NATURE AND AMOUNT OF CARRIERS IN CE-DOPED ND2CUO4 .2. LOW-TEMPERATURETRANSPORT AND XAS CHARACTERIZATION

This paper discusses the electrical (conductivity) and spectroscopic ( XAS) evidence of Ce-doped Nd2CuO4 samples in which the charge carrier density has been fixed to a known value by annealing in well known and well controlled conditions, and then quenching to room temperature. A il the experimental findings presented here are consistent with a band structure where no impurity levels are present in the energy gap. In addition, the XAS results indicate that the unoccupied band of lower e nergy is originated by a Mott-Hubbard splitting, It has been found tha t both Ce-doping and reduction of the oxygen excess delta concur in gi ving rise to the charge carriers density. For low charge carrier densi ty, the conductivity data indicate an Anderson localisation of the cha rge carriers, which is seemingly responsible of the semiconducting-lik e behaviour. As the charge carrier density increases, conductivity bec omes less dependent on temperature, and superconductivity (above 10 K) is found only when the charge carrier density is greater than the val ue of 0.082 electrons per copper atom.