Low-energy electrodynamics of ${\mathrm{SmB}}_6$

We have performed direct measurements of the low-temperature dynamical conductivity and dielectric permittivity of single crystalline ${\mathrm{SmB}}_6$ 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 $5\mathrm{K}<T<\mathrm{}20\mathrm{}\mathrm{K}$ the electrodynamic response and the dc conductivity of ${\mathrm{SmB}}_6$ 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 ${\mathrm{SmB}}_6$ are determined by the localized carriers in the narrow band with the typical signature of hopping conductivity.