STRUCTURAL AND SUPERCONDUCTING PROPERTIES OF BI2SR2CA(CU1-YCOY)2O8+DELTA SINGLE-CRYSTALS

In Bi2Sr2Ca(Cul-yCoy)2O8+delta single crystals, the Cu-ions have been substituted by Co up to the limit of existence of the Bi-2212 phase, which is at y=0.08. With increasing y, there results an increasing, strong deficiency of earth alkali-atoms compared to the stoichiometric composition. This increasing disorder with increasing y yields a decrease of the lattice constants and a broadening of the Raman-active phonons. The transition temperatures decrease linearly with increasing y with a slope of about 5 K/{Co/(Cu+Co)} at.%, i.e. for every percent Cu substituted by Co T(c) is reduced by 5 K. At the maximum value of y=0.08 we obtain T(c) = 45 K. The energy of the superconducting pp decreases with increasing y, but stronger than expected by the linear relation 2-DELTA parallel-to (0)/k(B)T(c)=5.7+/-0.2 observed for oxygen-deficient, pure Bi2Sr2CaCu2O8+delta (y=0) single crystals.