Epitaxial growth and superconductivity of optimally doped Sr1−xEuxCuO2 cuprate films

Superconducting infinite-layer ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Eu}}_{x}\mathrm{Cu}{\mathrm{O}}_{2}$ films at optimal doping ($x\ensuremath{\sim}0.15$) have been epitaxially grown on $\mathrm{SrTi}{\mathrm{O}}_{3}$(001) substrates with oxide molecular beam epitaxy. By combining x-ray diffraction and transport measurements, we have established a narrow window of growth conditions [i.e., ozone pressure, substrate temperature, ($\mathrm{Sr}+\mathrm{Eu}$)/Cu flux ratio, and postannealing] in which the superconducting infinite-layer structure can be readily achieved while eliminating any unwanted competing phases. A smaller out-of-plane lattice constant ($c<3.42\phantom{\rule{0.16em}{0ex}}\AA{}$) is found to be crucial to prompt superconductivity in the ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Eu}}_{x}\mathrm{Cu}{\mathrm{O}}_{2}$ films. Thickness-dependent electrical resistivity measurements reveal superconductivity even when the film thickness is thinned down to 5 nm. Furthermore, the superconducting state of the epitaxial infinite-layer films remains remarkably stable against air exposure, providing an ideal platform to explore the high-temperature superconductivity and unusual properties in cuprates.