Optical study of the charge-density-wave mechanism in 2H-TaS2 and NaxTaS2

We report an optical study of transition metal dichalcogenide 2H-TaS2 and the Na intercalated superconductor NaxTaS2 over a broad frequency range at various temperatures. A clear gap feature was observed for 2H-TaS2 when it undergoes the charge-density wave (CDW) transition. The existence of a Drude component in sigma(1)(omega) below T-CDW indicates that the Fermi surface of 2H-TaS2 is only partially gapped in the CDW state. The spectral evolution of two different NaxTaS2 crystals further confirms that the partial gap structure observed in 2H-TaS2 has a CDW origin. The CDW mechanism for 2H-TaS2 and the competition between CDW and superconductivity in the NaxTaS2 system are discussed.