Low Energy Electronic State and Optical Phonon in YbB₆

Abstract

Reflectivity spectrum of YbB₆ was measured in the energy region between 1 meV and 40 eV at 9 K and 300 K. Observed optical absorption structures in the energy region from 1 meV to 1 eV were decomposed into three parts. The first is a Drude part. The effective conduction electron number was evaluated to be about 0.9% per mole with weak temperature dependence. The second part consists of a weak absorption due to impurity centered 0.2 eV and strong interband transition between valence and conduction bands with the energy gap of about 0.5 eV. The last one is due to phonon absorption bands. By comparing with the theoretical phonon dispersion curve calculated by Takegahara et al., the absorption at about 13 meV was identified to be a T_1u-mode due to the relative motion of Yb and B₆ and the absorption at about 108 meV to a T_1u-mode due to an intra-molecular vibration of B₆.