X-ray scattering studies of strongly correlated materials: Orbital ordering in Potassium trifluorocuprate and stripe-like domain structures in Bismuth ferrite epitaxial thin films

The results of x-ray scattering experiments on orbitally ordered KCuF3 and periodic arrays of stripe-like ferroelectric domains in epitaxial thin films of multiferroic BiFeO3 are presented in this dissertation. Both resonant soft x-ray elastic scattering and non-resonant hard x-ray elastic scattering probes were used. Our experiments on KCuF3 have revealed a previously unidentified structural phase transition at T = 50K, involving GdFeO3 like rotations of the CuF6 octahedra. These rotations are quasi-ordered and exhibit glassy hysteresis, but serve to stabilize the Neel state at T = 39K. Based on these observations, we have formulated an orbital ordering model based on the archetypical Kugel-Khomskii model. Our modified Kugel-Khomskii model takes into account direct orbital exchange interactions due to a combination of electron-electron interactions and charge transfer effects. The effect of this term is to create a near degeneracy that dynamically frustrates the spin order that is lifted at low temperature by subdominant orbital-lattice interactions. A strong optical effect seen in the Cu L3 resonant scattering is also discussed. We also describe on-going studies of BiFeO3 epitaxial thin films. Based on non-resonant diffractin data, we have created simple charge density models that describe the structure of stripe-like ferroelectric domains with majority 109° and majority 71° domain walls. Resonant soft x-ray scattering near the Fe L3 transition edge has revealed a spatial modulation of predominantly Fe 3d valence states with a period equal to that of the ferroelectric domain structure. The possibility of a magnetic origin to the Fe L3 edge resonant scattering is discussed. No O K edge resonant scattering was observed, indicating that O 2p states are not significantly in uenced by the domain structure. The absence of O K edge resonant scattering is also discussed in terms of proposed mechanisms for the anomalously high electrical conductivity of 109° domain walls. We also brie y describe the observation of scattering near the substrate peak due to the thin film domain structure, which is causing a modulation of the substrate strain.