Isotope Effect in the High- T c Superconductors Ba 2 Y Cu 3 O 7 and Ba 2 Eu Cu 3 O 7

We have studied the effect of isotopic substitution on the superconducting ${T}_{c}$ in the 90-K superconductors ${\mathrm{Ba}}_{2}$Y${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and ${\mathrm{Ba}}_{2}$Eu${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ by replacing $^{16}\mathrm{O}$ with the heavier isotope $^{18}\mathrm{O}$. Samples with approximately 75% of the $^{16}\mathrm{O}$ replaced by $^{18}\mathrm{O}$ were prepared by gas-phase ion exchange. In these samples the phonon frequencies, measured by Raman spectroscopy, are reduced by the expected \ensuremath{\sim}4%. The transition temperatures, however, are found to change by less than 0.2%. This change in ${T}_{c}$ is much less than that expected for strongly coupled phonon-mediated superconductivity.