This thesis presents the results of resonant and non-resonant x-ray scattering experiments demonstrating the control of collective ordering phenomena in epitaxial nickel-oxide and copper-oxide based superlattices. Three outstanding results are reported: (1) LaNiO3-LaAlO3 superlattices with fewer than three consecutive NiO2 layers exhibit a novel spiral spin density wave, whereas superlattices with thicker nickel-oxide layer stacks remain paramagnetic. The magnetic transition is thus determined by the dimensionality of the electron system. The polarization plane of the spin density wave can be tuned by epitaxial strain and spatial confinement of the conduction electrons. (2) Further experiments on the same system revealed an unusual structural phase transition controlled by the overall thickness of the superlattices. The transition between uniform and twin-domain states is confined to the nickelate layers and leaves the aluminate layers unaffected. (3) Superlattices based on the high-temperature superconductor YBa2Cu3O7 exhibit an incommensurate charge density wave order that is stabilized by heterointerfaces. These results suggest that interfaces can serve as a powerful tool to manipulate the interplay between spin order, charge order, and superconductivity in cuprates and other transition metal oxides.

A one-of-a-kind text offering an introduction to the use of spectroscopic ellipsometry for novel material characterization In Introduction to Spectroscopic Ellipsometry of Thin Film Materials: Instrumentation, Data Analysis and Applications, a team of eminent researchers delivers an incisive exploration of how the traditional experimental technique of spectroscopic ellipsometry is used to characterize the intrinsic properties of novel materials. The book focuses on the scientifically and technologically important two-dimensional transition metal dichalcogenides (2D-TMDs), magnetic oxides like manganite materials, and unconventional superconductors, including copper oxide systems. The distinguished authors discuss the characterization of properties, like electronic structures, interfacial properties, and the consequent quasiparticle dynamics in novel quantum materials. Along with illustrative and specific case studies on how spectroscopic ellipsometry is used to study the optical and quasiparticle properties of novel systems, the book includes: Thorough introductions to the basic principles of spectroscopic ellipsometry and strongly correlated systems, including copper oxides and manganites Comprehensive explorations of two-dimensional transition metal dichalcogenides Practical discussions of single layer graphene systems and nickelate systems In-depth examinations of potential future developments and applications of spectroscopic ellipsometry Perfect for master’s- and PhD-level students in physics and chemistry, Introduction to Spectroscopic Ellipsometry of Thin Film Materials will also earn a place in the libraries of those studying materials science seeking a one-stop reference for the applications of spectroscopic ellipsometry to novel developed materials.

This thesis presents an experimental study of ordering phenomena in rare-earth nickelate-based heterostructures by means of inelastic Raman light scattering and elastic resonant x-ray scattering (RXS). Further, it demonstrates that the amplitude ratio of magnetic moments at neighboring nickel sites can be accurately determined by RXS in combination with a correlated double cluster model, and controlled experimentally through structural pinning of the oxygen positions in the crystal lattice. The two key outcomes of the thesis are: (a) demonstrating full control over the charge/bond and spin order parameters in specifically designed praseodymium nickelate heterostructures and observation of a novel spin density wave phase in absence of the charge/bond order parameter, which confirms theoretical predictions of a spin density wave phase driven by spatial confinement of the conduction electrons; and (b) assessing the thickness-induced crossover between collinear and non-collinear spin structures in neodymium nickelate slabs, which is correctly predicted by drawing on density functional theory.

In this thesis chemical and epitaxial degrees of freedom are used to manipulate charge and spin ordering phenomena in two families of transition metal oxides, while taking advantage of state-of-the-art resonant x-ray scattering (RXS) methods to characterize their microscopic origin in a comprehensive manner. First, the relationship of charge density wave order to both magnetism and the "pseudogap" phenomenon is systematically examined as a function of charge-carrier doping and isovalent chemical substitution in single crystals of a copper oxide high-temperature superconductor. Then, in copper oxide thin films, an unusual three-dimensionally long-range-ordered charge density wave state is discovered, which persists to much higher temperatures than charge-ordered states in other high-temperature superconductors. By combining crystallographic and spectroscopic measurements, the origin of this phenomenon is traced to the epitaxial relationship with the underlying substrate. This discovery opens new perspectives for the investigation of charge order and its influence on the electronic properties of the cuprates. In a separate set of RXS experiments on superlattices with alternating nickel and dysprosium oxides, several temperature- and magnetic-field-induced magnetic phase transitions are discovered. These observations are explained in a model based on transfer of magnetic order and magneto-crystalline anisotropy between the Ni and Dy subsystems, thus establishing a novel model system for the interplay between transition-metal and rare-earth magnetism.