M. Boffa

Directional solidification of bulk (Y,Sm,Nd)/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/

Rare Earth (Nd,Sm) elements, having relatively large ionic radius, have been substituted for Yttrium in Y123 superconducting melt textured bulk samples. In (Nd,Sm)123 a wider solidification range and higher recrystallization rate than Y123 significantly increase the solidification rate, making the whole process much faster. A preliminary comparison among directionally solidified (Y,Sm,Nd)/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ bulk bars fabricated by the Horizontal Bridgman method has been done. The different microstructural and superconducting features are studied by X-ray, SEM analysis, and ac and dc magnetic measurements. The (Nd,Sm)123 samples, appear comparable to Y123 grown with a pulling rate almost two orders of magnitude lower.

Microwave intermodulation study of YBa2Cu3O7−δ films in the presence of an external d.c. magnetic field

Abstract Two tone intermodulation measurements on microstrip resonators made of YBa 2 Cu 3 O 7−δ (YBCO) superconducting thin films are reported. The effect of an external d.c. magnetic field on third order products is investigated. The data show that the presence of the d.c. field affects the input power dependence of intermodulation signals consistently with changes of the surface impedance.

Microwave fluctuational conductivity in YBa\(\mathsf{_{2}}\)Cu\(\mathsf{_{3}}\)O \(\mathsf{_{7-\delta}}\)

We present measurements of the microwave complex conductivity at 23.9 and 48.2 GHz in YBa2Cu3O

Dynamic regimes in YBCO in applied magnetic field probed by swept frequency microwave measurements

_{7-\delta}

Microwave losses induced by vortex propagation in highly epitaxial YBa2Cu3O7−δ films

films, in the fluctuational region above Tc. With increasing temperature, the fluctuational excess conductivity drops much faster than the well-known calculations within the time-dependent Ginzburg-Landau theory [H. Schmidt, Z. Phys. 216, 336 (1968)]. Approaching the transition temperature, slowing down of the fluctuations is observed. We discuss the results in terms of a modified Gaussian theory for finite-frequency fluctuational conductivity, where renormalization is introduced in order to account for the

Microwave fluctuational conductivity in YBa2Cu3O7-δ

T\rightarrow T_{c}

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

regime, and a spectral cutoff is inserted in order to discard high-momentum modes. The data are in excellent agreement with the modified theory, when formulated for three-dimensional, anisotropic superconductors, in the whole experimentally accessible temperature range, and for both frequencies.We present measurements of the microwave complex conductivity at 23.9 and 48.2 GHz in YBa 2 Cu 3 O 7-δ films, in the fluctuational region above T c . With increasing temperature the fluctuational excess conductivity drops much faster than the well-known calculations within the time-dependent Ginzburg-Landau theory [H. Schmidt, Z. Phys. 216. 336 (1968)]. Approaching the transition temperature. slowing down of the fluctuations is observed. We discuss the results in terms of a modified Gaussian theory for finite-frequency fluctuational conductivity, where renormalization is introduced in order to account for the T → T c regime, and a spectral cutoff is inserted in order to discard high-momentum modes. The data are in excellent agreement with the modified theory, when formulated for three-dimensional, anisotropic superconductors, in the whole experimentally accessible temperature range, and for both frequencies.

Seeded directional growth of superconducting bulk Y123

We report measurements of the microwave resistivity in YBa2Cu3O7−δ (YBCO), in the presence of an applied magnetic field. Measurements are performed as a function of frequency, over a continuum spectrum between 6 and 20 GHz, by means of a Corbino disc geometry. These data allow for a direct identification of different dynamical regimes in the dissipation of YBCO in the presence of an applied magnetic field. While at high temperatures a frequency independent resistivity is observed, at lower temperatures we find a marked frequency dependence. The line in the (H,T) plane at which this change in the dynamical regime is observed is clearly identified and discussed in terms of vortex motion and fluctuational resistivity.

DC transport properties of epitaxial superconducting SmBa2Cu3O7−x films

The rf and microwave losses of devices based on YBa 2 Cu 3 O 7-δ superconducting films are dominated by extrinsic mechanisms that can be controlled by a careful device fabrication and design. Often the granular nature of the films shows up in the measurements of the residual surface resistance. We have measured the surface impedance of a large number of highly epitaxial, dc sputtered YBa 2 Cu 3 O 7-δ films patterned by argon ion milling as meanderline microstrip resonator (T c >90K and ΔT c <1K). The plot of R s vs. H rf shows that the excess surface resistance is determined by Josephson vortices propagating in the grain boundaries, in good agreement with the theoretical prediction of Halbritter. Further, the temperature and frequency dependence of Z, is consistent with this hypothesis.

Mixed state microwave resistivity of cuprate superconductors

We present measurements of the microwave complex conductivity at 23.9 and 48.2 GHz in YBa2Cu3O