Superoxygenation study of cuprate and iridate thin films*

High-pressure O₂ has been used to hole-dope and to stabilize high-oxidation phases of cuprates. We extend this superoxygenation technique to YBa₂Cu₃O_7-δ thin films, which are more reactive due to their large surface-to-volume ratio, and to the layered iridate Sr₂IrO₄, which is difficult to hole-dope by cation substitution. First, YBa₂Cu₃O_7-δ thin films grown by PLD are annealed in up to 700 atm O₂ and then characterized by TEM, XRD and XAS. The annealed films show phase conversion to Y₂Ba₄Cu₇O_15-δ and Y₂Ba₄Cu₈O₁₆, as well as regions of YBa₂Cu₅O_9-δ and YBa₂Cu₆O_10-δ. Second, epitaxial thin films of Sr₂IrO₄ are subjected to extended high-pressure annealing and similarly characterized. The post-annealed films show up to 3 order-of-magnitude drop in room temperature resistivity and an evolution towards semi-metallic behaviour. Furthermore, as film thickness is reduced, the annealed films show a structural transformation towards a quasi-cubic phase. Our results demonstrate the potential of using superoxygenation to stabilize exotic phases of transition metal oxides not achievable in bulk form. [1]
[1] H. Zhang et al., Phys. Rev. Materials 2, 033803 (2018).