Cause of T<SUB>c</SUB> depression in Sr substituted YBaSrCu<SUB>3</SUB>O<SUB>7-y</SUB> examined by isotope studies

We have measured oxygen isotope shifts in YBa2-xSrxCu3O7-y. With increasing Sr substitution, Tc decreases monotonically, while the oxygen isotope exponent &#945; 0 remains very small. This indicates that the hole concentration in the CuO2 planes is still optimal, since it is known that high temperature superconductors with non-optimum hole concentration have reduced Tc, but large isotope shifts. To confirm this, we carried out hole doping experiments on YBaSrCu3O7-y using high pressure O2 annealing, and also by Ca substitution for Y. In both cases, with the increased holes (overdoping), Tc further decreased as expected. Thus, the significant decrease in Tc with Sr substitution is not caused by reduced charge transfer into the CuO2 sheets, but rather is due to decreased ion size, polarizability and/or changes in the phonon spectrum