Designing Superlattices of Cuprates and Ferrites for Superconductivity

If a materials system tends to form metastable polymorphs, i.e., materials with exactly identical stoichiometries but different crystal structures, extrinsic forces, e.g., pressure or strain, are suitable to decisively control the synthesis of the target phase. The crystallographic assembly of cuprate superconductors that still holds record high superconducting transition temperatures (Tc) at ambient pressure is based on superlattice structures, and slabs of copper–oxygen planes are sandwiched by slabs of noncuprous oxides. Cuprates with Tc above 100 K are built up from slabs of the infinite-layer (IL) CaCuO2 phase and form superlattices with other oxides, e.g., HgBa2O2+δ, Bi2Sr2O4+δ, or Tl2Ba2O4+δ. However, if the infinite-layer CaCuO2 phase is synthesized without slabs of other oxides, superconductivity does not emerge. Here, we show that superconductivity can be induced in artificial superlattices built from IL-CaCuO2 that do not have a eutectic point in the thermodynamic phase diagram. In particular, we built superlattices from infinite-layer CaCuO2 and brownmillerite Ca2Fe2O5 using molecular beam epitaxy. Embedding slabs of the infinite-layer CaCuO2 cuprate into two-dimensional slabs of the brownmillerite Ca2Fe2O5 ferrite suppresses the formation of cationic defects in the CaCuO2 slabs of the superlattices. Effective suppression of cationic stripes within the CuO2 planes was directly confirmed using lateral high-resolution transmission electron microscopy methods. Complementary X-ray diffraction experiments of the superlattices confirmed that the emergence of superconductivity may correlate to the defect density of the IL-CaCuO2. HgBa2O2+δ, Bi2Sr2O4+δ, or Tl2Ba2O4+δ are diamagnetic insulators, whereas brownmillerite Ca2Fe2O5 is an antiferromagnetic insulator. Yet, magnetic ordering has no sway over the induction of superconductivity. Instead, similarly to HgBa2O2+δ, Bi2Sr2O4+δ, or Tl2Ba2O4+δ, the brownmillerite Ca2Fe2O5 slabs buffer local charge imbalances that arise from point defects during the synthesis of IL-CaCuO2 slabs. Here, Ca2Fe2O5 may be considered as an alternative to charge-reservoir layers in the cuprate superconductors synthesized from incongruent melts. Furthermore, superconductivity in such metastable, artificial superlattices of [(CaCuO2)n(Ca2Fe2O5)m]N is subject to the oxidizing environment applied during the cooling procedure after synthesis. Recuperation of vacant oxygen sites is crucial for the induction of superconductivity and highlights the significance of defect-free CuO2 planes. In contrast to the general notion that charge-reservoir layers are conceived to be a source of doping for adjacent CuO2 planes, our experiments show that the dominating contributions to the conduction layers are structural in nature, rather than from doping.