| Semiconductor technology is based on tuning the properties of devices by manipulating thin films and interfaces. Recently, this approach has been extended to complex oxides, where quantum many-body interactions give rise to emergent ground states not present in the parent materials. Rationally controlling and engineering correlated electronic phases has the potential to revolutionize modern electronics, but is hindered by the inability of current theory to account for the effects of many-body interactions on the underlying electronic structure. Manganites provide a particularly model system for studying many-body effects due to their complex electronic and magnetic phase diagrams, which give rise to many potentially useful properties. Despite extensive work on manganite films demonstrating numerous electronic phase transitions, little is directly known about how the electronic structure responds to the 'control parameters' accessible in thin films. This dissertation presents direct measurements of the electronic structure in La1-- xSrxMnO3 based thin films and interfaces through several phase transitions using a unique integrated oxide molecular-beam epitaxy and angle-resolved photoemission spectroscopy system.;We observe the full Fermi surface and near-EF electronic structure of the ferromagnetic and A-type antiferromagnetic metallic phases, reconciling first-principles calculations with experiment for the first time. Furthermore, our results provide key insights into the polaronic nature of the metallic charge carriers. We then explore the mechanism underlying the insulating ground state for La2/3Sr1/3MnO3 under strong tensile strain. Our measurements rule out the scenarios of bandwidth or localization-driven metal-insulator transitions, and reveal an instability of the strongly interacting metal towards an ordered insulating phase that can be accessed through epitaxial strain. By next studying atomically precise interfaces in (LaMnO3)2n/(SrMnO 3)n superlattices, we directly see how the interplay between dimensionality and strong many-body interactions drives large period superlattices into a pseudogapped insulating phase. Our results provide new insights into the physics of perovskite manganites, and illustrate in detail the nature and importance of phase competition in controlling the electronic properties of correlated thin films. These results should be applicable to correlated materials in general, and can help develop predictive models capable of realizing the full potential of oxide electronics. |
