Raman scattering from superconducting gap excitations in the presence of a magnetic field

We recently observed, using Raman scattering as a probe, the superconducting gap excitation in $2H-\mathrm{N}\mathrm{b}{\mathrm{Se}}_2$. This was seen by the appearance of two new Raman-active modes of different symmetry $A$ (18 ${\mathrm{cm}}^{-1\mathrm{}}$) and $E$ (15 ${\mathrm{cm}}^{-1\mathrm{}}$) when the sample is superconducting. In this paper we present further experimental details and results that confirm the theory of Balseiro and Falicov, which shows that the new modes borrow their Raman activity from low-lying charge-densitywave (CDW) phonons. Evidence of this direct coupling between superconducting electrons and the CDW phonon is seen by the behavior of the modes in a magnetic field. Such behavior of the Raman spectra is explained in terms of a model involving a complex gap. The effect of life-time broadening of the CDW phonon and consequences of varying coupling strength are presented.