Low-energy electrodynamics of SmB6

We have performed direct measurements of the low-temperature dynamical conductivity and dielectric permittivity of single crystalline SmB6 in the spectral range from 0.6 to 4.5 meV, i.e., below the hybridization gap. The obtained results together with the data of Hall-effect and infrared reflection measurements give evidence for a 19-meV energy gap in the density of states and an additional narrow donor-type band lying only 3 meV below the bottom of the upper conduction band. It is shown that at temperatures 5 K<T<20 K the electrodynamic response and the de conductivity of SmB6 are determined by quasifree carriers thermally excited in the conduction band. We evaluate the microscopic parameters of these carriers: the spectral weight, the concentration, the effective mass, the scattering rate, and the mobility. Below 8 K the concentration of carriers in the conduction band freezes out exponentially and finally the electronic properties of SmB6 are determined by the localized carriers in the narrow band with the typical signature of hopping conductivity. [S0163-1829(99)07603-1].