Optical reflection spectra of silicon clathrate compounds Ba8AgxSi46-x

Optical reflection spectra are measured for Ba8AgxSi46-x (0 less than or equal to x less than or equal to 6) at room temperature. A systematic decrease in the plasmon energy is found with increasing x, indicating that the carrier concentration decreases with increasing x. When x increases, the superconducting transition temperature T-c decreases. An origin of the decrease in T-c is assigned to the decrease in the density or states at the Fermi level due to the decrease in the carrier concentration.