Many transition metal oxides (TMOs) show complex physics, ranging from ferroelectricity to magnetism, high-Tc superconductivity and colossal magnetoresistance. The existence of a variety of ground states often occurs as different degrees of freedom (e.g. lattice, charge, spin, orbital) interact to form different competing phases which are quite similar in energy. The capability to epitaxially grow heterostructures of TMOs increased the complexity even more as new phenomena can emerge at the interface. One typical example is the two-dimensional electron system (2DES) at the interface of two insulating oxides, namely LaAlO3/SrTiO3, which shows metal-to-insulator transitions, magnetism or gate-tunable superconductivity. The origin of this thesis was the discovery of a similar 2DES at the bare surface of SrTiO3 fractured in vacuum, making it possible to study its electronic structure by angle-resolved photoemission spectroscopy (ARPES).In this thesis, the study of well-prepared surfaces, instead of small fractured facets, results in spectroscopic data showing line widths approaching the intrinsic value. This approach allows a detailed analysis of many-body phenomena like the renormalization of the self-energy due to electron-phonon interaction.Additionally, the understanding of the electronic structure of the 2DES at the surface of SrTiO3(001) was given an additional turn by the surprising discovery of a complex spin texture measured by spin-ARPES. In this thesis data is presented which contradicts these conclusions and discusses possible reasons for the discrepancy.One major motivation of this thesis was the question if and how the electronic structure and the properties of the 2DES can be changed or controlled. In this context, the study of 2DESs at (110) and (111) surface revealed that the electronic band structure of the 2DES (orbital ordering, symmetry of the Fermi surface, effective masses) can be tuned by confining the electrons at different surface orientations of the same material, namely SrTiO3.A major achievement of this thesis is the generalization of the existence of a 2DES in SrTiO3 to many other surfaces and interfaces of TMOs (TiO2 anatase, CaTiO3, BaTiO3) and even simpler oxides already used in modern applications (ZnO). In all these oxides, we identify oxygen vacancies as the origin for the creation of the 2DESs.In anatase and other doped d0 TMOs, both localized and itinerant electrons (2DES) can exist due to oxygen vacancies. Which of the two cases is energetically favorable depends on subtle differences as demonstrated by studying two polymorphs of the same material (anatase and rutile).In CaTiO3, the oxygen octahedron around the Ti ion is slightly tilted. This symmetry breaking results in the mixing of different d-orbitals demonstrating again why and how the electronic structure of the 2DES can be altered.In BaTiO3, the creation of a 2DES results in the coexistence of the two, usually mutual exclusive, phenomena of ferroelectricity and metallicity in the same material by spatially separating the two.Moreover, this work demonstrates that the 2DES also exists in ZnO which is - compared to the Ti-based oxides - rather a conventional semiconductor as the orbital character of the itinerant electrons is of s and not d-type.The main result of this thesis is the demonstration of a simple and versatile technique for the creation of 2DESs by evaporating Al on oxide surfaces. A redox reaction between metal and oxide results in a 2DES at the interface of the oxidized metal and the reduced oxide. In this thesis the study of such interfacial 2DESs was limited to photoemission studies in ultra high vacuum. However, this technique opens up the possibility to study 2DESs in functional oxides in ambient conditions by e.g. transport techniques, and might be an important step towards cost-efficient mass production of 2DESs in oxides for future applications.

This book summarizes the most recent and compelling experimental results for complex oxide interfaces. The results of this book were obtained with the cutting-edge photoemission technique at highest energy resolution. Due to their fascinating properties for new-generation electronic devices and the challenge of investigating buried regions, the book chiefly focuses on complex oxide interfaces. The crucial feature of exploring buried interfaces is the use of soft X-ray angle-resolved photoemission spectroscopy (ARPES) operating on the energy range of a few hundred eV to increase the photoelectron mean free path, enabling the photons to penetrate through the top layers – in contrast to conventional ultraviolet (UV)-ARPES techniques. The results presented here, achieved by different research groups around the world, are summarized in a clearly structured way and discussed in comparison with other photoemission spectroscopy techniques and other oxide materials. They are complemented and supported by the most recent theoretical calculations as well as results of complementary experimental techniques including electron transport and inelastic resonant X-ray scattering.

This book summarizes the current knowledge of two-dimensional oxide materials. The fundamental properties of 2-D oxide systems are explored in terms of atomic structure, electronic behavior and surface chemistry. The concept of polarity in determining the stability of 2-D oxide layers is examined, charge transfer effects in ultrathin oxide films are reviewed as well as the role of defects in 2-D oxide films. The novel structure concepts that apply in oxide systems of low dimensionality are addressed, and a chapter giving an overview of state-of-the-art theoretical methods for electronic structure determination of nanostructured oxides is included. Special emphasis is given to a balanced view from the experimental and the theoretical side. Two-dimensional materials, and 2-D oxides in particular, have outstanding behavior due to dimensionality and proximity effects. Several chapters treat prototypical model systems as illustrative examples to discuss the peculiar physical and chemical properties of 2-D oxide systems. The chapters are written by renowned experts in the field.

These volumes contain 365 of the 505 papers presented at the VUV-11 Conference, held at Rikkyo University, Tokyo, from August 27th to September 1st 1995. The papers are divided into three sections: atomic and molecular spectroscopy, solid state spectroscopy and instrumentation and technological applications. New aspects presented were both quantitative and qualitative improvements in fluorescence spectroscopy and magnetic circular dichroism measurements. The fluorescence data are complementary to those of photoemission in a sense but they appear to open up a new method to analyze the optical excitation and relaxation processes. The application of magnetic circular dichroism has proved to be useful not only in analyzing the electronic structures of magnetic materials but also in practical applications to material engineering as found in experiments combined with photoelectron microscopy. Excellent developments in applications are only found in the field of surface photochemistry, where the technique of etching using VUV light has been appreciably refined. Although the majority of distinctive scientific features in the VUV-11 Conference have been brought about by the application of synchrotron radiation, experiments using a different type of light source appear to have progressed steadily. This is evident in the studies of plasma radiation.

This book provides the first complete and up-to-date summary of the state of the art in HAXPES and motivates readers to harness its powerful capabilities in their own research. The chapters are written by experts. They include historical work, modern instrumentation, theory and applications. This book spans from physics to chemistry and materials science and engineering. In consideration of the rapid development of the technique, several chapters include highlights illustrating future opportunities as well.