Raman scattering by electronic excitations in high T-c superconductors

Electronic excitations have been observed in the normal and the superconducting states of most high T-c superconductors by means of Raman spectroscopy. The absolute scattering efficiencies measured in the various polarization configurations suggest that the scattering vertex is related to the effective masses at the Fermi surface. The interpretation of the ''normal'' scattering requires the introduction of a phenomenological scattering time which also accounts for the observed ''superconducting scattering'' well above the gap region. In the gap region, the scattering in polarized configurations contains information about the dependence of the gap on k-vector. Particularly informative in this respect is the asymptotic behavior at low frequencies. The peaks observed near gap frequencies, however, are difficult to interpret under the assumption of gaps with a simple dependence on k (such as k(x)(2) - k(y)(2)). The state of the art and outstanding problems are reviewed.