Photoelectron Spectroscopy of High Temperature Superconductors and Related Compounds

Detta är en avhandling från Henrik Nylén, Dept. of Synchrotron Radiation Research, Box 118, S-221 00 Lund

Sammanfattning: In this thesis, the high temperature superconductors (HTSC) Ba0.6K0.4BiO3, La2CuO4+x, Bi2Sr2Ca1-xYxCu2O 8+d, Bi2Sr2CuO6+d as well as semiconducting Ba0.9K0.1BiO3 and the strongly magnetoresistant material La1-xSrxMnO3 have been studied by photoelectron spectroscopy. By using the tunability of synchrotron radiation, techniques such as resonant photoemission and x-ray absorption spectroscopy have been applied. The aim of the studies has been to investigate the electronic structure of these compounds with an emphasis on strong electron-electron correlation, the distribution of valence states, band structure effects, the importance of oxygen nonstoichiometry, and the influence of metal overlayers. Experimental results from the high quality single crystals are compared to band structure calculations and atomic absorption calculations. Resonant photoemission was performed on Ba1-xKxBiO3 at photon energies in the vicinity of the Ba 4d, Ba 5p and O 1s core levels. The enhancements seen in the valence band at photon energies corresponding to the Ba 5p-5d excitation are assigned to Ba states. From the resonance at the Ba 4d threshold, Ba states were also identified close to the Fermi level whereas no resonant enhancement was seen for photon energies close to the O K absorption threshold. In addition, angle resolved measurements were used to identify the G and X high symmetry points in Ba0.6K0.4BiO3, and laterally resolved photoelectron microscopy showed a strongly varying Fermi level intensity on a micron length scale. For Bi2Sr2CaCu2O8+d and Bi2Sr2CuO6+d, a satellite enhancement was seen for photon energies close to the main oxygen K-edge absorption threshold. From the data, the values of the electron-electron interaction energies Udd and Upp, as well as the charge transfer energy D, can be found. The core level shifts and the valence-band shape in cuprate HTSC were also investigated by means of yttrium substitution and surface derived electronic bands. For La1-xSrxMnO3, core-level spectroscopy and x-ray absorption measurements indicated that x and temperature dependent changes in the electronic structure can be connected to the various magnetic phases. Temperature dependent resonant photoemission at the O K and Mn L3 edges showed enhanced spectral intensity close to the Fermi level and for photon energies around the Mn L edge, strong enhancements were seen in the valence band. The importance of small deviations in oxygen content is common for all compounds investigated in this thesis, although x-ray absorption and resonant photoemission measurements showed that O 2p holes play different roles in the doping mechanisms of cuprate and bismuthate high temperature superconductors as well as of magnetoresistant manganites.

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