MICROSCOPIC MODEL FOR THE ISOTOPE EFFECT IN THE HIGH-T-C OXIDES

An unconventional microscopic mechanism relating T-c and the isotope s ubstitution for the doped superconductors such as the high-T-c oxides is proposed. Strong nonadiabaticity, when it is impossible, strictly s peaking, to separate fully the nuclear and electronic degrees of freed om, leads to a peculiar dependence of the carrier concentration n on t he ionic mass M. This case corresponds, for example, to the isotopic s ubstitution of the axial oxygen in YBa2Cu3O7-x. Because of the depende nce of T-c on n, this leads to the dependence of T-c on M, that is to the isotope effect. The minimum value of the isotope coefficient corre sponds to T-c=T-c(max).