THE EFFECT OF CERIUM DOPING AND OXYGEN TREATMENT OF ND2-XCEXCUO4 - HALL-EFFECT MEASUREMENTS

We have measured the temperature dependent Hall coefficient R(H) and its variation with oxygen treatment and cerium concentration x in polycrystalline Nd2-xCexCuOy for x = 0.15, 0.17 and 0.18 (y almost-equal-to 4). We find that the heat treatment of Nd2-xCexCuOy in a reducing atmosphere delocalizes the doped charge carriers, transforming Nd2-xCexCuOy from a semiconductor into a superconducting metal. The apparent superconducting fraction in strongly reduced Nd2-xCexCuOy is a maximum for a particular cerium doping concentration x(optimal). For x = x(optimal), we find a negative Hall coeffient that saturates at low temperature, while for higher cerium doping concentrations, we find a positive Hall coefficient that continues to increase with decreasing temperature. The temperature dependence of the Hall coefficient at higher than optimal x appears to be dominated by the non-superconducting fraction. Contrary to some recent suggestions, the temperature dependence of the Hall coefficient is not evidence against the existence of n-type superconductivity in the "electron-doped" copper oxides.