Anomalous isotope effect in high-temperature superconductor LSCO with 3d-impurities

The influence of transition metal substitutions (Ni, Fe, Co) and Zn at the Cu sites on T-c and the oxygen isotope effect cuo has been studied in the system La(2-y)Sr(y)Cu(1-x)M(x)O(4) with the optimum (y = 0.15) and overdoped (y = 0.20) compositions. Essential distinctions in the behavior of parameter alpha(o) for systems with heterovalent Fe and Co substitutions and isovalent Ni and Zn substitutions have been revealed. For Fe doped compounds with y = 0.15 the isotope effect was found to be very large: alpha(o) approximate to 1.3 at x = 0.011. The dependence of the isotope effect on the substituted atom concentration changes substantially when the degree of strontium doping varies. The experimental data were theoretically analyzed on the basis of the phonon mechanism of superconductivity and the concept of the existence of a singularity in the electron density of states N(E(F)). It is shown that the influence of paramagnetic impurities on the T-c isotope shift of superconductors is enhanced when there is a sharp peak in N(E(F)). The difference in these effects for a system with the optimum strontium composition and for an overdoped system is also explained.