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Ordering Phenomena in Rare-Earth Nickelate Heterostructures
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.
Spin Spirals and Charge Textures in Transition-Metal-Oxide Heterostructures
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 experimen...
Uniaxial Pressure Study of Charge Density Waves in a High-T꜀ Cuprate Superconductor
This book presents comprehensive studies of charge density waves (CDW) in a high-Tc cuprate superconductor using x-ray scattering techniques under uniaxial pressure. Specifically, the work addresses inelastic x-ray scattering studies under uniaxial pressure performed on the underdoped cuprate YBa2Cu3O6.67(p=0.12, Tc=65K) with incoming photon energy in the resonant (E=931.3 eV, Cu-L3 edge) and non-resonant conditions (E=17.794 keV). This is a completely new approach to the investigation of charge density waves. It revealed new features of charge density waves in cuprates, whose properties had previously been inaccessible..
Controlling Collective Electronic States in Cuprates and Nickelates
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 dis...
