A small decrease in Tc of 0.3 to 0.5 K is observed when as much as 90% of the '^O in YBa2Cu307 is substituted with ^^O. This result is consistent with our observation that there is an oxygen isotope effect in Lai.85Sro.i5Cu04, but in contrast with previous reports that there is no isotope effect for YBa2Cu307. This new result suggests that phonons play an important role in the electron-pairing mechanism in YBa2Cu307. PACS numbers: 74.70.Ya Since the discovery of superconductivity above 90 K in YBa2Cu307,^''^ it has been debated whether the superconductivity in this material is phonon mediated as expected from conventional BCS theory,^ or whether a different mechanism is involved. Support for a nonphonon mechanism was provided when our group, ^ Batlogg et al.,^ reported that the superconducting transition temperature, Tc, does not change significantly when ^^O is replaced with ^^O in YBaiCuaOy. However, more recently we have observed an oxygen isotope effect in samples of Lai.85Sro.i5Cu04,^ Lai.85Cao.i5Cu04,^ and BaPbo.75Bio.2503.^ The shift in Tc to lower temperatures observed for Lai.85Sro.i5Cu04 when 75% of the ^^O was replaced with ^^O varied between 0.3 and 1.0 K depending on the sample. This shift is smaller than what is predicted by conventional BCS theory. The small decrease in Tc for Lai.85Sro.i5Cu04 prompted us to reexamine the effect of ^^O substitution on the transition temperature of YBa2Cu307. In this paper, we report the observation of a drop in Tc to lower temperatures by 0.3 to 0.5 K in three different samples of YBaiCuaOy when as much as 90% of the ^^O is replaced by ^^O. This shift in the onset of the diamagnetic transition due to ^^O substitution was reversible by resubstitution of ^^O for ^^O.According to the conventional BCS treatment,^ Tc should vary as M ~", where M is the oxygen mass. This assumes that the frequency of the important phonon mode depends only on the mass of oxygen. For a value of a =0.5, a shift in Tc to lower temperatures by approximately 5 K is predicted for a 90-K superconductor (on the assumption of complete substitution of ^^O for '^O). We note that this is the maximum shift expected and that because of the complexity of the structure, i.e., both chains and layers of Cu-O, this prediction is at best tenuous. Although the 0.3-to 0.5-K shift in Tc for YBaiCusOv is smaller than the predicted shift, the presence of an oxygen isotope effect demonstrates that phonons play an important role in the electron-pairing mechanism in this material.Three samples of YBa2Cu307, referred to below as samples I, II, and III, were synthesized via a nitrate route which has been described previously.^ This route involved our dissolving stoichiometric ratios of Y2O3, BaC03, and CuO in nitric acid and evaporating the solution to dryness. The resulting powders were decomposed near 800 °C in AI2O3 crucibles. To ensure homogeneous products, these powders were then ground and fired in air for 48 h at temperatures ranging from 750 to 950 °C, Sample I 11 11-18^ III III-
