Resonant Raman scattering in Bi 2Sr 2Ca 1−zY zCu 2O 8+δ single crystals

Abstract

Bi 2Sr 2Ca 1− z Y z Cu 2O 8+ δ single crystals have been investigated by means of Raman scattering as a function of doping (0≤ z≤0.85), incident laser energy, temperature and polarization directions of the incident and scattered light. Compared to pure Bi 2Sr 2CaCu 2O 8+δ, 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 CuO 2 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 ϵ res is independent of the Y-concentration z, the intensity of all the above resonating phonons at ϵ res is increased with increasing z, i.e., with a decreasing concentration of holes in the CuO 2 layers. The qualitative interpretation of the main resonant Raman-scattering features in Bi 2Sr 2Ca 1− z Y z Cu 2O 8+ δ 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.