Electronic Raman scattering from Bi2Sr2CaCu2O8+δ: Doping dependence of the pseudogap and anomalous 600 cm-1 peak

Raman spectra of both overdoped and underdoped single crystals of Bi₂Sr₂CaCu₂O_8+δ have been studied as a function of temperature. A substantial depletion of spectral weight at low energies is observed in the underdoped samples at temperatures well above T_c and a peak centered near 600 cm^-1 is observed in the normal-state spectra of the optimally and underdoped crystals, confirming a recent report of such a peak in underdoped crystals. This peak shows little dependence on doping or temperature and there is no apparent relationship between the doping dependence of the anomalous peak at 600 cm^-1 and the pair-breaking peak observed below T_c. In contrast, the onset temperature of the normal state depletion, the depth of depletion, and both the position and intensity of the pair-breaking peak are all dependent on sample doping. The depletion of normal-state spectral weight in the underdoped materials is attributed to the opening of the pseudogap but the origin of the 600 cm^-1 peak remains unclear.