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

We describe the results of electronic Raman-scattering experiments in diffe rently doped single crystals of YBa2Cu3O6+x and Bi2Sr2(CaxY1-x)Cu2O8 The da ta in antiferromagnetic insulating samples suggest that at least the low-en ergy 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 fo r the current response. Dynamical scattering rates Gamma(omega) = 1/tau(ome ga) and mass-enhancement factors 1 + lambda(omega) = m* (omega)/m of the ca rriers are obtained. It is found that a strong polarization dependence of t he carrier lifetime develops towards low doping. In B-2g (xy) symmetry sele cting predominantly electrons with momenta along the diagonals of the CuO2 planes the Raman data compare well with the results obtained from de and dy namical transport. In B-1g (x(2)-y(2)) symmetry projecting out momenta alon g the Cu-O bonds the de scattering rates of underdoped materials become tem perature independent and considerably larger than in B-2g symmetry. This in creasing anisotropy is accompanied by a loss of spectral weight in B-2g sym metry in the range between the superconducting transition at T-c and a char acteristic temperature T* of the order of room temperature which compares w ell with the pseudogap temperature found in other experiments. The energy r ange affected by the pseudogap is doping and temperature independent. The i ntegrated spectral loss is approximately 25% in underdoped samples and beco mes 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 direc t relation between the pseudogap and the superconducting gap.