Phase transformation by superoxygenation in cuprate and iridate thin films*

High-pressure O₂ has previously been used to hole-dope and stabilize high-oxidation phases of cuprates. We extend this superoxygenation technique to materials in thin film form since they are more reactive due to their large surface-to-volume ratio. YBa₂Cu₃O_7-δ (YBCO) 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 well as regions of YBa₂Cu₅O_9-δ and YBa₂Cu₆O_10-δ. 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 and to create novel materials by selectively transforming constituent layers in multilayer films. [1]
[1] H. Zhang et al., Phys. Rev. Materials 2, 033803 (2018).