（Out-of-plane effect on the superconductivity of Sr2-xBaxCuO3+σ with Tc up to 98)
W. B. Gao, Q. Q. Liu, L. X. Yang, Y. Yu, F. Y. Li, C. Q. Jin and S. Uchida
(Comments on this paper)
文章利用高温高压的方法制备出Sr2-xBaxCuO3+σ (0≤x≤0.6)超导样品，其中最高Tc为98K（x=0.6)，接近具有单层铜氧面的铜氧化物超导体的超导温度的最高值。 文章研究讨论铜氧面外Ba离子掺杂引起的顶角氧无序以及晶胞参数（顶角氧到铜氧面的距离、铜氧面内铜氧键长）的变化对超导Tc的影响。顶角氧的无序将降低Tc。随着Ba的掺杂浓度的增加，将进一步增加体系顶角氧的无序，但是Tc却显著上升。该实验结果说明，由于离子半径较大的Ba离子部分取代Sr离子后，顶角氧到铜氧面间的距离增大以及铜氧面内铜氧键长增大是该体系Tc显著增加的主要原因。
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A series of new Sr2-xBaxCuO3+σ (0≤x≤0.6) superconductors were prepared using high-pressure and hightemperature synthesis. A Rietveld refinement based on powder x-ray diffraction confirms that the superconductors crystallize in the K2NiF4-type structure of a space group I4/mmm similar to that of La2CuO4 but with partially occupied apical oxygen sites. It is found that the superconducting transition temperature Tc of this Ba substituted Sr2CuO3+σ superconductor with constant carrier-doping level, i.e., constant is controlled not only by order/disorder of apical-O atoms but also by Ba content. Tc max=98 K is achieved in the material with x =0.6 that reaches the record value of Tc among the single-layer copper oxide superconductors and is higher than Tc=95 K of Sr2CuO3+σ with optimally ordered apical-O atoms. There is Sr-site disorder in Sr2?xBaxCuO3+σ which might lead to a reduction in Tc. The result indicates that another effect surpasses the disorder effect that is related either to the increased in-plane Cu-O bond length or to elongated apical-O distance due to Ba substitution with larger cation size. The present experiment demonstrates that the optimization of local geometry out of the Cu-O plane can dramatically enhance Tc in the cuprate superconductors.