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 (Tc). Up to now, all of the families of high-Tc 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 Ln2–xCexCuO4–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 La2–xSrxCuO4 (refs 1, 2), and the T*-phase structure with Cu–O pyramids (Fig. 1c), as in Nd2–x–z Cex Srz CuO4 (ref. 3 and Y. T. et al., manuscript in preparation).

A superconducting copper oxide compound with electrons as the charge carriers

Although the vast majority of high-$T_c$ cuprate superconductors are hole-doped, a small family of electron-doped compounds exists. Under investigated until recently, there has been tremendous recent progress in their characterization. A consistent view is being reached on a number of formerly contentious issues, such as their order parameter symmetry, phase diagram, and normal state electronic structure. Many other aspects have been revealed exhibiting both their similarities and differences with the hole-doped compounds. This review summarizes the current experimental status of these materials, with a goal to providing a snapshot of our current understanding of electron-doped cuprates. When possible we put our results in the context of the hole-doped compounds. We attempt to synthesize this information into a consistent view on a number of topics important to both this material class as well as the overall cuprate phenomenology including the phase diagram, the superconducting order parameter symmetry, phase separation, pseudogap effects, the role of competing orders, the spin-density wave mean-field description of the normal state, and electron-phonon coupling.

Progress and perspectives on electron-doped cuprates

We examine the effect on the superconducting transition temperature ${(T}_{\mathrm{c}})$ of chemical inhomogeneities in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CuO}}_{6+\ensuremath{\delta}}$ and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ single crystals. Cation disorder at the Sr crystallographic site is inherent in these materials and strongly affects the value of ${T}_{\mathrm{c}}.$ Partial substitution of Sr by Ln (Ln = La, Pr, Nd, Sm, Eu, Gd, and Bi) in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{1.6}{\mathrm{Ln}}_{0.4}{\mathrm{CuO}}_{6+\ensuremath{\delta}}$ results in a monotonic decrease of ${T}_{\mathrm{c}}$ with increasing ionic radius mismatch. By minimizing Sr site disorder at the expense of Ca site disorder, we demonstrate that the ${T}_{\mathrm{c}}$ of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ can be increased to 96 K. Based on these results we discuss the effects of chemical inhomogeneity in other bulk high-temperature superconductors.

/pdf/effect-of-chemical-inhomogeneity-in-bismuth-based-copper-2d70j9dsxt.pdf

Effect of chemical inhomogeneity in bismuth-based copper oxide superconductors

The crystal structures of the known copper oxide superconductors are described, with particular emphasis on the manner in which they fall into structural families. The local charge picture, a framework for understanding the influence of chemical composition, stoichiometry, and doping on the electrical properties of complex structures, is also described.

https://science.sciencemag.org/content/sci/247/4943/656.full.pdf

Structural chemistry and the local charge picture of copper oxide superconductors.

Crystal chemistry and superconductivity of the copper oxides

Recently, a new copper oxide superconductor with an onset tem-perature of 28 K was found in the Nd–Ce–Sr–Cu–O system1. The superconducting phase was identified by X-ray diffraction and electron microscopy2, and the metal-atom positions are identical with those in the K2NiF4 or Nd2CuO4 structure3, except for the order of the cations Nd(Ce) and Sr. Here we report the refinement of the crystal structure using neutron powder diffraction. We find that it comprises alternating slabs of the K2NiF4 and Nd2CuO4 structures, and that all the copper atoms are crystallographically identical, with oxygen in fivefold (square-pyramidal) coordination. In having only one kind of copper site, this new compound is structurally simpler than the other CuO5-bearing superconductors such as YBa2Cu3O7, and may therefore be particularly helpful in the effort to understand the origins of high-temperature superconductivity.

Unusually simple crystal structure of an Nd-Ce-Sr-Cu-O superconductor

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 Nd/sup 3+/ sites with Ce/sup 4+/ ions introduces mobile electrons into the Nd/sub 2/CuO/sub 4/ structure which shows two-dimensional (2D) sheets of Cu-O squares with no apical oxygens. With further Sr doping into Nd/sub 2-//sub x/Ce/sub x/CuO/sub 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 Nd/sub 2-//sub x//sub -//sub y/ Ce/sub x/Sr/sub y/CuO/sub 4-//sub delta/ can be increased by controlling the composition (x,y) of non-copper cations and filling oxygen vacancies (delta) as well, which give rise to superconductivity with T/sub c/ up to 30 K.

Electron and hole doping in Nd-based cuprates with single-layer CuO2 sheets: Role of doped Ce ions and 30-K superconductivity.

A new family of layered copper oxide compounds with ordered rare-earth cations has been designed and synthesized. The compounds are partially Sr-substituted /ital L//sub 1/La/sub 1/CuO/sub 4/, where /ital L/(=Sm, Eu, and Gd) and La ions are sequentially ordered thus: (-/ital L/-/ital L/-La-La-). As a result, the Cu-O network of the new compound forms single-layer sheets of CuO/sub 5/ pyramids, which is essentially the same as in the 30-K superconductor (Nd,Ce,Sr)/sub 2/CuO/sub 4/ discovered recently by Akimitsu /ital et/ /ital al/. Extensive oxygen annealing makes the new compounds metallic, indicating their possibility as a new class of high-temperature superconductors with hole-type charge carriers.

New family of layered copper oxide compounds with ordered cations: Prospective high-temperature superconductors

A New Species of the Bathyal Lobster Genus Nephropsis (Crustacea : Decapoda : Nephropidae) from Australian Waters, with a Redescription of N. holthuisi

Hajime Watabe

Papers

Unusually simple crystal structure of an Nd-Ce-Sr-Cu-O superconductor

Electron and hole doping in Nd-based cuprates with single-layer CuO2 sheets: Role of doped Ce ions and 30-K superconductivity.

New family of layered copper oxide compounds with ordered cations: Prospective high-temperature superconductors

A New Species of the Bathyal Lobster Genus Nephropsis (Crustacea : Decapoda : Nephropidae) from Australian Waters, with a Redescription of N. holthuisi

Nephropsis hamadai, a new nephropid lobster (Decapoda: Nephropidae) from bathyal depth in Sagami Nada, central Japan