Effects of Zn and Mg in Cu sites of YBa/sub 2/Cu/sub 3/O/sub 7-[delta]/ single crystals on the resistive transition, fluctuation conductivity, and magnetic irreversibilities

We report on the resistive transition and fluctuation conductivity of an YBa2Cu2.97Zn0.03O7-δ single crystal and on the magnetic irreversibilities of this compound in two different oxygen states and another YBa2Cu3O7-δ single crystal in which (1 at.%) of Cu was substituted by Mg. Our measurements show a very drastic decrease of the superconducting transition temperature and the growth of granularity effects already in this low dopand concentration. In the temperature region immediately above Tc our results of fluctuation conductivity reveal the occurrence of critical and Gaussian regimes. The resistive transition of the YBa2Cu2.97Zn0.03O7-δ single crystal occurs in two steps. First superconductivity sets in within the grains and at somewhat lower temperature long-range superconducting order takes place over the whole sample. Between the zero resistance temperature Tc0(H) and the pairing transition temperature Tc(H) lays the magnetic irreversibility line Tirr(H), which follows the power law predicted by the flux creep theories in most of the high field range. However, in a low field region, dominated by Josephson flux dynamics, the irreversibility line exhibits two different regimes dominated by disorder and frustration and characterized by AT (de Almeida- Thouless) and GT (Gabay-Toulouse) power law behaviors. This Josephson flux dynamics is squeezed into a considerably smaller field region than in other granular YBa2Cu3O7-δ systems. We attribute the origin of the observed stepwise resistive transition and low field magnetic irreversibility regimes mainly to the granularity of the superconducting state, which is induced by the impurities at the Cu sites and increased by shortage of oxygen