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

The reflectivities of the mixed-valent compounds Ce(Formula presented) and Ce(Formula presented) 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 Ce(Formula presented), but not for high-carrier-density Ce(Formula presented), substantial spectral weight lost from the conductivity below 1000 (Formula presented) reappears in the frequency range 2000-10 000 (Formula presented), indicating that Ce(Formula presented) can be considered a lightly doped Kondo insulator. © 1996 The American Physical Society.