Superconductivity in infinite-layer CaCuO2-brownmillerite Ca2Fe2O5 superlattices

Abstract

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.