Magnetic-field irreversibility in superconductive Cu- m212 ( m=1, 2)

Abstract

The magnetic-field irreversibility ( H irr) characteristics of the CuBa 2YCu 2O 7− δ (Cu-1212 or so-called “123”) and Cu 2Ba 2YCu 2O 8− δ (Cu-2212 or so-called “124”) phases are studied with respect to the hole-doping level and the isovalent-substitution effect. For both the phases the H irr characteristics are enhanced with increasing overall hole-doping level. When comparing fully oxygenated samples, the H irr line of the Cu-2212 phase is located at magnetic fields lower than that of the Cu-1212 phase. The Sr(II)-for-Ba(II) substitution is not effective in enhancing the H irr characteristics of the Cu- m212 phases. It rather lowers the H irr line in the case of the Cu-1212 phase. On the other hand, the Yb(III)-for-Y(III) substitution enhances the H irr characteristics of the Cu-1212 phase. Both types of isovalent substitution, i.e. Sr-for-Ba and Yb-for-Y, result in contraction of the lattice dimensions, but work oppositely in terms of the effect on the H irr characteristics. Therefore it is considered that the control over the H irr characteristics is not simply achieved by the lattice-dimension change but rather by the change in the homogeneity of the hole-density distribution over the crystal.