Interfacial Phenomena in Atomically Engineered LaAlO3/SrTiO3 Heterostructures

Abstract

Complex oxides have phase diagrams of broad diversity, the increased interactions in their bulk resulting in spectacular phenomena, examples being colossal magnetoresistance, high transition temperature superconductivity, ferromagnetism and ferroelectricity. Additionally, the improved control over substrate surfaces and epitaxial deposition of complex oxides now allows the synthesis of abrupt interfaces, changing from one material to another over the distance of a single unit cell. In analogy to semiconductors where interfaces gave rise to novel interfacial states, there are tremendous opportunities in complex oxides to generate novel materials dictated by interfacial physics. A remarkable example was discovered by Akira Ohtomo and Harold Hwang in heterostructures composed of insulating perovskite constituents SrTiO3 and LaAlO3 where the polar heterointerface was shown to result in an electron gas with high electron mobility and intriguing magnetoresistance oscillations. The LaAlO3/SrTiO3 interface is often referred to as a prime example and model system for interface based multifunctional oxide heterostructures. Indeed, the system is a host to many scientific curiosities worth researching such as superconductivity, magnetism, spin-orbit coupling and high electron mobility. This thesis gives an overview of relevant literature and subsequently describes a synthesis method for epitaxial LaAlO3/SrTiO3 interfaces with examples for single interfaces, heterostructures and superlattices. For samples grown at relatively high oxygen partial pressures, a large negative magnetoresistance of the interface, together with a logarithmic temperature dependence of the sheet resistance is demonstrated. These effects are explained in terms of the scattering of conduction electrons off localized magnetic moments. Furthermore, oscillations showing an uncommon √B periodicity in the magnetoresistance are observed and evaluated. Using anomalous surface X-ray diffraction to study cation intermixing, deviations from ideal A-site and B-site compositions are demonstrated to exist at the interface. Heterostructures of LaAlO3/SrTiO3 bilayers on (001)-oriented SrTiO3 substrates have been used together with scanning tunnelling spectroscopy to study the coupling between sub-nanometer spaced electron and hole gases at these interfaces. In short, the work on LaAlO3/SrTiO3 interfaces described in this thesis studied its synthesis, magnetic effects, unconventional oscillations, interfacial structure and the interplay between spatially separated conducting sheets.