Transport properties of ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4+\mathrm{\delta }}$ crystals before and after reduction

We report in-plane resistivity and Hall-effect studies on the same ${\mathrm{Nd}}_{1.85}$ ${\mathrm{Ce}}_{0.15}$ ${\mathrm{CuO}}_{4+\mathrm{\delta }}$ crystals before and after reduction. A dramatic change in both magnitude and temperature dependence of ρ and ${\mathit{R}}_{\mathit{H}}$ has been observed. In particular, the magntiude of ${\mathit{R}}_{\mathit{H}}$ at T≥100 K drops about a factor of 2–3 after the reducing process but its T dependence is not affected, suggesting a large increase of mobile-carrier density. These observations are consistent with the proposal that excess oxygen exists in a prepared sample, which may cause localization of the electrons provided by Ce doping. The anomalous T dependence of ${\mathit{R}}_{\mathit{H}}$ in this system is also discussed within several proposed models.