Isotope effect in the high-Tc superconductors Ba2YCu3O7 and Ba2EuCu3O7

We have studied the effect of isotopic substitution on the superconducting Tc in the 90-K superconductors Ba2YCu3O7 and Ba2EuCu3O7 by replacing O16 with the heavier isotope O18. Samples with approximately 75% of the O16 replaced by O18 were prepared by gas-phase ion exchange. In these samples the phonon frequencies, measured by Raman spectroscopy, are reduced by the expected 4%. The transition temperatures, however, are found to change by less than 0.2%. This change in Tc is much less than that expected for strongly coupled phonon-mediated superconductivity.