Structural and Superconducting Properties of Bi sub 2 Sr sub 2 Ca(Cu sub 1--y Co sub y ) sub 2 O sub 8+ delta Single Crystals

In Bi sub 2 Sr sub 2 Ca(Cu sub 1--y Co sub y ) sub 2 O sub 8+ delta single crystals, the copper-ions have been substituted by cobalt up to the limit of existence of the bismuth-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 approx 5K/{Co/(Cu + Co)} at.%, i.e. for every percent Cu substituted by cobalt T sub c is reduced by 5K. At the maximum value of y = 0.08 T sub c = 45K is obtained. The energy of the superconducting gap decreases with increasing y, but stronger than expected by the linear relation 2 Delta sub || (0)/k sub B T sub c = 5.7 plus/minus 0.2 observed for oxygen-deficient, pure Bi sub 2 Sr sub 2 CaCu sub 2 O sub 8+ delta (y = 0) single crystals.