| In complex transition metal oxides, strong correlations between electrons lead to entangled ground states with many fascinating emergent phenomena, including magnetism and high-temperature superconductivity. Moreover, the interplay between structural, charge, spin, and orbital degrees of freedom in these systems opens up the possibility of inducing and influencing exotic phase behavior using state-of-the-art atomic layering techniques. In this thesis, I describe the engineering of electronic structure and transport properties of complex oxides through atomically precise control of dimensionality and interfacial structure using molecular beam epitaxy. Specifically, I present four studies focused on the rare-earth nick-elates (RNiO 3), an archetypal correlated system that features metal-insulator transitions and antiferromagnetism in the bulk.;First, the phase diagram of ultrathin nickelate thin films is studied, revealing a renormalization of phase boundaries due to enhanced orbital overlap induced by epitaxial strain. In addition, chemical doping is shown to be an effective way to control the metallic to insulating boundary. The second investigation concerns the thickness-induced metal-insulator transition in LaNiO3, in which the structural origin of the crossover as a result of polar surface distortions is revealed. Furthermore, the realization of two-dimensional conduction is demonstrated by atomic layer surface engineering. In the third experiment, the coincident metal-insulator and antiferromagnetic phase boundaries in bulk NdNiO3 are shown to diverge in confined heterostructures. In the single layer limit, long-range antiferromagnetic and charge order are fully suppressed as a result of orbital polarization and fluctuations in 2D. The last project focuses on the ability to manipulate the orbital configuration in nickelates. It is illustrated that unique three-component heterostructuring can be used to effectively change the nickelate orbital structure to emulate that of the high-temperature superconducting cuprates, and, in fact, can tune the orbital configuration between the bulk structures. This technique is based on simple physical mechanisms and represents a route to explore and enhance a wide variety of orbitally dependent phenomena in correlated oxides.;A combination of synchrotron-based x-ray diffraction and spectroscopy, magnetotransport measurements, and first-principles theory is employed in these studies to explore structure and properties in nickelate thin films and heterostructures. The results highlight the complex interplay of structure, chemistry, and electronic correlations in determining the competing phases in nickelates and demonstrate the efficacy of various atomic layering schemes to control their properties. In addition, the findings hold implications for the understanding and manipulation of electronic and magnetic phases in a variety of correlated oxide systems, setting the stage for novel device applications. |
