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We report the results of Raman scattering experiments on single crystals of La 2Ϫx Sr x CuO 4 that span the range from underdoped (xϭ0.10) to overdoped (xϭ0.22). 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. ͓S0163-1829͑99͒09913-0͔ PRB 59 9643 DOPING DEPENDENCE OF THE PSEUDOGAP IN . . .

Raman scattering studies of B x C 1Ϫx films have been carried out for boron concentrations in the range 0рxр0.17. The spectra exhibit two broad bands, a graphitic mode (G) centered between 1535 and 1590 cm Ϫ1 and a disorder-induced mode ͑D͒ centered between 1345 and 1370 cm Ϫ1 . The G mode is observed to soften as a result of boron substitution, and this behavior has been explained using a simple two-dimensional lattice dynamics model. As the boron concentration is increased, a decrease in the intensity of the D band is also observed. Furthermore, the variation of the intensity of the D band with x is found to be correlated with the interplaner spacing. It is thus concluded that the presence of the D mode is associated with the degree of disorder along the c axis. Finally, a phonon confinement model has been used to correlate the linewidth of the G band with the crystallite size in the materials. The results of fitting the calculated line shapes to the measured spectra provides an estimate for the crystallite dimension L a and also indicates that the frequency and linewidth of the G mode are most strongly influenced by the structural order within the a-b hexagonal planes. ͓S0163-1829͑96͒05722-0͔

We report the results of Raman scattering experiments on single crystals of La2–xSrxCuO4 (La214) as a function of temperature and doping. In underdoped compounds low-energy B1g spectral weight is depleted in association with the opening of a pseudogap on regions of the Fermi surface located near (±π, 0) and (0, ±π).The magnitude of the depletion increases with decreasing doping, and in the most underdoped samples, with decreasing temperature.The spectral weight that is lost at low energies (ω [Formula: see text] 800 cm–1) is transferred to the higher energy region normally occupied by multimagnon scattering. From the normal-state B2g spectra we have determined the scattering rate Γ(ω,T) of qausiparticles located near the diagonal directions in k-space. In underdoped compounds, Γ(ω,T) is suppressed at low temperatures for energies less than Eg(x) [Formula: see text] 800 cm–1. The doping dependence of both the two-magnon scattering and the scattering rate suppression suggest that the pseudogap is characterized by an energy scale Eg ~ J, where J is the antiferromagnetic super-exchange energy. Comparison with the results from other techniques provides a consistent picture of the pseudogap in La214.PACS Nos.: 74.25.Gz, 74.72.Dn, 78.30.Er

We report the results of electronic Raman-scattering experiments on an overdoped La 1.78 Sr 0.22 CuO 4 single crystal as a function of temperature. The scattering rate ⌫(⍀→0,T) has been determined from the normal state B 1g spectra in the range 50рTр300 K. ⌫(T) decreases linearly from 300 K to about 175 K and then undergoes a reduction with respect to the expected mean-field behavior. This trend suggests a crossover to a magnetic pseudoscaling regime at T cr Ӎ160 K. The results are in good agreement with the prediction of the nearly antiferromagnetic Fermi-liquid model. There is no evidence of a pseudogap in the spectra obtained from this overdoped sample. ͓S0163-1829͑98͒50918-6͔