Visualization of Dopant Oxygen Atoms in a Bi₂Sr₂CaCu₂O_{8+ δ} Superconductor

A proper amount of excess oxygen plays a significant role in hole-doped cuprate high-T_c superconductivity. Here, the dopant oxygen in Bi₂Sr₂CaCu₂O_{8+ δ} is directly imaged via integrated differential phase contrast combined with state-of-the-art scanning transmission electron microscopy. The location of dopant oxygen is observed to be consistent with the position inferred from local strain analysis of the incommensurate structure. The influence of dopant oxygen on the local atomic lattice and electronic structure is further explored using first-principle calculations. The dopant oxygen atoms not only aggravate the distortions of the local atomic arrangement but also alter the electronic states by transferring charge from the BiO planes to the CuO₂ planes. The underlying mechanism of charge transfer is resolved. The results may also be applicable to other oxygen-doped cuprates with high-T_c superconductivity.