In between matters: interfaces in complex oxides

Complex oxides are emerging as a versatile class of materials, exhibiting a wide variety of properties. In recent years, it has become increasingly clear that the properties of complex-oxide interfaces can differ considerably from those of the bulk. This opens up the possibility of tuning and improving interface properties and the stabilization of novel phases at the interface. This thesis therefore focuses on complex oxide interfaces. Their properties are studied for four different cases. In the first case, the influence of SrTiO3 substrates on the high-Tc superconductor and strongly correlated electron system La2-xSrxCuO4 is examined. It is found that an epitaxial-strain-enforced lattice modification induced by a phase transition in the substrate causes a change in the conductance anisotropy as well as in the behavior of the magnetoresistance. The results are interpreted in terms of the pinning of fluctuating stripes. The second case is a tunnel study on La2-xSrxCuO4. The obtained tunnel spectra indicate a suppressed carrier density at the interface between the superconductor and the tunnel barrier. The third case investigates the YBCO/LSMO interface. It is found that the resistance of this interface is large compared to a YBCO/Au interface, which is explained by the phenomenon of charge transfer across the interface. Resistance switching effects around Tc of LSMO/YBCO/LSMO trilayers are shown to arise from magnetic stray fields penetrating the superconductor. The conducting interface between the insulators LaAlO3 and SrTiO3¬ is the subject of the last case. The large magnetoresistance exhibited by this interface at low temperature is attributed to spin scattering off localized magnetic moments that are induced at the interface. The four cases identify strain, charge transfer, interplay between different kinds of magnetic ordering, stray fields, and structural as well as electronic reconstructions as factors influencing oxide behavior at the interface, which enable the manipulation of a wide range of phenomena.