VARIATION OF CARRIER CONCENTRATION IN ND1.85CE0.15CUO4-DELTA BY REDUCTION

We present the first Hall effect measurements of the dependence of the effective carrier density on the degree of reduction for the n-type superconductor Nd1.85Ce0.15CuO4-delta. We find that the apparent carrier density monotonically increases for samples heat treated in increasingly reducing atmospheres. We suggest that the small number of oxygen atoms removed by reduction come from key sites which when occupied, strongly suppress the number of extended states. We also find that under an optimal non-zero partial pressure of oxygen during reduction, a fully superconducting zero-resistance state can be produced. Non optimal reduction leads to reduced superconducting onset temperatures and/or residual resistances.