Origin of Unexpected Ir 3+ in a Superconducting Candidate Sr 2 IrO 4 System Analyzed by Photoelectron Holography.

The reason for the absence of superconductivity in SrIrO 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 IrO and SrO planes by photoelectron holography and density functional theory (DFT) calculations. In the Ir 4f spectrum of SrIrO, an unexpected Ir peak was observed as much as 50% of all of the Ir. The photoelectron hologram of Ir showed a displacement of about 0.15 Å. This displacement is thought to be due to the oxygen vacancies in the IrO plane. These oxygen vacancies and the associated local displacement of the atoms might inhibit superconductivity in spite of sufficient electron doping.