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 Sr 2 IrO 4 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 2 and SrO planes by photoelectron holography and density functional theory (DFT) calculations. In the Ir 4f spectrum of Sr 2 IrO 4 , an unexpected Ir 3+ peak was observed as much as 50% of all of the Ir. The photoelectron hologram of Ir 3+ showed a displacement of about 0.15 Å. This displacement is thought to be due to the oxygen vacancies in the IrO 2 plane. These oxygen vacancies and the associated local displacement of the atoms might inhibit superconductivity in spite of sufficient electron doping.