Electronic phase transitions in Pr$_{0.5}$Ca$_{0.5}$MnO$_3$ epitaxial thin films revealed by resonant soft x-ray scattering

We report the study of magnetic and orbital order in Pr$_{0.5}$Ca$_{0.5}$MnO$_3$ epitaxial thin films grown on (LaAlO$_3$)$_{0.3}$-(SrAl$_{0.5}$Ta$_{0.5}$O$_3$)$_{0.7}$ (LSAT) (011)$_c$. In a new experimental approach, the polarization and energy dependence of resonant soft x-ray scattering are used to reveal significant modifications of the magnetic order in the film as compared to the bulk, namely (i) a different magnetic ordering wave vector, (ii) a different magnetic easy axis and (iii) an additional magnetic reordering transition at low temperatures. These observations indicate a strong impact of the epitaxial strain on the spin order, which is mediated by the orbital degrees of freedom and which provides a promising route to tune the magnetic properties of manganite films. Our results further demonstrate that resonant soft x-ray scattering is a very suitable technique to study the magnetism in thin films, to which neutron scattering cannot easily be applied due to the small sample volume.

Resonant elastic soft x-ray scattering in oxygen-ordered YBa_2Cu_3O_{6+delta}

Static charge-density wave (CDW) and spin-density wave (SDW) order has been convincingly observed in La-based cuprates for some time. However, more recently it has been suggested by quantum oscillation, transport and thermodynamic measurements that density wave order is generic to underdoped cuprates and plays a significant role in YBa_2Cu_3O_{6+delta} (YBCO). We use resonant soft x-ray scattering at the Cu L and O K edges to search for evidence of density wave order in Ortho-II and Ortho-VIII oxygen-ordered YBCO. We report a null result -- no evidence for static CDW order -- in both Ortho-II and Ortho-VIII ordered YBCO. While this does not rule out static CDW order in the CuO_2 planes of YBCO, these measurements place limits on the parameter space (temperature, magnetic field, scattering vector) in which static CDW order may exist. In addition, we present a detailed analysis of the energy and polarization dependence of the Ortho-II superstructure Bragg reflection [0.5 0 0] at the Cu L edge. The intensity of this peak, which is due to the valence modulations of Cu in the chain layer, is compared with calculations using atomic scattering form factors deduced from x-ray absorption measurements. The calculated energy and polarization dependence of the scattering intensity is shown to agree very well with the measurement, validating the approach and providing a framework for analyzing future resonant soft x-ray scattering measurements.

Interface reconstruction in V-oxide heterostructures determined by x-ray absorption spectroscopy

We present an x-ray absorption study of the dependence of the V oxidation state on the thickness of LaVO$_3$ (LVO) and capping LaAlO$_3$ (LAO) layers in the multilayer structure of LVO sandwiched between LAO. We found that the change of the valence of V as a function of LAO layer thickness can be qualitatively explained by a transition between electronically reconstructed interfaces and a chemical reconstruction. The change as a function of LVO layer thickness is complicated by the presence of a considerable amount of V$^{4+}$ in the bulk of the thicker LVO layers.

Electronic structure of the SrTiO$_3$/LaAlO$_3$ interface revealed by resonant soft x-ray scattering

We investigated the electronic structure of the SrTiO$_3$/LaAlO$_3$ superlattice (SL) by resonant soft x-ray scattering. The (003) peak, which is forbidden for our ideal SL structure, was observed at all photon energies, indicating reconstruction at the interface. From the peak position analyses taking into account the effects of refraction, we obtained evidence for electronic reconstruction of Ti 3d and O $2p$ states at the interface. From reflectivity analyses, we concluded that the AlO$_2$/LaO/TiO$_2$/SrO and the TiO$_2$/SrO/AlO$_2$/LaO interfaces are quite different, leading to highly asymmetric properties.

Anisotropic quasiparticle renormalization in Na0.73CoO2: role of inter-orbital interactions and magnetic correlations

We report an angular resolved photoemission study of Na0.73CoO2 where it is found that the renormalization of the quasiparticle (QP) dispersion changes dramatically upon a rotation from GM to GK. The comparison of the experimental data to the calculated band structure reveals that the QP-renormalization is most pronounced along the GK-direction, while it is significantly weaker along the GM-direction. We discuss the observed anisotropy in terms of multiorbital effects and point out the relevance of magnetic correlations for the band structure of Na0.73CoO2.