Modification of electron spectrum and properties of HTSC during doping

Abstract

A mechanism for the influence of doping on the electron spectrum of La_2−xSr_xCuO₄ and Nd_2−xCe_xCuO₄ is proposed. In the framework of simple model taking into account the local nature of HTSC electron properties, the appearance of “midgap states” and the existence of critical concentration values (0.15 and 0.25) are explained. The effect of doping-induced changes of electron structure on transport and superconducting properties of La_2−xSr_xCuO₄ and Nd_2−xCe_xCuO₄ is considered. The dependence of the Hall resistance on doping concentration,x, is discussed. It is shown that in Nd_2−xCe_xCuO₄ the pair level of “negativeU-centers” arises at the top of the valence band only atx=0.15, and the dielectric-metal transition takes place. The superconductivity in Nd_2−xCe_xCuO₄ occurs in the hole-conductive phase.