MODIFICATION OF ELECTRON SPECTRUM AND PROPERTIES OF HTSC DURING DOPING

A mechanism for the influence of doping on the electron spectrum of La 2-xSrxCuO4 and Nd2-xCexCuO4 is proposed. In the framework of simple mo del taking into account the local nature of HTSC electron-properties, the appearance of ''midgap states'' and the existence of critical conc entration values (0.15 and 0.25) are explained. The effect of doping-i nduced changes of electron structure on transport and superconducting properties of La2-xSrxCuO4 and Nd2-xCexCuO4 is considered. The depende nce of the Hall resistance on doping concentration, x, is discussed. I t is shown that in Nd2-xCexCuO4 the pair level of ''negative U-centers '' arises at the top of the valence band only at x = 0.15, and the die lectric-metal transition takes place. The superconductivity in Nd2-xCe xCuO4 occurs in the hole-conductive phase.