Linearly polarized CuK-edge absorption spectroscopy of CuGeO3: Orbital population, band dispersion, and exchange interactions

We studied the unoccupied electronic states of CuGeO3 using linearly polari zed Cu It-edge absorption spectroscopy. The spectra show structures related to the Cu 4p(z) band with E parallel to e axis and to the Cu 4p(y) band wi th E parallel to b axis. The Cu 4p(z) band appears at higher energies than the Cu 4p(y) band due to the stronger interactions with the O 2p levels. Th e Cu 4p spectral weight is split about 5 eV into the so-called well screene d and poorly screened peaks. The Cu 4p bands exhibit changes as a function of the angle theta between the polarization vector and the c axis. The inte nsity changes reflect the polarization and the energy shifts reflect the di spersion of the Cu 4p bands. The pre-edge feature observed in the spectra i s directly related to the unoccupied Cu 3d band. The angular dependence of the pre-edge peak shows that the Cu 3d hole is completely polarized along t he c axis. The crystal axes direction seems to be more important for the an gular dependence than the local axes defined by the distorted oxygen octahe dra. The main character of the Cu 3d hole obtained from an orbital populati on analysis is 3d(xz). The dispersion of the Cu 3d band expected from previ ous calculations and confirmed by the experimental data is very small, abou t 0.6-0.7 eV. The small dispersion and orbital polarization help to explain the relatively weak and anisotropic exchange interactions in CuCeO3. [S016 3-1829(99)09719-2].