Raman Scattering by Superconducting-Gap Excitations and Their Coupling to Charge-Density Waves

$2H$-Nb${\mathrm{Se}}_2$ undergoes a charge-density-wave (CDW) distortion at 33 K which induces $A$ and $E$ Raman-active phonon modes. These are joined in the superconducting state at 2 K by new $A$ and $E$ Raman modes close in energy to the BCS gap $2\Delta$. Magnetic fields suppress the intensity of the new modes and enhance that of the CDW-induced modes, thus providing evidence of coupling between the superconducting-gap excitations and the CDW.