Mean free path effects on the electronic Raman spectra of d-wave superconductors

Because of its polarization dependence, Raman scattering appears to be particularly well suited to study the anisotropy of quasiparticle interactions. It is likely that these interactions are responsible both for finite mean free paths and for superconductivity. Since d-wave superconductivity in a single band is possible only in the presence of a strongly anisotropic interaction, direct evidence for such anisotropy would lend support to the currently favored notion of a pair state with d-wave symmetry. Comparison of the low frequency Big and Bag response in the normal state could provide an indication of the strength and anisotropy of the interaction. In the superconducting state a quasiparticle contribution to the Raman response, which would carry information on the anisotropy and temperature dependence of the interaction in the most easily identifiable way, should be observable in Bag Symmetry, if the experimental resolution at low frequencies is improved. Unfortunately, it is very unlikely that the corresponding data in Big symmetry will ever become available. In any case, high quality samples are required because disorder induced s-wave scattering would obscure effects due to the pairing interaction. The pair breaking peak also carries information on quasiparticle scattering, but it would take a careful quantitative analysis of the temperature dependence to find out whether pairing and scattering are described consistently by one interaction. The relative change in the Raman intensities at high frequencies should reflect the degree to which the quasiparticle interaction is separable with respect to frequency and momentum.