No description available.

In this dissertation, we utilized a wide range of complementary synchrotron-based X-ray spectroscopic and scattering techniques, notably X-ray absorption spectroscopy (XAS), hard X-ray photoelectron spectroscopy (HAXPES), standing-wave X-ray photoelectron spectroscopy (SW-XPS), and X-ray resonant magnetic reflectometry (XRMR), to understand and control the phenomenon of emergent interfacial ferromagnetism in strongly-correlated oxide heterostructures. This field holds great promise for the development of next-generation spintronic devices. In the heterostructures we investigated, neither of the parent oxide layers exhibits inherent ferromagnetism. Yet, when these layers are combined in an epitaxial film stack, charge-transfer phenomena give rise to an emergent ferromagnetic state at the interface. Throughout my graduate studies, I focused on studying such charge-transfer phenomena as the driving force for stabilizing interfacial ferromagnetism. This dissertation is structured around two main projects. The first project delves into the intriguing possibility of tuning the emergent interfacial ferromagnetism. More specifically, we investigated the mechanisms for suppressing interfacial charge transfer to gain control over and manipulate this magnetic phenomenon. In our second project, we explored a different facet of interfacial ferromagnetism, focusing on the origins of the imbalance in the magnitudes of the magnetic moment between the top and bottom interfaces in the same layer. Our investigation aimed to uncover the possible causes of this imbalance, ultimately leading us to scrutinize the role of defect states in this magnetic asymmetry. In the first part of this dissertation, we investigated the thickness-dependent metal-insulator transition within LaNiO3 and how it impacts the electronic and magnetic states at the interface between LaNiO3 and CaMnO3. We present a direct observation of a reduced effective valence state in the interfacial Mn cations. This reduction is most pronounced in the metallic LaNiO3/CaMnO3 superlattice, where the above-critical LaNiO3 thickness of 6-unit cells triggers this phenomenon, facilitated by the charge transfer of the itinerant Ni 3d eg electrons into the interfacial CaMnO3 layer. In contrast, when we examine the insulating superlattice with a LaNiO3 thickness below the critical value (2-unit cells), we observe a homogeneous effective valence state of Mn throughout the CaMnO3 layers. This homogeneity is attributed to the suppression of charge transfer across the interface. The second part of this dissertation delves deeply into the complexities of interfacial magnetism within the CaMnO3/CaRuO3 superlattices. Our experimental investigation unveiled an unexpected asymmetry in the strength of magnetism at these interfaces. Our findings suggest that within the superlattice CaMnO3/CaRuO3, the lower interface (CaRuO3/CaMnO3) exhibits a weaker magnetic moment when compared to the upper interface (CaMnO3/CaRuO3). This observation, supported by XRMR and XAS experimental data, was further clarified by first-principles density functional theory (DFT) calculations. Our calculations suggest that the observed magnetic asymmetry may be linked to the presence of oxygen vacancies at the interfaces. Our study significantly contributes to our understanding of interfacial ferromagnetism, potentially paving the way for controlling and manipulating this emergent property. This may be achieved by utilizing engineered defect states, offering exciting prospects for applications in the field of spintronics devices. / Physics

The intent of this thesis is to improve upon previously proposed tight-binding models for one dimensional black phosphorus, or phosphorene. Previous models offer only a qualitative analysis of the band structure of phosphorene, and fail to fully realize critical elements in the electronic band structure necessary for transport calculations. In this work we propose an improved tight-binding model for phosphorene by including up to eight nearest-neighbor interactions. The efficacy of the model is verified by comparison with DFT-HSE06 calculations, and the anisotropy of the effective masses in the armchair and zigzag directions is considered.

BSCCO-2212 is a high-temperature cuprate superconductor whose microscopic behavior is currently poorly understood. In particular, it is unclear whether its order parameter is consistent with s-wave or d-wave symmetry. It has been suggested that its order parameter might take one of several forms that are consistent with d-wave behavior. We present some calculations using the many-body theory approach to superconductivity that suggest that such order parameters would lead to a suppression of the critical temperature in the presence of impurities. Because some experiments have suggested the critical temperature of BSCCO-2212 is relatively independent of the concentration of impurities, this lends support to the hypothesis that its order parameter has s-wave symmetry.

Humanity has great energy demands, and must simultaneously combat climate change by curbing anthropogenic greenhouse gas emissions. Perovskite solar cells (PSC) provide a low-carbon energy source, at lower production costs than traditional silicon-based solar cells. PSC's suffer some issues with long-term stability. This report presents a measurement aimed at better understanding interfacial dynamics of PSC's, using X-ray Photo-electron Spectroscopy (XPS). By collecting data at the synchrotron BESSY II, material compositions at different depths in the cell were successfully measured. An unexplained shift in binding energy was observed for configurations with an external light source on and off.

No description available.

No description available.

No description available.

No description available.

No description available.