Electrons, holes, and spin in Nd2-xCexCuO4-delta

The spin nature of the states at the top of the valence band in Nd2CuO4 and Nd1.85Ce0.15CuO4-delta has been investigated with spin polarized resonant photoemission. A clear Zhang-Rice singlet state is observed at the top of the valence band in the undoped compound showing that the parent compound of this electron doped superconductor is similar to the parent compounds of hole doped cuprate superconductors. Upon doping, the stability of the singlet is destroyed and the optimum doped compound Nd1.85Ce0.15CuO4-delta does not display any well-separated singlet state. These results indicate that hole contribution to the superconductivity in this system is not important in contrast to what has previously been suggested on the bases of transport measurements.