RESONANT RAMAN-SCATTERING IN BI2SR2CA1-ZYZCU2O8+ DELTA SINGLE-CRYSTALS

Bi2Sr2Ca1-zYzCu2O8+delta single crystals have been investigated by means of Raman scattering as a function of doping (0 less-than-or-equal-to z less-than-or-equal-to 0.85), incident laser energy, temperature and polarization directions of the incident and scattered light. Compared to pure Bi2Sr2CaCu2O8+delta, additional phonons are found at 141, 326 and 434 cm-1. At the incident laser energy of 2.6 +/- 0.1 eV, the 463 cm-1 "in-phase out-of-plane" and the 275 cm-1 "out-of-phase out-of-plane" vibrations of the oxygens situated in the CuO2 planes as well as the Cu vibration at 165 cm-1 show a resonance. The resonant enhancement for the Cu vibration is larger by a factor of 20 than that for the oxygen modes. While the resonance energy epsilon(res) is independent of the Y-concentration z, the intensity of all the above resonating phonons at epsilon(res) is increased with increasing z, i.e., with a decreasing concentration of holes in the CuO2 layers. The qualitative interpretation of the main resonant Raman-scattering features in Bi2Sr2Ca1-zYzCu2O8+delta can be given within the framework of a model, which takes into account the possibility of the creation of additional, "optically pumped" Zhang-Rice singlets formed through the excitation of electrons from the valence (O 2p) band into the upper Hubbard band of Cu.