Physics of Modified (La, Sr) sub 2 CuO sub 4 and Ba sub 2 YCu sub 3 O sub 7 Superconductors

Results of studies will be presented in which (La, Sr) sub 2 CuO sub 4 and Ba sub 2 YCu sub 3 O sub 7 based compounds are modified either by oxygen isotope exchange, metal-ion substitution or oxygen stochiometry changes. In an extensive investigation of the oxide superconductors with transition temperatures between 10K and 90K, the dependence of T sub c on the oxygen mass (isotope effect) is found to decrease with increasing T sub c.