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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

INVESTIGATIONS OF STRONGLY-CORRELATED COMPLEX METAL OXIDES AND INTERFACES USING SYNCHROTRON X-RAY SPECTROSCOPY

Chandrasena, Ravini Udeshika January 2019 (has links)
In this dissertation, we used a combination of several synchrotron-based x-ray spectroscopic techniques to investigate the effects of strain, ionic defect formation, and heteroengineering in strongly-correlated electronic systems. First, we introduce a method to control and stabilize oxygen vacancies in complex transition-metal oxide thin films. In our approach, we utilized atomic layer-by-layer pulsed laser deposition (ALL laser PLD) from two separate targets to synthesize high-quality single crystalline CaMnO3 films under coherent tensile strain, varying systematically from +0.8% to +4%. An increase of the oxygen vacancy content in the single-crystalline CaMnO3 thin films with applied in-plane strain was experimentally observed using high-resolution soft x-ray absorption spectroscopy (XAS) in conjunction with bulk-sensitive hard x-ray photoelectron spectroscopy (HAXPES). Our experimental results were verified using first-principles theory and atomic core-hole multiplet calculations. Furthermore, our results highlight the importance of protecting the surfaces of CaMnO3 thin-films with thin Pt layers in-situ in order to stabilize the oxygen vacancy content. Next, we discuss the role of oxygen vacancies in driving the metal-insulator transition in LaNiO3 thin films. Here, we also use atomic layer-by-layer pulsed laser deposition (ALL laser PLD) from two separate targets to synthesize high-quality single-crystalline LaNiO3 films with systematically varying thicknesses, ranging from 1 u.c. to 50 u.c. An increase in the oxygen vacancy content was observed with the decreasing LaNiO3 film thickness using XAS. A higher concentration of oxygen vacancies was observed for the ultrathin insulating films (<1.5 u.c.). The experimental results were compared to first-principles theoretical calculations. We found that LaNiO3 exhibits room-temperature metallic behavior for thicknesses down to 1.5 u.c., which is the lowest value reported to date. Finally, we have investigated an atomically-abrupt interface between the paramagnetic LaNiO3 and the antiferromagnetic CaMnO3 thin films. The interface between these two complex oxides exhibits interfacial ferromagnetism, which can be tuned via a thickness-dependent metal-insulator transition in LaNiO3. Here, we used depth-resolved standing-wave photoemission spectroscopy (SW-XPS), scanning transmission electron microscopy (STEM), and XAS to observe a depth-dependent charge reconstruction occurring at the LaNiO3/CaMnO3 interface. Our elemental standing-wave rocking-curve analysis revealed the depth-dependent changes of the Mn and Ni valence states at the interface, yielding increased amounts of Mn3+ and Ni2+ cations at the interface. These results suggest Mn4+-Mn3+ ferromagnetic double exchange and Ni2+-Mn4+ superexchange as possible underlying causes of the emergent interfacial ferromagnetism. / Physics

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