SUPERCONDUCTIVITY IN Y1-xCaxBa2Cu3-xMxO7-δ (M=Fe, Ni) SYSTEMS

The relation of crystal structure, oxygen content, normal state resistivity with. temperature and superconducting critical temperature of Y1-xCaxBa2Cu3-xMxO7-δ(M = Fe, Ni) systems have been studied. In comparison with YBa2Cu3-xMxO7-δ (M = Fe,Ni) systems, it is found that the superconducting Tc of Y1-xCaxBa2Cu3-xFexO7-δ system is remarkably higher than that of YBa2Cu3-xFexO7-δ system with the same x value. On the contrary, the Tc of Y1-xCaxBa2Cu3-xNixO7-δ system is lower than that of YBa2Cu3-xNixO7-δ system. These results show that the variation of the carrier concentration (nH) due to Ca and Fe substitutions has been compensated and the sharp decreases of nH and Tc are prevented when Ca and Fe simultaneously substitute for Y and Cu(I). But in the case of Ca and Ni simultaneous substitutions, the compensation of the carrier concentration is not important and the correlation of Ca and Ni substitutional effects is weak. We suggest that the carrier concentration is an essential factor to determine Tc when the element substitution occurs out of the CuO planes, such as in Y1-xCaxBa2Cu3-xFexO7-δ system, but the pair-breaking effect is more important when Ni2+ ions substitute mainly into Cu(II) sites in Y1-xCaxBa2Cu3-xNixO7-δ system and Tc is directly suppressed by the pair-breaking effect.