Origin of Unexpected Ir³⁺ in a Superconducting Candidate Sr₂IrO₄ System Analyzed by Photoelectron Holography

The reason for the absence of superconductivity in Sr₂IrO₄ was estimated by photoelectron spectra and photoelectron holograms. The analysis of the La photoelectron hologram concluded that La atoms are substituted to Sr sites. Two O 1s peaks were observed and were identified as the oxygens in the IrO₂ and SrO planes by photoelectron holography and density functional theory (DFT) calculations. In the Ir 4f spectrum of Sr₂IrO₄, an unexpected Ir³⁺ peak was observed as much as 50% of all of the Ir. The photoelectron hologram of Ir³⁺ showed a displacement of about 0.15 Å. This displacement is thought to be due to the oxygen vacancies in the IrO₂ plane. These oxygen vacancies and the associated local displacement of the atoms might inhibit superconductivity in spite of sufficient electron doping.