ANOMALOUS ISOTOPE EFFECT IN HIGH-TEMPERATURE SUPERCONDUCTOR LSCO WITH3D-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 studie d 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 C o substitutions and isovalent Ni and Zn substitutions have been reveal ed. 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 depend ence of the isotope effect on the substituted atom concentration chang es substantially when the degree of strontium doping varies. The exper imental data were theoretically analyzed on the basis of the phonon me chanism of superconductivity and the concept of the existence of a sin gularity in the electron density of states N(E(F)). It is shown that t he influence of paramagnetic impurities on the T-c isotope shift of su perconductors is enhanced when there is a sharp peak in N(E(F)). The d ifference in these effects for a system with the optimum strontium com position and for an overdoped system is also explained.