc) down to Tc.

Published
2000
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32. Angular dependence of the resistively-determined irreversibility line in YBCO

Author

Enrico Silva, Stefano Sarti, M Giura, R Fastampa, Giura, M, Fastampa, R, Sarti, S, and Silva, Enrico

Subjects
Materials science, Field (physics), Condensed matter physics, Energy Engineering and Power Technology, Conductivity, Condensed Matter Physics, Electronic, Optical and Magnetic Materials, Magnetic field, Electrical resistivity and conductivity, Electrical and Electronic Engineering, Thin film, Scaling, Critical field, Line (formation)
Abstract

We present a wide set of resistive measurements taken in a YBCO thin film, as a function of the temperature, magnetic field and field orientation ϑ with respect to the ( a,b ) planes. We focus the attention on the low portion of the resistance curves. Contrary to what is found close to T c , no angular scaling is possible. We examine the angular behavior of the irreversibility line, conventionally taken as the field and angle dependent temperature T i ( H , ϑ) at which the resistivity exceeds some sensitivity threshold. It is found that T i does not follow a simple 2D or 3D behavior, and that its angular dependence is affected by the extended defects present in the film (twin planes) and by the layered structure. We also show that the irreversibility line as a function of the angle, taken at constant reduced field h = H/H c 2 ( H c 2 is the temperature- and angle- dependent upper critical field, as determined from the fluctuation conductivity), presents a noticeable angular dependence.

Published
1997
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