Doping Dependence of the Pseudogap in La(2-x)Sr(x)CuO(4)
  We report the results of Raman scattering experiments on single crystals of La(2-x)Sr(x)CuO(4) that span the range from underdoped (x = 0.10) to overdoped (x =0.22). The spectra are consistent with the existence of a strong anisotropic quasiparticle interaction that results in a normal state depletion of spectral weight from regions of the Fermi surface located near the zone axes. The strength of the interaction decreases rapidly with increasing hole concentration and the spectral evidence for the pseudogap vanishes when the optimum doping level is reached. The results suggest that the pseudogap and superconducting gap arise from different mechanisms. 
J. G. Naeini, X. K. Chen, J. C. Irwin, M. Okuya, T. Kimura, K. Kishio