Electron and hole doping in Nd-based cuprates with single-layerCuO2sheets: Role of doped Ce ions and 30-K superconductivity

Abstract

We have attempted electron and hole doping in Nd-based cuprates with single-layer sheets of Cu-O squares and pyramids. It was found that partial substitution of ${\mathrm{Nd}}^{3+}$ sites with ${\mathrm{Ce}}^{4+}$ ions introduces mobile electrons into the ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_4$ structure which shows two-dimensional (2D) sheets of Cu-O squares with no apical oxygens. With further Sr doping into ${\mathrm{Nd}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Ce}}_{\mathrm{x}}$ ${\mathrm{CuO}}_4$, the crystalline lattice undergoes a structural transformation into the new hole-conductive compound with 2D sheets of Cu-O pyramids, which is identical with the superconducting phase discovered recently by Akimitsu et al. The hole concentration in ${\mathrm{Nd}}_{2\mathrm{-}\mathrm{x}\mathrm{-}\mathrm{y}}$ ${\mathrm{Ce}}_{\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{y}}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ can be increased by controlling the composition (x,y) of non-copper cations and filling oxygen vacancies (δ) as well, which give rise to superconductivity with $T_c$ up to 30 K.