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.