Isotope Effect in Superconducting YBa2Cu3O7-δ System

An oxygen isotope effect is observed by a measurement of the resistivity when 16O is substituted for 18O in the YBa2Cu3O7-δ compound. The superconducting transition temperature Tc is lowered by 0.4 K upon substitution. The results suggest that the phonon related to oxygen in the two-dimensional CuO2 plane contributes to the mechanism of high-Tc superconductivity. When 63Cu is substituted for 65Cu, no appreciable isotope effect is observed.