Realizing metallicity in Sr2IrO4 thin films by high-pressure oxygen annealing

Abstract

Perovskite iridates are a promising material platform for hosting unconventional superconductivity. Transport measurements of Sr2IrO4 thin-film field-effect transistors are expected to provide irrefutable evidence for the existence of superconductivity. However, these experiments have revealed a remarkably robust insulating state over wide electron and hole doping ranges; this finding is in contrast to the case of the bulk material, in which metallicity appears upon moderate electron doping by substituting cations in place of Sr. The nature of this robust insulating state and whether any metallic state can be realized in the Sr2IrO4 thin film are two remaining challenges that preclude further progress in the search for superconductivity in this system. Here, we show that this insulating state is enhanced in Sr2IrO4 thin films by thermal annealing under vacuum conditions, while it can be destroyed upon annealing in an oxygen atmosphere within restricted ranges of oxygen pressure, annealing temperature and ion substitution levels. The resulting films exhibit metallic transport behavior near room temperature and a metal–insulator crossover at ~200 K. Our results point to the potentially important roles of the oxygen vacancies at different atomic sites in the formation of the robust insulating state and the new metallic state and to their interplay in the Sr2IrO4 thin film. This finding opens new possibilities in the search for unconventional superconductivity by further tailoring the as-found metallic state in properly oxygen-annealed Sr2IrO4 thin films. Despite enormous efforts by many research groups, Sr2IrO4 was found to stay remarkably insulating in thin film form. Now, a high-pressure oxygen annealing treatment on the Sr2IrO4 thin film realized the long-sought metallicity for the first time. An emerging transport phase diagram was deduced from the experiment that features an interplay between two states: the robust insulating state, which is likely dominated by the defect scattering effect of planar oxygen vacancies O(2), and the new metallic state, which likely reflects an intrinsic bulk-like property of the IrO2 planes with effective electron doping due to apical oxygen vacancies O(1).