SHIELDING CURRENTS INFLUENCE ON INTERGRANULAR COUPLING IN HIGH Tc SUPERCONDUCTORS

In this paper we analyze the weakly coupled structure of high Tc superconducting granular samples in terms of a Josephson junctions network. We start from the well known results on superconducting loops closed by Josephson junctions, and show that the energy barrier for entrapping or detrapping flux quanta is essentially determined by the shielding currents crossing the junctions which remove the states generation. This effect corresponds to the absence of degeneration of the thermodynamic potential as a function of different number of flux quanta trapped in the superconducting loops. Following this approach we remark that the superconducting glass model neglects the effects of shielding currents and may be a poor approximation. On the contrary a correct description of superconducting granular systems leads to a model very similar to the usual flux creep model.