Electronic Raman Scattering Intensity in Layered High-Tc Superconductors: Threshold Energy, Symmetry and Anisotropy of Energy Gap

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At very low temperatures, low-frequency electronic Raman scattering in a superconductor arises from pair-breaking excitations across the energy gap. In principle, this allows the possible determination of the nature of the energy gap, including its symmetry. To examine this carefully, a simplified general expression was derived for the electronic Raman scattering intensity, suitable for high-T c layered superconductors with widely open Fermi surfaces in the k z direction, perpendicular to the k x , k y reciprocal layer plane. The unscreened part of the single-particle scattering from both charge-density fluctuations (CDF) and spin-density fluctuations (SDF) was taken into account. The shape of the resulting electronic Raman spectrum depends crucially on the directions of the incident and scattered light polarizations, e i and e s , the wavevector transfer q=q i -q s , the nature of constant normal-state single-particle energy surfaces near the Fermi energy and the form of the energy gap function Δ(k). A careful analysis of the results for different symmetries of the gap function Δ and interaction vertex functions |Y CDF | and |Y SDF |, averaged over k z , for the tetragonal D 4h point group, showed that currently available experimental data for different high-T c cuprates do not allow one to determine the symmetry of the gap function uniquely. Although d x 2 -y 2 symmetry for the gap function in these materials may be a strong possibility, a highly anisotropic s-wave type gap function cannot be ruled out completely.