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Novel Phases in Hetero-Epitaxial and Super-Oxygenated Thin Films of Complex Oxides

It is well-known that microstructural defects in heteroepitaxial oxide thin films can significantly affect the electronic and superconducting properties of these films. In this three-part talk, I discuss structural phase transitions and defect structures in epitaxial cuprate and iridate films, induced by either heterostructuring or superoxygenation.

In the first part of the talk, I examine the effects of heteroepitaxial strain on the superconducting critical temperature (Tc) of YBa2Cu2O7-δ (YBCO-123) thin films. By comparing scanning transmission electron microscopy and electrical resistance measurements on different YBCO-123 thin-film heterostructures, I show that heteroepitaxial strain, rather than long-range proximity effect, is responsible for the long length scales of Tc reduction in manganite/cuprate heterostructures.

The next part of the talk focuses on superoxygenation of Y-Ba-Cu-O thin films. YBCO-123 films annealed in high-pressure oxygen in conjunction with Cu enrichment show clear evidence of phase transformation to Y2Ba4Cu7O15-δ and Y2Ba4Cu8O16, as well as regions of exotic phases containing multiple CuO or Y layers. These results demonstrate a novel route of material synthesis towards discovering more complex oxide phases.

In the last part of the talk, I extend the superoxygenation technique to Sr2IrO4 films in an effort to hole-dope the iridates. High-pressure oxygen annealed Sr2IrO4 films show a progressive drop in room-temperature resistivity of up to 3 orders of magnitude, and an evolution towards metallic behavior. The evolution towards metallicity is attributed to structural phase transformation, interstitial oxygens, and Ir vacancies.