Superconductivity at 110 K in the infinite-layer compound (Sr_1-xCa_x)_1-yCuO₂

Abstract

THE 'infinite-layer' parent structure¹ of the copper oxide superconductors (Fig. 1) is the simplest structure containing the CuO₂ sheets that are apparently essential to high-transition-temperature (high-T_c) superconductivity. At ambient pressure only Ca_1-x Sr_xCuO₂ with x≈O.l can be stabilized in this structure^1,2but at high pressures and temperatures compounds ranging from Ba¹/₃Sr²/₃CuO₂ to Ca²/₃Sr¹/₃CuO₂ through SrCuO₂ can be synthesized³. We have previously reported superconductivity with T_c = 40–100 K in the Ba–Sr–Cu–O system^4,5 but have not until now been able to isolate a superconducting phase. Here we report the isolation of an alkaline-earth-deficient infinite-layer phase, (Ca_1-xSr_x)_1-yCuO₂ (y~O.l), with T_c up to 110 K. In contrast to Sr_1-xR_xCuO₂ (with R a rare-earth element and T_c⩽43 K), which from the composition dependence of the lattice constants is thought to be an n-type superconductor^6,7 our data suggest that the present superconductor is of p-type, with the carriers arising from calcium and strontium vacancies. High-resolution electron micrographs reveal defect layers, which we suggest are where the calcium and strontium vacancies are concentrated.