First Principles Studies of Magnetic Oxides, Spin-Driven Ferroelectricity, and the Effect of Polarization in the Chemistry of Functional Heterointerfaces

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Doctor of Philosophy (PhD)
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Density Functional Theory
Electronic structure
Ferroelectric surfaces
Functional heterointerfaces
Magnetic oxides
Condensed Matter Physics
Mechanics of Materials
Physical Chemistry
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Achieving accurate description and understanding of the chemical and physical properties of complex materials enables the further development of their technological applications. Employing density functional theory (DFT) with rotationally invariant Hubbard corrections, we present an extensive study of binary manganese oxides modeling their noncollinear spin patterns and computing their electronic structures in agreement with experimental results. Leveraging on our success in predicting accurately magnetic properties, we explore the noncollinear cycloidal magnetic order in CaMn${7}$O${12}$, which breaks inversion symmetry generating one of the largest spin-driven ferroelectric polarizations measured to date. Based on a generalized spin-current model with Heisenberg-exchange and Dzyaloshinskii-Moriya interaction energetics we explain the microscopic origin of the polarization, including its direction, coupling to the spin helicity, charge density redistribution, and magnetic exchange interactions. Our experimental collaborators synthesize the proposed material, CaMn${7}$O${12}$, in films, reporting experimental evidence of its remarkable high temperature charge ordering phase transition and our atomistic insights through DFT calculations elucidate on the structural and electronic coupling of this phase transition. Symmetry breaking and chemical potential mismatch at an interface could lead to novel phenomena and multifunctional properties inaccessible in the bulk; therefore, interfacial engineering of functional heterostructure geometries could guide devices by design. We propose the functional interface between graphene and polydomain ferroelectrics as platform for novel field effect transistors. Here, we present both a theoretical understanding of how ferroelectric polarization direction affects the graphene carrier density and with help from our experimental collaborators we show evidence of our explanations. We predict that the graphene can be \emph{n}- or \emph{p}-type depending on the polarization direction and quantify the changes in carrier density. The functional complex oxide heterostructure, LaAlO${3}$/SrTiO${3}$ - known for the emergence of two-dimensional electron gas (2DEG) at the interface, is investigated to explain the experimental observation of a surface chemically switchable luminescence process after water treatment. We determine that water dissociates at the surface of LaAlO${3}$/SrTiO${3}$, which leads to protons on the surface cancelling the ``polar catastrophe" and providing accessibility to optical transitions from the 2DEG system. Finally, motivated by the instability of organometal halide perovskites once exposed to water, we study the interaction of water with the (001) surfaces of CH${3}$NH${3}$PbI$_{3}$ under low and high water concentrations concluding that the orientation of the dipole from the methylammonium molecules heavily influenced the surface interaction with water.

Andrew M. Rappe
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