Superconductivity in the infinite-layer Sr$_{1-x}$Ca$_{x}$CuO$_{2}$ phase

The CuO$_{2}$ planes are the fundamental building blocks of cuprate superconductors where Cu assumes three types of copper coordinations, i.e., octahedral, pyramidal, and square-planar. For cuprates with the infinite layer structure, Cu is stabilized in a square-planar environment and this structure is known to show superconductivity. The square-planar coordination is also known to cuprates with Nd$_{2}$CuO$_{4}$ structure and we have shown earlier that doping is not a relevant parameter in inducing superconductivity, quite in contrast to cuprates with octahedral- or pyramidal coordinated copper. Moreover, for cuprates with infinite-layer structure the induction of superconductivity has been associated to reconstruction processes rather than doping in CaCuO$_{2}$/SrTiO$_{3}$ superlattices. Here we show that the superconductivity in Sr$_{1-x}$Ca$_{x}$CuO$_{2}$ is predominantly subject to defects arising either from cation- and/or oxygen- disorder. Using molecular beam epitaxy we synthesized high quality single crystalline thin films with 100 nm thickness of Sr$_{1-x}$Ca$_{x}$CuO$_{2}$. High angle annular dark field scanning transmission electron tomographs are used to link the degree of cation disorder in this thermodynamically unstable phase to the induction of superconductivity