Systematic variation of transport and thermodynamic properties with degree of reduction in ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$

Superconductivity only occurs in ${\mathrm{Nd}}_{1.85}$ ${\mathrm{Ce}}_{0.15}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ after heat treatment in a reducing at- mosphere; yet we find the oxygen loss during reduction to be remarkably small. To better understand the physical role of reduction, we have systematically varied the degree of reduction in a series of ceramic samples by varying the partial pressures of oxygen and argon in the reducing atmosphere, keeping the total pressure constant. We find that small increases in the degree of reduction strongly increase the apparent carrier density and the superconducting Meissner fraction. We also find that there is an optimal degree of reduction to produce a single-phase sample with a zero-resistance superconducting state. These results call into question many previous experiments performed on samples of ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ce}}_{\mathit{x}}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ in which the effect of varying the degree of reduction was not systematically studied.