Charge dynamics of Ce-based compounds: Connection between the mixed valent and Kondo-insulator states.

The reflectivities of the mixed-valent compounds CeSn3 and CePd3 have been measured to obtain the optical conductivity as a function of temperature. Both compounds show a renormalization of the low-energy Drude conductivity at low temperatures. In addition, for the low-carrier-density compound CePd3 , but not for high-carrier-density CeSn3 , substantial spectral weight lost from the conductivity below 1000 cm 21 reappears in the frequency range 2000–10 000 cm, indicating that CePd3 can be considered a lightly doped Kondo insulator.