Gap excitation in Raman spectrum of an anisotropic charge-density-wave superconductor

In the presence of a static lattice distortation of wave vector Q, a direct coupling between electron density and dynamical CDW fluctuation exists in the first anharmonic term of the electron-ion potential. Based on this interaction a theory is developed for Raman scattering of light from an anisotropic CDW superconductor using a partial gapping model. The renormalization both by the Coulomb interaction at small q limit and by the residual coupling between electrons is taken into account. It is shown that this coupling not only always induces a pole at frequency ohms ≈ 2Δ (Δ is the superconducting gap) in the phonon spectrum but also contributes to the enhancement of Raman activity at gap frequency, if the coupling parameter g2(k) is anisotropic.