TRANSITION-TEMPERATURE AND ISOTOPE EFFECT NEAR AN EXTENDED VAN HOVE SINGULARITY

Using Eliashberg theory and a model density for alpha(2)F the transition temperature T-c and the isotope effect a are calculated near an extended van Hove singularity. We show that, at least in the one-particle and the Migdal approximation, even the considered strong van Hove singularity cannot yield large enhancements of T-c and strong reductions of alpha of the kind observed in experiment around optimal doping.