Atomic Stripe Formation in Infinite-Layer Cuprates

High-temperature superconductivity appears in cuprate materials that have been tuned in a way where the copper-oxygen bond configuration and coordination is in a state of minimal energy. In competition with the Jahn-Teller effect, which impedes the formation of infinitely connected CuO₂ planes, the state of minimal energy persists for planar copper-oxygen bond length variations of up to 10%. We have synthesized the infinite-layer phases of CaCuO₂ and SrCuO₂ as single-crystalline films using molecular beam epitaxy and performed in-plane scanning transmission electron microscopy mapping. For the infinite-layer phase of CaCuO₂ with a short Cu-O bond length, the CuO₂ planes maintain their minimal energy by forming distinguished atomic stripes. In contrast, atomic stripe formation does not occur in the infinite-layer phase of SrCuO₂, which has a larger Cu-O bond length. The polar field provided by the charge reservoir layer in cuprates with infinitely connected CuO₂ planes holds the key over the emergence of superconductivity and is vital to maintain infinitely connected CuO₂ planes themselves.