Abstract
It is shown that isotope effects observed in the high-Tc cuprates are compatible with unconventional d-wave superconductivity due to a nonphonon mechanism. Because of the high-Tc, a considerable amount of quasi-particle damping exists at Tc, thereby giving rise to pair-breaking effect. The electron-phonon interaction is assumed to contribute only to the pair-breaking effect. The oxygen isotope effects in YBa2Cu3O7 and Bi2Sr2CaCu2O8 are consistently explained by a moderate amount of the phonon contribution to the pair breaking. Using the parameters determined for these compounds, we obtain the isotope exponents comparable with the experimental results for the oxygen and copper isotope exchange in La1.85Sr0.15CuO4. The increase of isotope exponent α with decreasing Tc by impurity-substitution to YBa2Cu3O7 is also explained qualitatively. The oxygen isotope effect on the antiferromagnetic transition temperature in La2CuO4 is shown to be related with the tilting of CuO6 octahedra and have nothing to do with the isotope effect on Tc.
