Local Symmetry and Domain Patterns: Ordering Phenomena in Transition-Metal Oxides

Transition-metal oxides (TMOs) show rich phase diagrams with various magnetic and electronic phases that arise from the complex interplay between spin, charge, orbital, and lattice degrees of freedom. The most prominent class of materials, which has attracted a lot of attention in the last decades, are probably high-temperature superconductors (HTS), in which the resitivity drops to zero below a critical temperature. Further examples are different kind of complex ordering phenomena, for example a modulation of the charge, spin or orbital density. In some systems a coexistence of energetically close phases is found. X-ray diffraction experiments are sensitive to a periodic arrangement of an electron-density modulation. If tuned to an absorption edge of one of the contained elements, a virtual transition from the core states to the valence states probes the occupation of the intermediate state and makes the process spectroscopically sensitive to the scattering center. Many of the interesting x-ray absorption edges of TMOs are in the energy interval between 400 and 3000 eV, in which also the absorption in air is strong. In Chap. 2 we report about commissioning and building up a novel ultar-high vacuum (UHV) diffractometer for soft x-ray diffraction experiments, which is meant as an user endstation at P04 beamline of the new synchrotron facility PETRA III. The instrument is based on a sufficiently large vacuum chamber with two differentially pumped rotary feedthroughs for an independent sample and detector rotation. The large detector circle allows to use complex detector setups, like an in-vacuum CCD and polarization analyzer. Therefore, the diffractometer is well suited for new techniques of gaining importance, like coherent x-ray diffraction (CXD), in which the finestructure of a diffraction reflex reveals information about the domain-wall dynamics, and like resonant soft x-ray diffraction (RSXD), which shows its full power when combined with a polarization analysis of the diffracted photons. In first experiments at DESY, BESSY II, and FLASH, the new machine has been successfully operated and helped to obtain data in unprecedented precision. Data recorded from multilayer reflectors, which will be used for the setup of the new polarization analyzer, and data that were recorded with the new CCD detector are presented in this chapter as well. Further data, from a RSXD experiment on magnetite are presented in Chap. 4. In Chap. 3 we report about a moderately surface-sensitive RSXD experiment in which for the first time charge stripe order in the proto-typical HTS La1.88Sr0. 12CuO4 (LSCO) was found. The strong similarity of the resonant diffraction spectra from the charge order reflex in LSCO to those from an La1.48Nd0. 4Sr0.12CuO4(LNSCO) reference sample, in which the existence of charge stripe order is well established, clearly shows the similar electronic origin. While charge stripe peaks were found in all LSCO samples that were studied, including in-situ cleaved and ex-situ polished ones, we could not find a reflex with bulk-sensitive hard x-ray diffraction experiments. From this we conclude that LSCO is close to the formation of a charge stripe order that already the reduced dimensionality in the near-surface region is sufficient to stabilize them in a layer of at least 5 nm thickness. In Chap. 4 we present a RSXD experiment from the (0 0 1/2) charge-orbital order reflex in Fe3O4. The azimuthal dependence of the linear dichroism shows a low-symmetry pattern, which reflects a strong monoclinic symmetry of the electronic3dwave func-tion. We found this monoclinic deviation prior to the refinements from the pseudo-orthorhombic to the monoclinic crystal structure, which was published this year. From the orbital ordering models that were proposed so far only a calculation, which is based on a complex-coefficient orbital ordering, could predict our experiment correctly. This theory further predicts a circular dichroism in the diffraction signal, which we indeed found in our experiment. In Chap. 5 we report about our results of CXD experiments on doped titanates, in which a coexistence of charge-ordered insulating an metallic regions was found. Charge ordered regions are characterized by superstructure reflections. Coherent illumination of these regions allows to produce an interference pattern that reflects information on the spatial arrangement of the insulating regions. The temporal evolution of this pattern provides insight into the dynamics due to domain reorganizations and domain-wall movements. First experiments at the ESRF and BESSY II were seriously limited due to the lacking stability of the sample setup. CXD experiments at BESSY II and PETRA III with a new sample setup, which was developed in cooperation with the University of Amsterdam, have shown that domain-wall dynamics take place on even shorter time scales than accessible. RSXD and CXD are powerful tools to study various properties of TMOs, ranging from the local electronicdwave symmetry in magnetite to the reorganization of mesoscopic domains. The new UHV diffractometer will allow to perform these kind of experiments with high precision and modern detector setups. --- This dissertation contains reprints of figures with permission from the American Physical Society. Readers may view, browse, and/or download material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. 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