Epitaxial growth and superconductivity of optimally doped Sr_1-xEu_xCuO₂ cuprate films

Superconducting infinite-layer Sr1-xEuxCuO2 films at optimal doping (x similar to 0.15) have been epitaxially grown on SrTiO3(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, (Sr + 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 angstrom) is found to be crucial to prompt superconductivity in the Sr1-xEuxCuO2 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.