The temperature dependence of the low energy excitation (LEE) mode in
cation superionic conductors is explained based upon an ionic plasma m
odel. We regard the lattice ions as a uniform negative charge distribu
tion and consider the LEE mode as a plasma oscillation of cations unde
r the anion background. The model explains qualitatively the character
istic features of the LEE mode, A comparison with the boson peak obser
ved in amorphous solids is also presented.