We study the polarization-dependent electronic Raman response of untwinned YBa2Cu3O7−δ superconductors employing a tight-binding band structure with anisotropic hopping matrix parameters and a superconducting gap with a mixing of d- and s-wave symmetry. Using general arguments we find screening terms in the B^{\}_{1g} scattering channel which are required by gauge invariance. As a result, we obtain a small but measurable softening of the pair-breaking peak, whose position has been attributed for a long time to twice the superconducting gap maximum. Furthermore, we predict superconductivity-induced changes in the phonon line shapes that could provide a way to detect the isotropic s-wave admixture to the superconducting gap.
@article{arxiv.cond-mat/0607421,
title = {Screening in (d+s)-wave superconductors: Application to Raman scattering},
author = {Andreas P. Schnyder and Christopher Mudry and Dirk Manske},
journal= {arXiv preprint arXiv:cond-mat/0607421},
year = {2007}
}