Addition by Subtraction: Visualizing and Tailoring Vacancies for Designer Materials
Complex oxides inhabit a major section of today’s research space, as the choice of cations and crystal structures allow diverse functional properties such as ferroelectricity, high-temperature superconductivity, giant-magnetoresistance and colossal ionic conductivity. In particular, the use of transitional metals in oxide crystals (so-called transition metal oxides, or TMOs) bring about useful physical phenomena, including spin-orbit coupling, half-metallicity, and metal-insulator transitions.
TMO thin-films typically have finite concentrations of oxygen vacancies, which have traditionally been viewed as detrimental to their functional properties. However, in this talk I will show how oxygen vacancies can be used as an extra degree of freedom to control the functionalities of TMOs. The presence of a strong coupling between vacancies and the electric polarization in oxides can be used to drive polar ordering in a superlattice made up of two materials that lack any polar ordering in their bulk. Vacancies localized at the surface of an oxide film can stabilize an exotic phase where the surface layer displays metallic and ferromagnetic behavior, while the bulk remains antiferromagnetic and insulating. Oxygen-decorated copper vacancies can lead to a dilute magnetic behavior in a high-Tc superconductor: YBa2Cu3O7−δ, a highly unusual phenomenon as ferromagnetism and superconductivity are generally antagonistic to each other. I will end the talk by discussing examples of observation and manipulation of vacancies at the atomic scale using a scanning transmission electron microscope. Overall, the talk will demonstrate the importance of “seeing” the distribution of vacancies and relating them to the properties of TMOs and highlight the need to have the ability to manipulate vacancies at the atomic-scale to control new functionalities.