Intertwined Order Parameters in a Charge-Ordered Superconductor

We present a spectroscopic study of the low temperature state in NbSe$_2$, exhibiting charge-density wave and superconductivity orders. Raman scattering reveals that the spectrum of quasi-particle excitations out of the condensate is characterized by an energy gap derived from both order parameters in a way that suggests intertwining between them. Supported by a calculation of NbSe$_2$ Raman vertices, and by earlier photoemission studies, we conclude that in NbSe$_2$ an isotropic superconductivity interplays with a strongly anisotropic charge-density wave order on selected parts of the Fermi surface, as characterized by admixture of particle-particle and particle-hole excitations.