Browsing by Author "Zegkinoglou, Ioannis"

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Resonant and high-energy X-ray scattering studies on strongly correlated electron systems in transition metal oxides
(2007) Zegkinoglou, Ioannis; Keimer, Bernhard (Prof. Dr.)
The strongly correlated electron systems in Ca_{2-x}Sr_xRuO_4, RuSr_2GdCu_2O_8 and YBa_2Cu_3O_{6+x} transition metal oxides were investigated with resonant and high-energy x-ray scattering techniques. The main results and conclusions of the work are briefly summarized in the following. Single crystals of Ca_2RuO_4 and Ca_{1.9}Sr_{0.1}RuO_4 were investigated with resonant x-ray diffraction at the Ru LII and LIII absorption edges, i.e. at photon energies E=2.968 keV and E=2.838 keV, respectively. At the magnetically allowed (1 0 0) and (0 1 1) reciprocal space positions a new ordered phase was discovered above the magnetic phase transition at T_N=110 K. The scattering from this phase has a strongly resonant character, with non-zero intensity only at energies very close to the absorption edges. It is observed exclusively in the sigma-pi polarization channel, hence is characterized by a polarization which is parallel to the scattering plane. The scattering intensity in the new phase decreases smoothly with increasing temperature, without showing any anomalies, and vanishes at a phase transition at approximately 260 K. Based on the polarization and temperature dependences, as well as on complementary muon spin rotation measurements, which indicate no ordered magnetic moment above the Néel temperature, we can draw the conclusion that the resonant scattering in the new phase originates from the ordering of the Ru 4d orbitals. The orbital ordering phase transition is also observed in the strontium-doped Ca_{1.9}Sr_{0.1}RuO_4 system, but at a lower temperature of approximately 130 K. The propagation vector of the orbital order remains unchanged in the doped compound, despite the change of the magnetic structure. Furthermore, an additional resonant signal was observed in both investigated Ca_{2-x}Sr_xRuO_4 systems at the (1 1 0) position, which is forbidden both magnetically and by the space-group. The intensity of this signal decreases smoothly with increasing temperature with no anomalies up to the metal-insulator transition. Based on the temperature and polarization dependences of the scattering intensity at (1 1 0), this can be attributed to the tilt order of the RuO_6 octahedra. Micrometer-sized single crystals of RuSr_2GdCu_2O_8 were investigated with resonant x-ray diffraction at the Ru LII absorption edge. Based on the azimuthal dependence of the scattering intensity at the (1/2 1/2 1/2) magnetic position, the exact direction of the magnetic moment in the material could be determined. The magnetic moment was found to be aligned not along the crystallographic c-direction, as initially suggested by neutron scattering investigations, but instead along a direction which forms an angle of approximately 53 degrees with the c-axis. In addition, the magnetic order parameter of the Ru spin-system was determined. The experimental data indicate a possible influence of the onset of superconductivity on the magnetic order parameter, but this is still to be confirmed. Single crystals of underdoped, optimally doped and overdoped YBa_2Cu_3O_{6+x}, as well as stoichiometric YBa_2Cu_4O_8, were investigated with high-energy non-resonant x-ray diffraction at photon energies of 100 keV and 115 keV. A new superstructure with periodicity equal to four unit cells was discovered in optimally doped YBa_2Cu_3O_{6.92} and overdoped Y_{0.8}Ca_{0.2}Ba_2Cu_3O_{6.95}. The superstructures are practically identical in the two compounds, which have almost the same oxygen content, but very different charge carrier concentrations. This indicates that the superstructures do not originate from electronic stripes in the CuO_2 planes, but rather from oxygen vacancy ordering in the Cu-O chains. This conclusion is supported by two more independent observations: the persistence of the superstructures up to temperatures well above room temperature; and the absence of diffuse scattering features in the stoichiometric YBa_2Cu_4O_8 compound, which contains no oxygen vacancies and should therefore make the observation of stripes easier. If such stripes do exist in YBa_2Cu_3O_{6+x}, then the intensity of the associated diffuse features must be at least one order of magnitude smaller than that of the aboved described signatures of oxygen order. The comparison of the diffuse scattering patterns of YBa_2Cu_3O_{6.92} samples containing different oxygen isotopes (O-16 vs. O-18) revealed that these are identical. This means that variations in the oxygen order in the Cu-O chains cannot be responsible for isotope effects reported in previous work on this material.