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 andphotoelectronholograms. The analysis of the La photoelectron hologram concludedthat La atoms are substituted to Sr sites. Two O 1s peaks were observedand were identified as the oxygens in the IrO2 and SrOplanes 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+ showeda displacement of about 0.15 angstrom. This displacement is thoughtto be due to the oxygen vacancies in the IrO2 plane. Theseoxygen vacancies and the associated local displacement of the atomsmight inhibit superconductivity in spite of sufficient electron doping. High concentration of oxygen vacancieswas found in theIrO(2) plane even in the parent material of high-temperaturesuperconductor candidate Sr2IrO4 by analyzingthe photoelectron hologram.