Visualizing the atomic configuration of carbonate groups in a (Cu,C)Ba 2 Ca 3 Cu 4 O 11+δ superconductor.

Carbonates (CO 3 2- ) have always been known as impurities to degrade the superconductivity in cuprate high-Tc superconductors. Herein, the atomic arrangement of carbonates is directly visualized in (Cu,C)Ba 2 Ca 3 Cu 4 O 11+δ via integrated differential phase contrast (iDPC) combined with state-of-the-art scanning transmission electron microscopy. The carbon atoms replace Cu atoms in the charge-reservoir layers, contributing to the formation of carbonates through strong orbital hybridization with the surrounding oxygen atoms. Using first-principles calculations, the spatial configuration of carbonate groups is confirmed and their influence on local crystal lattice and electronic states are further investigated. The carbonates not only accommodate the distortions by improving the flatness of outer CuO 2 layers but also reduce the density of states at the Fermi level. These two factors play competitive roles to affect the superconductivity. Our study provides direct evidence of the presence of CO 3 2- groups and gains an insight into the underlying mechanism of superconductivity in oxycarbonate superconductors. This article is protected by copyright. All rights reserved.