ISOTOPE EFFECT FOR VERY LARGE COULOMB PSEUDOPOTENTIAL

In view of the existence of superconductivity in highly correlated systems with reduced screening, we have studied the variation of the critical temperature Tc, within conventional Eliashberg theory, when the Coulomb repulsion pseudopotential * is very large. We find that small and even negative values of the total isotope effect can be achieved for Tcf100 K with a mass renormalization 6 if *(E) is 0.5 and, at the same time, the coupling is exclusively to high-energy phonons with E90 meV. © 1990 The American Physical Society.