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 ∼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.

• Received 24 April 1987

DOI:https://doi.org/10.1103/PhysRevLett.58.2333