Screening in (d+s)-wave superconductors: Application to Raman scattering

TL;DR

This paper investigates the polarization-dependent Raman response in untwinned YBa2Cu3O7−δ superconductors, revealing screening effects and predicting phonon line shape changes due to mixed d+s wave symmetry.

Contribution

It introduces a detailed analysis of screening effects in Raman scattering and predicts observable phonon line shape modifications caused by s-wave admixture.

Findings

Screening terms in B1g channel are necessary for gauge invariance.

Softening of the pair-breaking peak is observed.

Superconductivity induces detectable changes in phonon line shapes.

Abstract

We study the polarization-dependent electronic Raman response of untwinned YBa$_2$Cu$_3$O$_{7-\delta}$ 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.