The Isotope Effect in the High T sub c Superconductors Ba sub 2 Ln Cu sub 3 O sub 7.

We have studied the effect of isotopic substitution on the superconducting T sub c in the 90K superconductors Ba sub 2 YCu sub 3 and Ba sub 2 EuCu sub 3 O sub 7 by replacing O sup 16 with the heavier isotope O sup 18. Samples with approximately 75% of the oxygen O sup 16 replaced by O sup 18 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 sub c is much less than than expected for strongly coupled phonon mediated superconductivity.