ON THE ISOTOPE EFFECT IN THE E-L MODEL OF HIGH-T-C LAYERED CUPRATES

The change of the isotope effect is studied in the framework of the mo del based on the assumptions of the dominant role of extended saddle-p oint singularities in the electron spectrum and a long-range phonon-me diated interaction between electrons (E-L model). The idea is that if the distance between the Fermi energy and the saddle point is less tha n the Debye energy, this distance defines the effective energy scale, and hence there is no isotope effect, whereas in the opposite case the Debye energy defines the cut off, and the usual isotope effect is res tored. The cross-over between these regimes is studied, A comparison w ith existing experimental data shows that the concept reflects correct ly the observed trends.