The optical properties of the intermediate valence semiconductor ${\mathrm{YbB}}_{12}$ are studied at frequencies $8{\mathrm{cm}}^{-1}\text{–}10000{\mathrm{cm}}^{-1}$ (quantum energies 1 meV to 1.2 eV) and temperatures 5 K–300 K. Microscopic parameters of charge carriers are determined: effective mass, mobility, relaxation rate, relaxation time, plasma frequency. A strong decrease of the relaxation rate is observed when cooled down to 70 K, indicating that scattering is mediated mainly by phonons and not by the Kondo effect. Below the coherence temperature of $T^{*}=70\mathrm{K}$ a Drude-like response of renormalized quasiparticles is observed with a scattering rate $\gamma$ following the Fermi liquid behavior $\gamma \propto T^2$, an effective mass $m^{*}\approx 34m_0$, and an enhanced relaxation time $\tau =4\times {10}^{-13}\mathrm{s}$, implying that coherent effects play a crucial role in formation of the ground state in ${\mathrm{YbB}}_{12}$. At $T=5\mathrm{K}$ an absorption peak is found at 2.7 meV whose origin can be ascribed to an exciton-polaronic bound state.

• Received 30 September 2005

DOI:https://doi.org/10.1103/PhysRevB.73.045207