Perovskite oxide heteroepitaxy : strain and interface engineering

Perovskite oxides are naturally suitable for heteroepitaxy. The new functionality of the heterostructures can be attributed to two fundamental effects in heteroepitaxy. At first the crystal structure of the layers is changed, due to the matching of the in-plane lattice constants to those of the substrate. The matching results in strain in the layers, the magnitude of which can be controlled with the use of an appropriate substrate. This is called strain engineering. Furthermore, the interfaces between different layers break symmetry and therefore new functionality can be expected at the interfaces. Heteroepitaxy allows for direct intervention, e.g. with dopant insertion, at the interface during growth, which is called interface engineering. In this thesis, an exploration of the possibilities with strain and interface engineering is made for both the LaAlO3/SrTiO3 (LAO/STO) interface and the fully spin polarized metal La0.67Sr0.33MnO3 (LSMO).\ud At first, the conducting interface between the two band insulators LAO and STO is studied. The main experimental result is that the expected electrostatic potential buildup in the LAO layer is not observed. This indicates that the most widely used model to explain the conductivity at the interface, the electronic reconstruction due to the polar discontinuity, is not applicable to the experiments. An alternative model for the conductivity is proposed.\ud In LSMO thin films, the crystal structure is determined by the strain from the underlying substrate. With the choice of a specific substrate surface, the magnetic properties of the LSMO layer can be controlled. A model was developed which predicts the magnetocrystalline anisotropy. The model was verified with measurements of the\ud magnetic anisotropy of LSMO thin films grown on different surfaces of NdGaO3 single crystal substrates.\ud The use of LSMO in spintronic devices requires spin polarized conductivity at the interface. Due to interface reconstructions, generally the spin polarized conductivity is reduced at the interface. Interface engineering was applied to study and improve the properties of the LSMO

In-gap features in superconducting LaAlO3_3-SrTiO3_3 interfaces observed by tunneling spectroscopy

We identified quasiparticle states at well-defined energies inside the superconducting gap of the electron system at the LaAlO$_3$-SrTiO$_3$ interface using tunneling spectroscopy. The states are found only in a number of samples and depend upon the thermal-cycling history of the samples. The states consist of a peak at zero energy and other peaks at finite energies, symmetrically placed around zero energy. These peaks disappear, together with the superconducting gap, with increasing temperature and magnetic field. We discuss the likelihood of various physical mechanisms that are known to cause in-gap states in superconductors and conclude that none of these mechanisms can easily explain the results. The conceivable scenarios are the formation of Majorana bound states, Andreev bound states, or the presence of an odd-frequency spin triplet component in the superconducting order parameter.Comment: 11 pages, 5 figure

Structural phases driven by oxygen vacancies at the La0.7Sr0.3MnO3/SrTiO3 hetero-interface

An oxygen vacancy driven structural response at the epitaxial interface between La0.7Sr0.3MnO3 films and SrTiO3 substrates is reported. A combined scanning transmission electron microscopy and electron energy loss spectroscopy study reveal the presence of an elongated out-of-plane lattice parameter, coupled to oxygen vacancies and reduced manganese oxidation state at the La0.7Sr0.3MnO3 side of the interface. Density functional theory calculations support that the measured interface structure is a disordered oxygen deficient brownmillerite structure. The effect of oxygen vacancy mobility is assessed, revealing an ordering of the vacancies with time

Bacterial carbon sources in coastal sediments: a cross-system analysis based on stable isotope data of biomarkers

International audienceCoastal ecosystems are typically highly productive, and the sediments in these systems receive organic matter from a variety of local and imported sources. To assess if general patterns are present in the origin of carbon sources for sedimentary bacteria and their relation to the origin of the sediment organic carbon pool, we compiled both literature and new data on ?13C of bacterial biomarkers (the phospholipid derived fatty acids i+a15:0), along with ?13C data on sediment organic carbon (?13CTOC) and macrophyte biomass from a variety of typical near-coastal systems. These systems included mangroves, salt marshes (both C3 and C4-dominated sites), seagrass beds, and macroalgae-based systems, as well as unvegetated sediments. First, our ?13Ci+a15:0 data showed large variability over the entire range of ?13CTOC, indicating that in many settings, bacteria may depend on carbon derived from various origins. Secondly, systems where local macrophyte production is the major supplier of organic carbon for in situ decomposition are generally limited to organic carbon-rich, peaty sites (TOC>10 wt%), which are likely to make up only a small part of the global area of vegetated coastal systems. These carbon-rich sediments also provided a field based estimate of isotopic fractionation between bacterial carbon sources and biomarkers (-3.7±2.1), which is similar to the expected value of about -3 associated with the biosynthesis of fatty acids. Thirdly, only in systems with low TOC (below ~1 wt%), we consistently found that bacteria were selectively utilizing an isotopically enriched carbon source, which may be root exudates but more likely is derived from microphytobenthos. In other systems with between ~1 and 10 wt% TOC, bacteria appear to show on average little selectivity and ?13Ci+a15:0 data generally follow the ?13CTOC, even in systems where the TOC is a mixture of algal and macrophyte sources that generally are believed to have a very different degradability