R. Marcon

The effective surface resistance of superconductor/dielectric/metal structures

We 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, , 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//metal structure to stress the nonintrinsic behaviours and to test the proposed model.

ANISOTROPIC RENORMALIZED FLUCTUATIONS IN THE MICROWAVE RESISTIVITY IN YBA2CU3O7-DELTA

We discuss the excess conductivity above Tc due to renormalized order-parameter fluctuations in YBCO at microwave frequencies. We calculate the effects of the uniaxial anisotropy on the renormalized fluctuations in the Hartree approximation, extending the isotropic theory developed by Dorsey [Phys. Rev. B 43, 7575 (1991)]. Measurements of the real part of the microwave resistivity at 24 and 48 GHz and of the dc resistivity are performed on different YBCO films. The onset of the superconducting transition and the deviation from the linear temperature behavior above Tc can be fully accounted for by the extended theory. According to the theoretical calculation here presented, a departure from gaussian toward renormalized fluctuations is observed. Very consistent values of the fundamental parameters (critical temperature, coherence lenghts, penetration depth) of the superconducting state are obtained.

Magnetic 3D-2D crossover in Bi2Sr2CaCu2O8+x

Abstract The angular and temperature dependence of the critical field H ∗ , defined as the onset magnetic field for the DC resistive dissipation, is used to test the 3D-2D crossover in Bi 2 Sr 2 CaCu 2 O 8+ x . We show that in a narrow temperature range a magnetically induced dimensional crossover is present. Information about the layer coupling energy is obtained.

Effective surface impedance of Y Ba2Cu3O7−δ films on silicon substrates

We study the effect of the substrate on the effective surface impedance of a Y Ba2Cu3O7?? thin film grown over a silicon substrate. The microwave measurements, performed with a resonant cavity technique at two distinct frequencies (24 and 48?GHz), show anomalies both in the temperature and in the frequency dependences. By properly taking into account a relevant complex substrate contribution, with an important role played by the finite thickness effect, the whole set of data finds a satisfactory explanation in terms of standard impedance transformations. Using independent measurements at a single temperature, it is possible to extract the temperature dependence of the Y Ba2Cu3O7?? bulk surface impedance, which shows the expected frequency and temperature behaviours.

High Frequency Resistivity in YBa2Cu3O7-δ: Effects of Anisotropic Renormalized Fluctuations

We discuss the excess conductivity above Tc due to renormalized order-parameter fluctuation in YBCO at microwave frequencies. We show that a phenomenological cut-off in the fluctuative spectrum can extend the results of the anisotropic theory of renormalized fluctuations up to the high temperature region. Measurements of the real part of the microwave resistivity at 24 GHz are performed on two YBCO films. The onset of the superconducting transition and the deviation from the linear temperature behavior can be fully accounted for by the extended theory. Very consistent values of the fundamental parameters (critical temperature, coherence lenghts, penetration depth) of the superconducting state are obtained.

Flux-flow dissipation in GdBa2Cu3O7 at microwave frequencies

Abstract We report measurements of the magnetic-field-dependent microwave dissipation in high-purity GdBa2Cu3O7-δ 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 ⪆ 0.02 T, and to weak-links at low fields (H 0.02 T) has to be ascribed to the free motion of flux line

Anisotropy in Bi2Sr2CaCu2O8+x films at microwave frequencies : enhancement of dissipation in parallel magnetic field

Magneto-resistive measurements at microwave frequencies have been carried out in c-oriented Bi2Sr2CaCu2O8+x films. When the angle between the magnetic-field direction and the (a, b)-plane is varied a large anisotropy is observed in the microwave absorption. In the range 4° < ≤ 90°, the dissipation scales as H sin , but below 4° an anomalous enhancement of dissipation is present. The behaviour of the enhancement is measured as a function of the temperature in the range (70 ÷ 85) K.

Fluxon Motion and Critical Field Hc2 in Sintered YBa2Cu3O7

We report the results of microwave absorption measurements in a magnetic field in sintered YBa2Cu3O7. The data are interpreted in terms of intragrain fluxon motion and the behaviour of Hc2 near the transition temperature is obtained. The critical temperature of these effects has been found close to the value of the transition temperature measured in a single crystal (89 K) well below the d.c. transition temperature of the sample (91.5 K).

Response of a YBa2Cu3O7-x Superconducting System to a Microwave Field with and without an External Magnetic Field

The behaviour of a high-Tc superconductor YBa2Cu3O7-x subjected to a microwave e.m. field is presented. The experimental results, regarding the Q-factor of a microwave cavity (of which the sample is a part) as a function of temperature and external magnetic, show a sharp transition at 91 K and the presence of other superconducting phases at 60 K and 35 K.

Preparation and characterization of textured thick films of the 2212 phase in the BSCCO and BPSCCO systems

Abstract The preparation of textured thick films of the 2212 phase in the BSCCO and BPSCCO systems by a partial melting process of powders deposited on Ag substrates by drying alcoholic suspensions is reported. The quality of the films in the BPSCCO system can be improved as far as microstructure and electrical properties is concerned, by starting from non-stoichiometric compositions, in particular by adding PbO to 2212 BSCCO powders. The influence of the PbO addition on structural, electrical and magnetic properties is discussed. Very high critical current densities (5 × 10 6 −9 × 10 6 A cm −2 at 10 K and for H = 5 kOe ), comparable to or quite greater than those reported for single crystals, can be obtained by this new preparation method.