Götz Berner (PhD thesis)

Götz Berner (PhD thesis): Funktionelle oxidische Heterostrukturen aus dem Blickwinkel der Spektroskopie

Abstract

Oxide heterostructures exhibit a manifold of unexpected physical properties due to charge and spin rearrangement. Because of the possibility to control the conductivity, magnetism or high-temperature superconductivity, these artificial materials are prospective candidates for future application in microelectronics. However, this needs a fundamental understanding of the mechanism leading to such effects. This thesis addresses the investigations of such systems by photoassisted spectroscopy providing a direct access to the electronic structure. The further aim of this study is to establish applicable spectroscopic methods for characterizing the buried layers in heterostructures.
The study presented here deals with two prototypical oxide heterostructures. In the prominent LaAlO₃/SrTiO₃ heterostructure the formation of a two-dimensional electron system at the interface is observed, if the LaAlO₃ layer exceeds a critical thickness. The electronic reconstruction is discussed as the driving mechanism. In this study the twodimensional electron system is characterized by photoelectron spectroscopy and resonant inelastic x-ray scattering. The comparison of the charge carrier densities determined from spectroscopy with data from transport measurements indicates the coexistence of localized and mobile charge carriers at the interface. The analysis of core-level spectra as well as valence band spectra show that a modified electronic reconstruction picture is needed to explain the experimental observations. In such a scenario oxygen vacancies in the LaAlO₃ surface layer might provide the extra charge. By using resonant photoelectron spectroscopy momentum-resolved measurements were performed to observe the metallic states at the chemical potential. For the first time a mapping of the buried Fermi surface of an oxide heterostructure has been accomplished. Additionally, some Titanium-derived states were identified in the spectra which are probably localized by surrounding oxygen vacancies in the SrTiO₃. They are interpreted as a possible source of the ferromagnetism, which coexists with the superconducting phase in the LaAlO₃/SrTiO₃ heterostructure.
The other multilayer system studied here is the LaNiO₃-LaAlO₃ superlattice structure. Due to its closeness to the correlation-induced insulating phase the integration of the metallic LaNiO₃ in a heterostructure possibly opens the way to novel phases. Actually, a continuous metal-insulator transition is observed below a LaNiO₃ layer thickness of four unit cells, which is manifested in a loss of quasiparticle coherence in the valence band spectra. Even though the momentum-resolved data is affected by photoelectron diffraction, a Fermi surface can be identified. Its topology provides the possibility of Fermi surface nesting and the formation of a spin density wave. Thus, the results support the indication for a magnetic ordering in the two-dimensional ground state.

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