The pseudogap in La2-xSrxCuO4: A raman viewpoint

We report the results of Raman scattering experiments on single crystals of La2-xSrxCuO4 (La214) as a function of temperature and doping. In underdope d compounds low-energy B-1g spectral weight is depleted in association with the opening of a pseudogap on regions of the Fermi surface located near (/- pi,0) and (0, +/- pi). The magnitude of the depletion increases with dec reasing doping, and in the most underdoped samples, with decreasing tempera ture. The spectral weight that is lost at low-energies (omega less than or equal to 800 cm(-1)) is transferred to the higher energy region normally oc cupied by multimagnon scattering. From the normal-state B-2g spectra we hav e determined the scattering rate Gamma(omega,T) of qausiparticles located n ear the diagonal directions in k-space. In underdoped compounds, Gamma(omeg a,T) is suppressed at low temperatures for energies less than E-g(x) simila r or equal to 800 cm(-1). The doping dependence of both the two-magnon scat tering and the scattering rate suppression suggest that the pseudogap is ch aracterized by an energy scale E-g similar to J, where J is the antiferroma gnetic super-exchange energy. Comparison with the results from other techni ques provides a consistent picture of the pseudogap in La214.