Thesis (M.Sc.)--Chulalongkorn University, 1992

The oxygen isotope effect in studied in the high-Tc superconductors. It is found that the isotope effect exponent α is almost zero. This nearly absent oxygen isotope effect seems to suggest that the conventional electron-phonon interaction cannot play a key role in high-Tc superconductivity. In this thesis, we introduce a double-square well potential for the pairing interaction and include a logarithmic van Hove singularity in the density of states within the framework of the BCS theory. The isotope effect exponent α is modified. At higher critical temperatures α goes to zero, and the isotope effect vanishes. We find a strong correlation between the transition temperature and the electron-phonon coupling interaction. At highTc where the oxygen isotope effect is nearly absent, the electron-phonon coupling interaction is found to be small, in contrast to being quite large at low-Tc. Our results suggest that the phonon coupling mechanism still plays a role in the high-Tc superconductors, but it may be supplemented by other purely electronic mechanisms. Together with the known isotope effect exponent for the oxide super-conductors, we can consider these high-Tc superconductors in two groups, firstly the La-Sr Cu-O group and scecondly in Y-Ba-Cu-O, Bi-based and Tl-based groups. Our calculations also gives a distinct range for the values of the electron-phonon coupling interaction supporting the existence of each group.