Electron-spectroscopy study of correlation mechanisms inCuGeO3ssingle crystals

X-ray (Al-Kα) and resonant Cu 2p→3d and Cu 3p→3d photoemission valence-band spectra of high-quality ${\mathrm{CuGeO}}_3$ single crystals are reported and interpreted. In addition, an attempt is given to evaluate the charge transfer (Δ), the d-d Coulomb interaction (${\mathrm{U}}_{\mathrm{dd}}$ ) energies, and the superexchange term (J) on the basis of ${\mathrm{L}}_{2,3}$ ${\mathrm{M}}_{4,5}$ ${\mathrm{M}}_{4,5}$ -${\mathrm{L}}_{2,3}$ ${\mathrm{M}}_{2,3}$ ${\mathrm{M}}_{4,5}$ 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not account for the band gap ≈3.7 eV [M. Bassi, P. Camagni, R. Rolli, G. Samoggia, F. Parmigiani, and A. Revcolevschi (unpublished)] and the emission arising from many-body effects (correlated ${\mathrm{d}}^8$ and ${\mathrm{d}}^8$ -${\mathrm{d}}^9$ L hybridized states), while Δ and ${\mathrm{U}}_{\mathrm{dd}}$ , found to be ≈4.2 and ≈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 ≈1.6 eV, which justifies the blue color of ${\mathrm{CuGeO}}_3$ , and the band gap is ≈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.