Dissertation Defense: Alexandru Bogdan Georgescu, Yale University, “New Methods and Phenomena in The Study of Correlated Complex Oxides”

Transition metal oxides have long been an important subject of study, both theoretically and experimentally. The wide array of phases possible in their bulk forms (high T$_c$ superconductivity, colossal magnetoresistance, ferroelectricity, etc.) makes them of scientific and technological significance, while relatively recent materials deposition techniques have allowed researchers to grow new, ‘artificial’ materials in the form of heterostructures and thin films. These structures offer a rich array of parameters to explore, as interfaces and thin films often show patterns of behavior that are quite different from their parent bulk compounds. From the point of view of electronic structure theory, this offers a rich playground where one can search for new physical phenomena. What makes transition metal oxides physically interesting is also what makes them difficult to study theoretically: the transition metal d-orbitals that dictate the wide array of phases in this class of materials cannot always be treated appropriately within band theory due to strong local electron-electron interactions. The local interactions are most often treated with a multi-band Hubbard model ‘glued’ on top of the first principles calculation. In this thesis, we have explored both a variety of complex oxide heterostructures and phenomena as well as advanced the computational framework used to describe them. We have analyzed the effect of local electrostatic fields at a ferroelectric-manganite interface as seen by electron energy loss spectroscopy, found a dimer-Mott state in a cobaltate-titanate interface, and identified new sources of orbital polarization at a nickelate-aluminate interface. We have also developed a generalized slave-boson formalism for multi-band Hubbard models that can be applied in large scale calculations involving complex oxide heterostructures and thin films.