Electronic and optical properties of titanate-based oxide heterostructures

In this thesis we study properties of transition metal oxide heterostructures and superlattices, including electronic structures, optical responses, and metal-insulator transitions. We start with a general discussion of the properties of transition metal oxides, primarily ABO₃ (A: rare earth ion, B: transition metal, O:oxygen) perovskites. We introduce the effect of A-site substitution on the electronic and magnetic properties in bulk perovskites, followed by the basic properties of oxide heterostructures and superlattices composed of two different ABO₃ perovskites focusing on the metal insulator transitions and properties of the interface electron gas. Next, we present calculations of the charge density profile, subband occupancy and ellipsometry spectra of the electron gas at the LaAlO₃/SrTiO₃ interface. The calculations employ self-consistent Hartree and random phase approximations. We discuss the dependence of spatial structure and subband occupancy on the magnitude of the polarization charge at the interface and spatial structure of the dielectric constant. The response to applied AC electric fields is calculated and the results are presented in terms of the ellipsometry angles. Our results show a dip in the ellipsometry spectrum near the longitudinal optic phonon frequency of the SrTiO₃ and a peak at higher energy, which are related to the in-plane Drude response and the out-of-plane plasmon excitation, respectively. The relation of the features to the subband occupancies and the in-plane conductivities is given. We conclude with the study of thickness dependent metal-insulator transitions in superlattices composed of Mott insulating GdTiO₃ and band insulating SrTiO₃ using a first-principles GGA+U method. The structural and metal-insulator phase diagrams with respect to the number of unit cells, n, of SrTiO₃ and on-site correlation U are presented, showing that there are two different insulating phases for n>1 and n=1. For superlattices with n>1 the insulating phase involves both charge and orbital ordering with associated structures in Ti-O bond lengths but for n=1 superlattices, we find an insulating phase driven by orbital ordering within the quasi one-dimensional bonding bands across the SrO layer. The inconsistencies with experiment suggests the importance of the many-body correlations.