Photoelectron Spectroscopy of Cuprates and Manganites

Detta är en avhandling från Department of Synchrotron Radiation Research, Lund University

Sammanfattning: Abstract In this thesis, the high temperature superconductors (HTSC) La2CuO4+x and Bi2Sr2CaCu2O8+d and the colossal magnetoresistive (CMR) La1-xSrxMnO3 have been studied by photoelectron spectroscopy. Techniques such as angular resolved photoemission (ARPES), resonant photoemission spectroscopy (RESPES) and x-ray absorption spectroscopy have been applied, using synchrotron radiation. The aim of the study has been to investigate the electronic structure of these materials. By using resonance photoemission spectroscopy on La2CuO4+x single crystals, we have found the presence of La states in a large region in the valence band and strong evidence for the existence of surface derived La-O electronic bands in the c^surface orientation. We have also investigated the influence of surface termination effects by using different methods of cleaving the samples. This reveals that the topmost layer can be either La-O or Cu-O, giving rise to remarked different photoelectron spectra. The generally accepted picture of the Fermi-surface topology of B2Sr2CaCu2O8, is large hole-like crossings centered around the X(Y) symmetric point. We have performed angular resolved measurements showing that by choosing a proper photon energy and experimental geometry, an electron-like Fermi surface can clearly be seen centered at the G-point. We have also found that the ARPES holelike Fermi surface is photon energy dependent. Magnetoresistant materials such as the La1-xSrxMnO3 exhibit a complicated phase diagram as function of doping and temperature. Many questions remain to be answered regarding their electronic and magnetic structure. By core level spectroscopy on single crystal in a wide doping region and for different temperatures temperature. Doping and temperature dependent shifts were seen in the core levels, reflecting the changes in screening and band filling connected to the different regime in the phase diagram. The properties of the states in the valence band and close to the Fermi level are of fundamental importance of describing the electronic properties of the complex metal oxides. Up to now, few systematic studies of the valence band with respect to doping and temperature have been performed. By angular resolved and angular integrated valence band spectroscopy on single crystals we have observed binding energy shifts of the spectral features in the valence band for increasing Sr concentration, as well as an increase of the density of states close to the Fermi level. A feature, which has not previously been observed, was found in the valence band. Resonant photoemission close to the Mn 2p threshold shows that this feature is related to Mn states. The nature of this feature is discussed in terms of band structure and Mn 3d orbital orientation.

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