Effect of disorder in the charge-density-wave compounds LaTe _1.95 and CeTe _1.95-xSe _x (x=0 and 0.16) as revealed by optical spectroscopy

We present optical spectroscopy measurements on rare-earth ditelluride single crystals of LaTe _1.95 and CeTe _1.95-xSe _x (x=0 and 0.16). The measurements reveal formation of charge-density-wave energy gaps at rather high-energy levels, for example, 2Δ∼8500 cm ^-1 for LaTe _1.95 and 6800 cm ^-1 for CeTe _1.95. More strikingly, the study reveals that, different from the rare-earth tritellurides, the Te vacancies and disorder effect play a key role in the low-energy charge excitations of ditelluride systems. Although an eminent peak is observed between 800 and 1500 cm ^-1 in conductivity spectra for LaTe _1.95, and CeTe _1.95-xSe _x (x=0 and 0.16), our analysis indicates that it could not be attributed to the formation of a small energy gap, instead it could be well accounted for by the localization modified Drude model. Our study also indicates that the low-temperature optical spectroscopic features are distinctly different from a semiconducting charge-density-wave (CDW) state with entirely gapped Fermi surfaces.