Electron Spectroscopy Study of Correlation Mechanisms in CuGeO3 Single Crystal

X-ray (Al-K alpha) and resonant Cu 2p-->3d and Cu 3p-->3d photoemission valence-band spectra of high-quality CuGeO3 single crystals are reported and interpreted. In addition, an attempt is given to evaluate the charge transfer (Delta), the d-d Coulomb interaction (U-dd) energies, and the superexchange term (J) on the basis of L(2,3)M(4,5)M(4,5)-L(2,3)M(2,3)M(4,5) Auger transitions, and core-level spectra analyzed within the frame of the Anderson Hamiltonian in the impurity limit. The results clearly show that one-electron band-structure calculations do not account for the band gap approximate to 3.7 eV [M. Bassi, P. Camagni, R. Rolli, G. Samoggia, F. Parmigiani, and A. Revcolevschi (unpublished)] and the emission arising from many-body effects (correlated d(8) and d(8)-d(9) L hybridized states), while Delta and U-dd, found to be approximate to 4.2 and approximate to 6.7 eV, respectively, allow us to classify this compound as a charge-transfer insulator with a strong ionic character. In addition, energy-dependent electron-energy-loss measurements suggest that the forbidden d-d intraband transitions are centered at approximate to 1.6 eV, which justifies the blue color of CuGeO3, and the band gap is approximate to 3.7 eV, as required by the transparency of the crystal in the visible region of the electromagnetic spectrum. Finally, in the approximation allowed by the present models, J results to be of the order of -7 meV.