Infrared spectroscopy of the intermediate-valence semiconductor YbB12

The dynamic conductivity and permittivity spectra of the intermediate-valence compound YbB12 are measured in the frequency range (6-104) cm-1 (quantum energy 0.75 meV-1.24 eV) at temperatures of 5-300 K. Analysis of the spectral singularities associated with the response of free charge carriers has made it possible for the first time to determine the temperature dependences of their microscopic parameters, viz., concentration, effective mass, relaxation frequency and time, mobility, and plasma frequency. It is shown that the relaxation frequency decreases upon cooling from 300 K to the coherence temperature T* = 70 K for YbB12, which is mainly associated with the phonon mechanism of scattering of charge carriers. For cooling below the coherence temperature T* = 70 K, the temperature dependence of the relaxation frequency for charge carriers of the Fermi-liquid type is found to be γ ∼ γ0 + T2, while their effective mass and relaxation time increase, respectively, to m*(20 K) = 34m0 (m0 is the free electron mass) and τ(20 K) = 4 x 10-13 s, indicating the establishment of coherent scattering of carriers from localized magnetic moments of the f centers. At a temperature of T = 5 K, the conductivity spectrum contains an absorption line at a frequency of 22 cm-1 (2.7 meV); the origin of this line can be associated with the exciton-polaron bound state. Since such a state was observed earlier in other intermediate-valence semiconductors (such as SmB6, TmSe1-xTe, and (Sm, Y)S), it is probably typical of this class of compounds