Large copper isotope effect in oxygen depleted YBa2Cu3Oy: Importance of Cu-dominated phonon modes in the pairing mechanism

We have measured the copper isotope effect in YBa₂Cu₃O_y. We find a large positive copper-isotope exponent (α_Cu~=0.37) in the underdoped plateau region (y~6.6). The exponent decreases to zero (or slightly negative) in the optimally doped region (y~6.94). The α_Cu in YBa₂Cu₃O_6.6 is similar to the α_O (oxygen-isotope exponent) in YBa_2-xLa_xCu₃O_y with the same T_c. These comparable values of α_Cu and α_O show that phonons which involve Cu motion as well as the (high-frequency) modes of the oxygen in the CuO₂ planes are important in the pairing mechanism.