Kondo effect and superconductivity in ${\mathrm{Nd}}_{2\mathrm{-}\mathrm{x}}$${\mathrm{Ce}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ compounds

Samples of ${\mathrm{Nd}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Ce}}_{\mathrm{x}}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ (0≤x≤0.2) were prepared by annealing both in air and under vacuum (${10}^{\mathrm{-}5}$ Torr). Powder x-ray-diffraction patterns indicated that all samples consisted of a single-phase ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_4$-type tetragonal structure with crystal symmetry I4/mmm. Electrical resistivity measurements showed that all Ce-doped ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ samples annealed in air and those annealed in vacuum with x<0.17 demonstrate a resistivity minimum (Kondo effect) resulting from the combination of magnetic and phonon scattering. Magnetic susceptibility and electrical resistivity measurements reflect the influence of crystal-field effects below 55 K. The linear temperature-dependent term of the resistivity typical of metallic behavior in the high-$T_c$ oxides exists across the whole range of Ce concentration, with the exception of the host compound ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$, which is an insulator. This indicates that a model of metal-insulator transition is not appropriate in describing the occurrence of superconductivity in ${\mathrm{Nd}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Ce}}_{\mathrm{x}}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$.