Optical study of the charge-density-wave mechanism in 2H-TaS₂ and Na_xTaS₂

We report an optical study of transition metal dichalcogenide 2$H$-TaS$_2$ and Na intercalated superconductor Na$_x$TaS$_2$ over a broad frequency range at various temperatures. A clear gap feature was observed for 2$H$-TaS$_2$ when it undergoes a CDW transition. The existence of a Drude component in $\sigma$$_1$($\omega$) below T$_{CDW}$ indicates that the Fermi surface of 2$H$-TaS$_2$ is only partially gapped in CDW state. The spectral evolution of two different Na$_x$TaS$_2$ crystals further confirms that the partial gap structure observed in 2$H$-TaS$_2$ has a CDW origin. The CDW mechanism for 2$H$-TaS$_2$, the competition between CDW and superconductivity in Na$_x$TaS$_2$ system are discussed.