Carrier relaxation, pseudogap, and superconducting gap fn high-Tc cuprates:: A Raman scattering study

We describe the results of electronic Raman-scattering experiments in differently doped single crystals of YBa2Cu3O6+x and Bi2Sr2(CaxY1-x)Cu2O8 The data in antiferromagnetic insulating samples suggest that at least the low-energy parts of the spectra of metallic samples originate predominantly from excitations of free carriers. We therefore propose an analysis of the data in terms of a memory function approach which has been introduced earlier for the current response. Dynamical scattering rates Gamma(omega) = 1/tau(omega) and mass-enhancement factors 1 + lambda(omega) = m* (omega)/m of the carriers are obtained. It is found that a strong polarization dependence of the carrier lifetime develops towards low doping. In B-2g (xy) symmetry selecting predominantly electrons with momenta along the diagonals of the CuO2 planes the Raman data compare well with the results obtained from de and dynamical transport. In B-1g (x(2)-y(2)) symmetry projecting out momenta along the Cu-O bonds the de scattering rates of underdoped materials become temperature independent and considerably larger than in B-2g symmetry. This increasing anisotropy is accompanied by a loss of spectral weight in B-2g symmetry in the range between the superconducting transition at T-c and a characteristic temperature T* of the order of room temperature which compares well with the pseudogap temperature found in other experiments. The energy range affected by the pseudogap is doping and temperature independent. The integrated spectral loss is approximately 25% in underdoped samples and becomes much weaker towards higher carrier concentration. In underdoped samples, superconductivity-related features in the spectra can be observed only in B-2g symmetry. The peak frequencies scale with T-c. We do not find a direct relation between the pseudogap and the superconducting gap.