Strain control of oxygen stoichiometry in epitaxial perovskites*

Many physical properties of transition metal oxides (TMOs) are critically dependent upon the oxidation state of transition metals. Thus, a precise control of oxygen stoichiometry is critical to unambiguously understand many intriguing properties and functionalities. Based on a recent discovery of TMO-based oxygen sponges that can shed or absorb oxygen at highly reduced temperatures as low as 200 $^{\circ}$C [Jeen \textit{et al}., Nature Mater. \textbf{12}, 1057 (2013) and Choi \textit{et al}., Phys. Rev. Lett. \textbf{111}, 097401 (2013)], we have explored various complex oxide materials to control the oxygen stoichiometry and, thereby, the critical physical properties. The latter include superconductivity in doped La$_{2}$CuO$_{4}$, metal-insulator transition in VO$_{2}$, and electronic and ionic conductivity as well as magnetism in SrCoO$_{3-d}$. In particular, by tuning strain systematically via lattice mismatching, we found that the epitaxial strain is a great tool to create functional defects that are critical in discovering new functionalities and/or improving the performance of materials especially for electronic and ionic conduction in complex oxides.