LARGE COPPER ISOTOPE EFFECT IN OXYGEN DEPLETED YBA2CU3OY - IMPORTANCEOF CU-DOMINATED PHONON MODES IN THE PAIRING MECHANISM

We have measured the copper isotope effect in YBa2Cu3Oy. We find a lar ge positive copper-isotope exponent (alpha Cu approximate to 0.37) in the underdoped plateau region (y similar to 6.6). The exponent decreas es to zero (or slightly negative) in the optimally doped region (y sim ilar to 6.94). alpha(Cu) in YBa2Cu3O6.6 is similar to the alpha(O) (ox ygen-isotope exponent) in YBa2-xLaxCu3Oy with the same T-c. These comp arable values of alpha(Cu) and alpha(O) show that phonons which involv e Cu motion as well as the (high-frequency) modes of the oxygen in the CuO2 planes are important in the pairing mechanism.