[en] It is shown that the value of the 1/ψ critical exponent measured on sintered high-T_c granular superconductors (La_1.85Sr_0.15CuO_4-δ and YBa₂Cu₃O_7-δ) can be understood within the framework of a fractal model. Using the fractal cluster model, it is shown that under the hypothesis of ν = 1.33, the fractal dimension of these systems can be always straightforwardly derived. The comparison between the values of the SuG critical exponents and those of the spin glasses (SG) suggests the inclusion of both the ensembles of critical systems in a unique universality class whose definition differs from the usual one. (author)

[en] Some aspects of magnetism in high T_c superconductors studied during the last year are critically reviwed. The ''hierarchy'' of the critical fields, the consequences of the crystal anisotropy on flux pinning and critical currents, the vortex creep, the Superconducting Glass state and the low field measurement are discussed and relevant results and open problems are pointed out

[en] We show that the zero field cooling (ZFC) M/H curves of both the YBaCuO and the LaSrCuO granular superconductor glasses (SuG) are subjected to scaling when plotted against the reduced variable t/H^1/ψ. The breaking of the scaling for too weak or too strong magnetic fields is discussed and justified by the introduction of a phenomenological fractal picture, describing the behaviour of the disordered intergranular junction network

[en] Published in summary form only

[en] In this paper an analysis is presented of the ground-state of a planar array of superconducting grains. In particular, the dependences on external magnetic field, positional disorder and link disorder are examined. Strong dependence on the history is clearly seen as the field is increased beyond a certain critical value, and butterfly-like diagrams are shown for the magnetic moment versus the field

[en] We have carried out torque measurements on sintered samples of oxide superconductors. The results show different regimes: we observe a fairly abrupt crossover from weak torque to strong torque, starting at a field of the order of 150 gauss in La_1.85Sr_0.15CuO₄. The torque signals are rigid- or viscous-like depending on the filed and on the history, and there are associated strong creep relaxation effects. We suggest this behaviour is related to the granular nature of the superconductivity, and that the crossover field represents onset of penetration of flux lines into the superconducting grains. (author). 6 refs, 3 figs

[en] We have submitted to a close scrutiny the genesis of the glassy (or better the fully incommensurate) state in arrays of superconducting junctions at zero temperature. From our numerical simulations, using the XY hamiltonian, it comes out that frustration by itself is a necessary but not a sufficient condition. The other needed element is the disorder. However only the positional disorder is effective in the formation of a truly incommensurate state. The oscillation damping in the ground state energy and in the magnetic moment, as a function of the frustration, is not linear. The critical frustration (characteristic of the short range ''order-disorder'' crossover) decreases almost linearly with the root mean square of plaquette area, but not in a ''universal'' manner. (orig.)

[en] The preparation procedure used to obtain superconducting thin films by radio frequency magnetron sputtering of a single mosaic target is described in detail. The single mosaic target is composed of (Y-Er), BaF2, and Cu

[en] We compare the superconductor glass phase diagram in the (H,T)-plane, deduced from computer simulations performed on a 2D system of superconducting clusters, with experiments performed on a LaSrCuO granular perovskite. It is shown that real high-T_c granular systems in low fields (≤ 10G) are 3D and behave in a much more complicated manner than expected from an idealized model

[en] Magnetic critical current densities of an YBaCuO single crystal have been investigated using both magnetometry and torque. For currents along the C axis, J_c is estimated to be 10³ and 10⁵ A/cm² respectively from the two techniques, compared to 10⁵ and 10⁶ A/cm² for currents in the AB plane. Differences between the two estimates are ascribed to the intrinsic inhomogeneity of the flux pinning