Out-of-plane effect on the superconductivity ofSr2−xBaxCuO3+δwithTcup to 98 K

Abstract

A series of new ${\text{Sr}}_{2\ensuremath{-}x}{\text{Ba}}_{x}{\text{CuO}}_{3+\ensuremath{\delta}}$ $(0\ensuremath{\le}x\ensuremath{\le}0.6)$ superconductors were prepared using high-pressure and high-temperature synthesis. A Rietveld refinement based on powder x-ray diffraction confirms that the superconductors crystallize in the ${\text{K}}_{2}{\text{NiF}}_{4}$-type structure of a space group $I4/mmm$ similar to that of ${\text{La}}_{2}{\text{CuO}}_{4}$ but with partially occupied apical oxygen sites. It is found that the superconducting transition temperature ${T}_{c}$ of this Ba substituted ${\text{Sr}}_{2}{\text{CuO}}_{3+\ensuremath{\delta}}$ superconductor with constant carrier-doping level, i.e., constant $\ensuremath{\delta}$ is controlled not only by order/disorder of apical-O atoms but also by Ba content. ${T}_{c}^{\text{max}}=98\text{ }\text{K}$ is achieved in the material with $x=0.6$ that reaches the record value of ${T}_{c}$ among the single-layer copper oxide superconductors and is higher than ${T}_{c}=95\text{ }\text{K}$ of ${\text{Sr}}_{2}{\text{CuO}}_{3+\ensuremath{\delta}}$ with optimally ordered apical-O atoms. There is Sr-site disorder in ${\text{Sr}}_{2\ensuremath{-}x}{\text{Ba}}_{x}{\text{CuO}}_{3+\ensuremath{\delta}}$ which might lead to a reduction in ${T}_{c}$. 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 ${T}_{c}$ in the cuprate superconductors.