Isotropy of critical currents with respect to magnetic induction in grained superconductors

The effect of discreteness of the Josephson medium on the electrodynamics of high-Tc superconducting ceramics has been studied both theoretically and experimentally. It is shown that two scenarios are possible for the penetration of magnetic field into grained superconductors. The first is in line with the conventional pattern for a continuous type-II superconductor, considered within the framework of the flux-line-cutting concept sensitive to the anisotropy induced by a constant field. The second, corresponding to a Josephson penetration depth much less than the characteristic grain size, is described by an isotropic local current-voltage characteristic. For these two models, expressions are presented describing the harmonic spectrum of a response to an oscillating field. Experiments with a linearly polarized oscillating field orientated at various angles to a constant field and also with a rotating oscillating field superimposed on a constant field have shown that the isotropic model is preferable for describing high-Tc superconducting ceramics. (author)