Effect of conduction electrons on the polarized Raman spectra of copper oxide superconductors

The line shape, laser-wavelength dependence, coupling to phonons, and doping dependence of the Raman continua in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ single crystals differ for photons polarized parallel and perpendicular (ZZ geometry) to the Cu-O planes. The ZZ continuum in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ can be accurately fit to a formula that describes scattering by a ‘‘dirty’’ free electron gas with frequency-independent damping. The doping dependence of the ZZ continuum appears to be consistent with electronic scattering. We have found evidence against scattering by conduction electrons being the origin of the plane-polarized continua in copper oxides: in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ and in ${\mathrm{Sm}}_2$ ${\mathrm{CuO}}_4$ materials the a-axis-polarized continuum does not change much with doping (a continuum in the insulating phases is just as strong as in the superconductors), whereas ir reflectivity measurements show that the density of the electronic states is strongly affected by doping.