Cause of T_c depression in Sr substituted YBaSrCu₃O_7-y examined by isotope studies

Abstract

We have measured oxygen isotope shifts in YBa_2-xSr_xCu₃O_7-y. With increasing Sr substitution, T_c decreases monotonically, while the oxygen isotope exponent α 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 T_c, but large isotope shifts. To confirm this, we carried out hole doping experiments on YBaSrCu₃O_7-y using high pressure O₂ annealing, and also by Ca substitution for Y. In both cases, with the increased holes (overdoping), T_c further decreased as expected. Thus, the significant decrease in T_c 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.