[en] In recent years epitaxial growth of transition metal oxides became possible and exotic phenomena were found at their interfaces. A prominent example is the formation of a high-mobility two-dimensional electron gas at the interface of LaAlO₃ grown on TiO₂-terminated SrTiO₃ substrates. Such heterointerfaces fabricated by pulsed laser deposition under various growth conditions were investigated with soft X-ray photoemission spectroscopy. We focused on stoichiometric aspects as well as in-gap states and differences in the core levels. Deviations from an abrupt interface without defects were found.

[en] The structure of LaAlO₃/SrTiO₃ interfaces is under intense discussion due to the appearance of metallic conductivity, magnetism and superconductivity. The most important phenomena supposed to be crucial for the observed effects are the polar catastrophe, the formation of oxygen vacancies and the intermixing of La and Sr at the interface. However, at present there is no consensus about the driving force of the unusual properties. Here we apply on and off resonant photoemission to differently prepared LaAlO₃/SrTiO₃ interfaces. In particular we study the behavior of the valence band, the Ti 2p, Sr 3d and La 4d lines. Significant differences are found as a function of sample treatment and photon energy. A model of the interface structure is developed.

No abstract available

[en] A large oxypnictides family LnFeAsO (Ln=La,Ce, Pr,Nd,Sm,Gd) has been found to be superconducting with a transition temperature up to 55 K and high upper critical fields. The onset of the superconducting critical temperature T_c in these compounds increases with the reduction of the rare-earth ionic size, and the highest T_c obtained so far is 55 K in doped SmFeAsO. In this contribution we present results of X-ray absorption (XAS) spectroscopy measurements on the soft X-ray regime for polycrystalline LaFeAsO, CeFeAsO, SmFeAsO and GdFeAsO at various temperature as well as for Ba_1-xK_xFe₂As₂ single crystals.

No abstract available

[en] We present first-time measurements of the Fermi surface and low-energy electronic structure of intermetallic compounds Gd₂PdSi₃ and Tb₂PdSi₃ by means of angle-resolved photoelectron spectroscopy (ARPES). We show that the Fermi surface in both compounds consists of an electron barrel at the Γ point surrounded by spindle-shaped electron pockets originating from the same band, with the band bottom of both features lying at 0.5 eV below the Fermi level. From the experimentally measured band structure, we estimate the momentum-dependent RKKY coupling strength and demonstrate that it is peaked at the 1/2Γ K wave vector. Comparison with neutron diffraction data from the same crystals shows perfect agreement of this vector with the propagation vector of the low-temperature in-plane magnetic order, thereby demonstrating the decisive role of the Fermi surface geometry in explaining the complex magnetically ordered ground state of ternary rare earth silicides.

[en] The mixed valence compound SmB_6 has been well known for its anomalous low temperature resistivity behavior for decades: At temperatures below 50 K, SmB_6 transmutes from a metal to an insulator but shows residual resistivity for temperatures less than 5 K. Renewed interest in this material comes from theoretical proposals, predicting topological protected surface states making this compound the prime candidate for the new material class of ''Topological Kondo Insulators''. Indeed, elaborate transport experiments have evidenced that the residual conductivity occurs only at the surface. However, it is generally well known that the surface of f-systems undergoes valence changes and reconstructions, which may also influence the surface properties of this material. Applying surface sensitive soft X-ray photoemission spectroscopy, we have investigated the surface properties of freshly cleaved SmB_6 single crystals at 15 K monitoring the Sm valance, the chemical state of boron as well as the surface stoichiometry, and also the development of these over time and with increased temperature: We have found that the surface shows an unexpected complexity stemming from both intrinsic and extrinsic changes.

[en] LaAlO₃ and NdGaO₃ thin films of different thicknesses have been grown by pulsed laser deposition on TiO₂-terminated SrTiO₃ single crystals and investigated by soft X-ray photoemission spectroscopy. The surface sensitivity of the measurements has been tuned by varying photon energy hν and emission angle Θ. In contrast to the core levels of the other elements, the Sr 3d line shows an unexpected splitting for higher surface sensitivity, signaling the presence of a second strontium component. From our quantitative analysis we conclude that during the growth process Sr atoms diffuse away from the substrate and segregate at the surface of the heterostructure, possibly forming strontium oxide