Granular superconductivity in a Ca-dopedYBa2Cu3O7−δsingle crystal: Role of divalent impurities and the effect of applied fields on the grain coupling

We investigate the effect of the divalent Ca atoms at the trivalent Y site of YBa2Cu3O7− on the magnetoconductivity and magnetic irreversibility of single crystals and a polycrystalline sample. The magnetic irreversibility displays the de Almeida–Thouless and Gabay-Toulouse power laws, the signature of a frustrated superconducting grain aggregate for the H c axis as well as for the H ab plane. The resistive transition for measuring current along the ab plane and H c axis is a clear two step process. While the upper temperature step is only weakly affected by applied fields, the lower temperature transition is visibly broadened and shifted down to lower temperatures. We impute the upper temperature step to the pairing transition within grains while the lower temperature step is due to grain coupling and coherence transition. For the J ab and H ab planes, the resistive transition is a one step process and the effect of the applied fields is very weak for H J as well as for H J. We explain these observations in terms of the directional anisotropy of the phase displacements between the superconducting grains and their weakening effect on the grain coupling, caused by the applied field. We also compare our present results with those of YBa2Cu3O7− single crystals doped with the trivalent Pr at the Y site and with divalent elements such as Zn or Mg at the Cu site and Sr at the Ba site and suggest that the valence as well as the local breaking of the orbital symmetry by impurities plays an important role in the induction of superconducting granularity.