The pseudogap in La2-x Srx CuO4: A Raman viewpoint

We report the results of Raman scattering experiments on single crystals of La2-x Srx CuO4 (La214) as a function of temperature and doping. In underdoped 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 decreasing doping, and in the most underdoped samples, with decreasing temperature. The spectral weight that is lost at low-energies (omega < 800 cm^-1 ) is transferred to the higher energy region normally occupied by multimagnon scattering. From the normalstate B-2g spectra we have determined the scattering rate T(omega, T) of qausiparticles located near the diagonal directions in k-space. In underdoped compounds, T(omega, T) is suppressed at low temperatures for energies less than Eg(x)~ 800 cm ʹ. 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.