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Abstract:

The 'infinite-layer' compound1-3 AcuO2 (where A stands for cations such as strontium or calcium), has the simplest structure of all superconducting copper oxides, with only bare cations separating the CuO2 planes. Accordingly, an understanding of the doping mechanism(s) that lead to superconductivity in this compound may facilitate the elucidation of the same phenomenon in the other copper oxide superconductors. Recently, Azuma and co-workers2,4 observed planar defects in an infinite-layer phase synthesized at high oxygen pressure, and proposed that the defects are A-cation deficient, and lead to superconductivity (with transition temperature T c≈ 100-110 K) in this compound. Here, based on quantitative X-ray and high-resolution electron-microscopic analysis of the planar defects in (Sr, Ca)CuO2, we propose that the defects consist of a corrugated Sr-O layer substituted for a CuO2 layer, with the incorporation of apical oxygen atoms (which are absent in the parent structure) at roughly half the available sites in the neighbouring Sr layers. This is equivalent to an insertion of a Sr3O2 ± x block in an otherwise infinite-layer sequence. The variable oxygen stoichiometry of our defect model can account for the occurrence of p-type superconductivity (following high-pressure oxygenation), n-type superconductivity (high-pressure reduction) or lack of superconductivity (high-pressure neutral-atmosphere annealing) in this system, depending on the synthesis conditions4. © 1994 Nature Publishing Group.

Abstract:

The NQR spectra of Cu in several ErBa2Cu3O6+x powder samples (0.1≤x≤0.9), characterized by particularly careful annealing, shows a limited number of frequency peaks. This fact is taken as evidence of a high degree of ordering of O(4) in the chain Cu(1) layer of the perovskite (formation of long full chains separated by empty chains as dictated by the stoichiometry), leading to a few regular patterns in the local configurations. © 1994.

Abstract:

The phase diagram of the title system is remarkably rich, exhibiting regions of long-range antiferromagnetic order, short-range (spin-glass-like) order, high temperature superconductivity, or none of these. Here, the phase diagram is reviewed, with an emphasis on our recent studies using 139La NQR, powder and single crystal neutron diffraction, and magnetization measurements for 0≤χ≤0.15 and 0≤δ≤0.1. © 1994.

Abstract:

Herein, we review the magnetic, superconducting and structural phase diagrams of the title systems, with an emphasis on our recent results form magnetic and structural neutron diffraction, magnetic susceptibility and system, and indicate the occurrence of frustrated phase separation on a nanoscopic length scale in the La2-xSrxCuO4 system with O < x ≲ 0.05. ¹³⁹La nuclear quadrupole resonance measurements. The results clarify the miscibility gap in the La2CuO4+δ. © 1994.

Abstract:

We have extended the calcium solid-solubility limit in La2-xCaxCuO4 to x = 0.2 using synthesis at high oxygen pressure. The superconducting transition temperature increases with doping to 34 K at x = 0.15 and is suppressed for x > 0.15, similar to the Sr-substituted material. At a fixed doping level, the optimum superconducting properties (the highest Tc) are found for flat and square CuO2 planes. © 1994.