The Electronic Structure of Spin Ladders and Spin Chains Studied by Ellipsometry

Within the scope of this thesis, a spectroscopic ellipsometer has been put into operation and equipped with a UHV liquid Helium flow cryostat to enable low temperature optical measurements down to 10 K in the energy range between 0.8 to 5 eV. After successful installation of the ellipsometer, a systematic investigation of the electronic structure of one-dimensional copper-oxide spin ladders and spin chains was carried out. Ellipsometric optical conductivity data of the biaxial copper oxide compounds La5.2Ca8.8Cu24O41 (LCO) with 0.2 holes per formula unit and Sr14-xCaxCu24O41 for x = 0 (SCO) and x = 5, 8 (SCCO), doped with 6 holes per formula unit, were derived in all three crystallographic axes. We investigated the temperature, hole-doping and Ca-doping dependence of the interband excitations. We find a multipeak structure in the interband region in the two crystallographic directions a and c. A pronounced temperature dependence in the optical conductivity along c is observed. We investigated the dependence of those absorptions on temperature and Ca content and identified them with CT excitations of different character by using a Drude-Lorentz parameter fit and by comparison with the literature. We find a new interpretation of the lowest interband excitation in the optical conductivity along c that we identify with a ZN excitonic resonance (a ZN exciton is described as a bound pair of empty site Cu+ and a Zhang-Rice singlet state on a neighboring site) and the corresponding continuum at higher energy. A new absorption observed in x = 0 shows a strong dependence on temperature that could be related to charge-ordering temperatures in SCO. To further investigate the temperature dependence of the optical spectra along c at low energy, we combined the ellipsometric data with far and mid-infrared reflectance and transmittance data from FT spectroscopy and performed a Kramers-Kronig transformation followed by a spectral-weight analysis. This showed a spectral-weight transfer from the CT-region to the low-energy region with rising temperature in x = 0, 5, and 8. Moreover, a spectral weight shift with Ca substitution x is observed below 2.5 eV. Another compound investigated by ellipsometry within the scope of this thesis was the 1D zigzag chain SrCuO2. We investigated the optical conductivity along the chains and perpendicular to the chains as a function of temperature. The spectrum shows a prominent absorption feature at 2 eV that is assigned to a charge-transfer excitonic resonance and one with lower intensity at 4.2 eV that is assigned to one-center charge-transfer excitation similar to those observed in basically all low dimensional copper oxides. In summary, the data show that a unified picture might be found for the electron hole excitations in low dimensional cuprates