On the theory of isotopic effects in high-temperature superconducting compounds

Isotopic effects in high-$T_{c}$ systems are considered. For strong spin-phonon coupling, the calculated isotopic effect index is substantially lower than the classical index for low-$T_{c}$: superconductors. The isotopic effect indices of high-$T_{c}$: systems such as $\rm{La}_{2-x}\rm{Sr}_x\rm{CuO}_4$ and $\rm{YBa}_2 \rm{Cu}_3 \rm{O}_{7-y}$ are calculated from the dependence of the inverse exchange correlation length on the ion mass. This dependence can be obtained in the limit of maximum amplification of effective electron-phonon coupling by spin fluctuations of exchange nature. The theoretical estimates are compared with the experimental data.