title: Unveiling Electronic Structure Evolution in Complex Oxides: Tuning Charge, Spin and Structural Order
Creators: name: Wu, Yu-Mi
Publisher: name: UNSPECIFIED
Year: 2023

description: The interplay between the electron, spin, orbital, and lattice degrees of freedom in complex oxide materials generates an abundance of macroscopic physical properties. Small external perturbations to these materials influencing the coupling between these fundamental degrees of freedom can induce a huge response to their physical behaviors. Recent advances in synthesis techniques have provided a fertile ground to create model systems and allowed to systematically tune specific interaction parameters of the materials. This has therefore provided opportunities for understanding, controlling, and predicting new ground states utilizing different synthesis approaches, important for applications in next-generation devices. Here scanning transmission electron microscopy (STEM) combined with electron energy-loss spectroscopy (EELS) is used to study the electronic structure by tuning electronic, spin, and structural orders in complex oxides. First, the strain-induced charge disorder-to-order transition in mix-valent spinel LiV2O4 films are studied. Using spatially-resolved EELS, local charge distribution coupled with lattice distortion induced by epitaxial strain is observed. Three different electronic states are found: first a metallic charge-disordered state in the unstrained film; second an insulating charge-ordered state in a tensile-strained film, which has yet to be observed in bulk material; and third an intermediate state possessing short-range charge order in a compressive-strained film. An ordering pattern related to Fe3O4 Verwey-type charge ordering is identified in the film under tensile strain. Moreover, the compressively strained film showing a spatial evolution of the confinement of V 3d electrons evidences the strong correlation between the electronic and lattice structures in the system. Second, the combined strain and interface effects on the electronic and magnetic properties of La2CuO4–La0.5Sr0.5MnO3–La2CuO4 trilayer heterostructures are investigated. Using EELS, systematic variations of local charge distribution under strain disentangle the contributions from lattice distortions, atomic stacking sequences, and chemical intermixing at the interface. The charge rearrangement of Mn ions results in two different magnetic phases: an interfacial ferromagnetically reduced layer and an enhanced ferromagnetic layer away from the interface. Furthermore, the magnitude of charge redistribution can be effectively controlled by epitaxial strain together with charge transfer at the interface, which in turn influences the magnetic interaction in the trilayers. The third part of the thesis turns focus on the topotactically-induced structural order in rare-earth nickelate single crystals. In La compounds, distinct changes in the lattice and electronic structures between the perovskite and the infinite-layer phase after oxygen reduction are observed. The removal of apical oxygen atoms forming NiO2 planes in the infinite-layer phase leads to physical behaviors deviating from bulk properties in powders with similar compositions. On the other hand, the topotactic reduction in Pr compounds removing parts of apical oxygen atoms generates a brownmillerite-related structure. The oxygen vacancy ordering forms alternating NiO6 octahedral and NiO5 pyramidal layers in the structure, which was previously not observed in oxygen-deficient nickelates. Finally, the doping-controlled electronic structure transition in antimonates is studied. In K-doped BaSbO3, the system undergoes a metal–insulator transition accompanied by a structural phase transition with electron doping. Low-energy excitations probed by EELS reflect three different components: the interband transition across the band gap, free-carrier plasmon, and the bipolaron state. The systematic changes in these three components explain the apparent trends in conductivity. The absence of the excitation to the bipolaron state upon cooling evidences the bipolaron formation in the intermediate semiconducting phase. This observation suggests that changes in carrier concentration drive the metal–insulator transition through a bipolaron mechanism, where the bipolaron condenses into a charge-ordered insulating state in antimonates. Overall, the thesis covering four different systems establishes a picture of how the electronic structure evolves by tuning charge, spin, and structural orders using different approaches, from epitaxial strain, and interface engineering, to topotactic reduction and chemical doping. This combination of approaches provides insights into the origin of physical behaviors and opportunities of adding new functionalities in complex oxides.