A superconducting copper oxide compound with electrons as the charge carriers

Abstract

Since the discovery of high-temperature superconductivity in cop-per oxide compounds¹, there has been much effort to understand what is required for a compound to have a high transition tem-perature (T_c). Up to now, all of the families of high-T_c copper oxide superconductors have contained two-dimensional sheets of Cu–O pyramids or octahedra, and the carriers of the superconduct-ing current have been electron vacancies, or 'holes'. By contrast, we report here the discovery of a family of superconducting copper oxides in which the carriers are electrons. The new superconductors are Ce⁴⁺-doped compounds, with the formula Ln_2–xCe_xCuO_4–y, where Ln stands for the lanthanides Pr, Nd or Sm. The compounds have the Nd₂CuO₄ (T′-phase) structure², which is composed of sheets of Cu–O squares (see Fig. la). This structure has no apical oxygen atoms, in contrast to the T-phase structure with Cu–O octahedra (Fig. 1b), as observed in La_2–xSr_xCuO₄ (refs 1, 2), and the T^*-phase structure with Cu–O pyramids (Fig. 1c), as in Nd_2–x–z Ce_x Sr_z CuO₄ (ref. 3 and Y. T. et al., manuscript in preparation).