  High-temperature cuprate superconductors have naturally a superlattice
structure. Infinite-layer CaCuO2 is the common ingredient of cuprates with
superconducting transition temperatures above 100 K. However, infinite-layer
CaCuO2 by itself does not superconduct. Here we show that superconductivity
emerges in artificial superlattices built from infinite-layer CaCuO2 and
brownmillerite Ca2Fe2O5 grown by molecular beam epitaxy. X-ray diffraction and
electron microscopy characterizations showed that the crystal quality of the
infinite-layer CaCuO2 in the superlattices significantly improved compared to
bare thin-films of CaCuO2. We found that the induction of superconductivity in
[(CaCuO2)n(Ca2Fe2O5)m]N superlattices is also subject to the oxidizing
environment used during the cool-down procedure and therefore to a minimization
of oxygen vacancies within the CuO2 planes. The inserted Ca2Fe2O5 layers buffer
charge imbalances triggered by point defect formation during growth, minimizing
cationic defects in the infinite-layer CaCuO2 layers thus stabilizing
monolithic infinite-layer CaCuO2 slabs; embedding CaCuO2 within a superlattice
enables extended two-dimensional CuO2 planes and therefore superconductivity
while Ca2Fe2O5 serves similar to the charge-reservoir layers in the cuprate
superconductors synthesized from incongruent melts.
