Kondo coupling induced charge gap in ${\mathrm{Ce}}_3$${\mathrm{Bi}}_4$${\mathrm{Pt}}_3$

Measurements of the infrared reflectivity of the Kondo insulator ${\mathrm{Ce}}_3$ ${\mathrm{Bi}}_4$ ${\mathrm{Pt}}_3$ are reported. Near room temperature the charge dynamics are comparable to those of a heavy fermion compound in the incoherent regime; however, below 100 K the depletion of the low frequency conductivity singifies the development of a charge gap at low frequency (∼300 ${\mathrm{cm}}^{\mathrm{-}1}$). The temperature dependence of the depleted spectral weight scales with the quenching of the Ce 4f moments, demonstrating that the gap formation is due to the local Kondo coupling of charge carriers to the Ce magnetic moments. The spectral weight which disappears as the gap forms must be displaced to energies much larger than the gap.