THEORY OF THE ISOTOPE EFFECT AND SUPERCONDUCTING TRANSITION TEMPERATURE IN HIGH-Tc OXIDES

Analytical expressions for the superconducting transition temperature T-c and the isotope coefficient alpha have been obtained for the case where the Fermi level is close to the van Hove singularity. In this approach, we consider two interactions, the first related to the phonons and the second relevant to the magnetic excitations. Our result shows that the isotope coefficient alpha decreases with the superconducting transition temperature T-c in qualitative agreement with experimental data.