Untersuchung der elektronischen Struktur von Hochtemperatursupraleitern und verwandten Uebergangsmetalloxiden mit Nahkanten-Roentgenabsorptionsspektroskopie

The unoccupied electronic structure and its orbital character has been studied with polarization-dependent near-edge x-ray absorption spectroscopy (NEXAFS) for selected high-temperature superconductors (HTSC) and related transition metal oxides. Although YBa_2Cu_3O_7_-_#delta# (Y-123) is arguably the best-investigated HTSC a conclusive NEXAFS study on how partial substitution of Cu by other transition metals affects the electronic structure has sorely been missing. The study presented here on a series of well characterized YBa_2Cu_3_-_xFe_xO_y single crystals shows that the cause for T_c suppression is not at all magnetic pair breaking but charge carrier depletion, primarily in the chains; effects from disorder cannot be excluded. Annealing at high oxygen pressure increases along with oxygen content both the hole concentration and T_c. Fe 3d-O 2p-derived states contribute prominently to the spectra for all polarizations a few eV above E_F. Iron prefers a trivalent state in Y-123; upon reduction a spin transition can be observed. As YBa_2Cu_3_-_xFe_xO_y single crystals cannot be detwinned it is very difficult to distinguish between contributions from planes and chains to the spectra. In this situation thin films grown with a reduced degree of twinning ('twin-poor') allow more detailed investigations. An extended self-absorption correction was developed for fluorescence yield NEXAFS on epitactical HTSC thin films. Its application to twin-poor Y-123 thin films demonstrates that (apart from the effect of residual twins) the spectral information is equivalent to that of detwinned single crystals for a range of optimum film thicknesses, and this in turn allows to augment the NEXAFS study of YBa_2Cu_3_-_xFe_xO_y with spectra for corresponding twin-poor thin films. The system Ca_2_-_x(Sr,La)_xRuO_4 is structurally related to the HTSCs; the development of its unoccupied electronic structure with x was investigated in this work, with emphasis on the metal-insulator transitions. The NEXAFS spectra reflect well most changes in the crystal structure; furthermore, the Hubbard-like splitting of a near-E_F band can be observed for increasing Ca content, showing directly that correlation effects become more pronounced due to band narrowing. (orig.)Available from TIB Hannover: ZA 5141(6575) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman