Evidence of charge-density waves in the optical properties of SmNiC₂

In this study, we have measured the optical reflectivity spectra of low-dimensional SmNiC₂ (T_C=18.5 K and T_CDW=151 K) in the terahertz to ultraviolet frequency range at various temperatures. The optical conductivity spectra were transformed from the reflectivity data using the Kramers-Kronig relation. The optical conductivity of the normal state is well described by the Drude-Lorentz model assuming two bands. In nonmagnetic states below T_CDW, two clean gaps opening on top of the Drude response are observed at around 500 and 750 cm^-1. In the ferromagnetic state, the higher-side gap opening remains at the same frequency but the lower-side gap opening disappears, instead an additional absorption due to magnon gap formation is observed at around 70 cm^-1.