New Evidences for the observation of the Higgs boson in the Superconductor 2H-NbSe$_2$

We provide here new evidences for the observation of the amplitude mode of the superconducting order parameter, the so-called Higgs Boson, in 2H-NbSe$_2$. We report quantitatively comparative electronic Raman measurements on the dichalcogenides 2H-NbSe$_2$, whose superconductivity (SC) coexists with a charge density wave order (CDW), and 2H-NbS$_2$, which exhibits only the SC. A SC pair breaking peak develops below T$_c$ in 2H-NbS$_2$ whose intensity is much smaller than the peak associated with the SC in 2H-NbSe$_2$. Thus, the peak observed in 2H-NbSe$_2$ below T$_c$ certainly doesn't get its intensity only from the superconducting condensate. Moreover, we measure precisely a spectral weight transfer from the amplitude mode of the CDW to the SC peak in 2H-NbSe$_2$, versus decreasing temperature. The total spectral weight for both peaks is constant within $\pm~3\%$. This result is consistent with the theory of the observation of a Higgs mode thanks to its coupling with an amplitudon developed by Littlewood and Varma. This result complements what was firstly observed by Sooryakumar et Klein under magnetic field.