RESONANT RAMAN-SCATTERING OF SUPERCONDUCTING BI2SR2CACU2O8+DELTA SINGLE-CRYSTALS

We report on polarized Raman spectra taken on the ab-plane of Bi2Sr2CaCu2O8+delta single crystals for incident laser energies ranging from 1.83 eV to 2.70 eV. We find a resonance at 2.62 +/- 0.02 eV for the "in-phase out-of-plane" (A(g)-symmetry) and "out-of-phase out-of-plane" (B1g-symmetry) vibrations of the oxygens in the CuO2-layers. This resonance can be explained by a two-band Raman scattering process between the {Cu 3d-O 2p} bands below the Fermi energy epsilon-F and the Upper Hubbard band (UHB) above epsilon-F and vice versa. From the onset of this resonance the "charge transfer gap" DELTA-CT can be estimated to be DELTA-CT = 2.5 +/- 0.1 eV. In B1g-symmetry, the electronic scattering continuum is strongly enhanced for incident laser energies larger than 2.5 eV and thus points at a three-band Raman scattering process occurring at this energy. The latter might involve the hybridized {O 2p-Cu 3d} valence bands, the UHB and correlated singlet band-like states (width almost-equal-to 60 meV) in the vicinity of epsilon-F.