Synchrotron studies of electronic and magnetic properties of oxide heterostructure interfaces
This thesis focuses on two projects involving resonant x-ray experiments on transition metal oxide heterostructures. The first project is the study of the interface between the anti-ferromagnetic Mott-Hubbard insulator LaTiO3 and the anti-ferromagnetic charge transfer insulator LaFeO3, which has previously been observed to exhibit interfacial charge transfer. The goal is an understanding of complex charge transfer and magnetic properties of the interface, which may provide insight into useful functionalities of the interface and possible use in electronic and magnetic devices. First, background information on the 3d elements and components forming this interface is given, the experimental techniques used to study the interface and methods to analyze the acquired data by Prof. Green are described. A combination of resonant x-ray reflectometry and x-ray absorption spectroscopy allowed us to verify the structural quality of the interface, detect the interface charge transfer, and determine the spin state of relevant atoms. For the second project of this thesis, we study the role of oxygen vacancies in forming 2D electron liquid (2DEL) at the interface of LaAlO3/SrTiO3, a conducting sheet of charge which emerges between two otherwise insulating materials. Previous studies of this interface have revealed that it exhibits electrical conductivity, superconductivity and ferromagnetism. The fields of oxide interfaces in general, and oxide 2D electron liquids in particular, are currently of high interest as they show great potential for device applications. Our experiments study the luminescence of these interface oxygen vacancies, using X-ray-excited optical luminescence (XEOL), in a set of samples where variation in growth conditions were used as an attempt to control the distribution profile of oxygen vacancies. By modifying the incident photon energy during XEOL experiments, we control the probing depth of our x-rays which stimulate the luminescence of the oxygen vacancies, and thus probe their depth profile. Our resulting depth profiles can distinguish if the oxygen vacancies are fully confined to the interface where the 2DEL resides, or if they are distributed further into the substrate. These results give key insight into the interaction between vacancies and conduction electrons, and how to achieve high mobility electron liquids.
oxygen vacancies, oxide heterostructures, resonant x-ray reflectometry, 2D electron liquid (2DEL), LaAlO3/SrTiO3, X-ray-excited optical luminescence (XEOL)
Master of Science (M.Sc.)
Physics and Engineering Physics